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SCHOLAR Study Guide CfE Higher Chemistry Unit 1: Chemical Changes and Structure Authored by: Emma Maclean (George-Heriot’s School) Reviewed by: Diane Oldershaw (Menzieshill High School) Previously authored by: Peter Johnson Brian T McKerchar Arthur A Sandison Heriot-Watt University Edinburgh EH14 4AS, United Kingdom. First published 2014 by Heriot-Watt University. This edition published in 2016 by Heriot-Watt University SCHOLAR. Copyright © 2016 SCHOLAR Forum. Members of the SCHOLAR Forum may reproduce this publication in whole or in part for educational purposes within their establishment providing that no profit accrues at any stage, Any other use of the materials is governed by the general copyright statement that follows. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, without written permission from the publisher. Heriot-Watt University accepts no responsibility or liability whatsoever with regard to the information contained in this study guide. Distributed by the SCHOLAR Forum. SCHOLAR Study Guide Unit 1: CfE Higher Chemistry 1. CfE Higher Chemistry Course Code: C713 76 ISBN 978-1-909633-20-9 Print Production and Fulfilment in UK by Print Trail www.printtrail.com Acknowledgements Thanks are due to the members of Heriot-Watt University's SCHOLAR team who planned and created these materials, and to the many colleagues who reviewed the content. We would like to acknowledge the assistance of the education authorities, colleges, teachers and students who contributed to the SCHOLAR programme and who evaluated these materials. Grateful acknowledgement is made for permission to use the following material in the SCHOLAR programme: The Scottish Qualifications Authority for permission to use Past Papers assessments. The Scottish Government for financial support. The content of this Study Guide is aligned to the Scottish Qualifications Authority (SQA) curriculum. All brand names, product names, logos and related devices are used for identification purposes only and are trademarks, registered trademarks or service marks of their respective holders. i Contents 1 Reaction rates - collision theory 1.1 Prior knowledge . . . . . . . 1.2 Introduction . . . . . . . . . 1.3 Rate of reaction . . . . . . . 1.4 Collision theory . . . . . . . 1.5 Summary . . . . . . . . . . . 1.6 Resources . . . . . . . . . . 1.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 3 4 6 10 10 10 2 Reaction rates - reaction profiles 2.1 Prior knowledge . . . . . . . 2.2 Interpreting graphs . . . . . 2.3 Activation energy . . . . . . 2.4 Summary . . . . . . . . . . . 2.5 Resources . . . . . . . . . . 2.6 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 19 19 25 30 30 31 3 Catalysis 3.1 Prior knowledge . . . . . . 3.2 Introduction to catalysis . . 3.3 Catalyst mechanism . . . 3.4 Catalysts in industry . . . . 3.5 Potential energy diagrams 3.6 Catalysts and energy . . . 3.7 Summary . . . . . . . . . . 3.8 Resources . . . . . . . . . 3.9 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 37 37 38 39 43 52 60 61 62 . . . . . . . 67 69 69 70 75 79 79 80 5 Bonding and structure 5.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 85 86 . . . . . . . . . 4 The Periodic Table 4.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Arrangement of elements in the Periodic Table: Introduction 4.3 History of the Periodic Table . . . . . . . . . . . . . . . . . . 4.4 Trends and patterns (periodicity) . . . . . . . . . . . . . . . . 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii CONTENTS 5.3 5.4 5.5 5.6 5.7 5.8 Metallic bonds . Covalent bonds Ionic bonds . . Summary . . . . Resources . . . End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Periodic Table trends 6.1 Prior knowledge . . . . . . . . . . . . . . 6.2 Covalent radius . . . . . . . . . . . . . . 6.3 Ionisation energies . . . . . . . . . . . . 6.4 Electronegativity . . . . . . . . . . . . . . 6.5 Summary of trends in the Periodic Table 6.6 Summary . . . . . . . . . . . . . . . . . . 6.7 Resources . . . . . . . . . . . . . . . . . 6.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Bonding continuum and polar covalent bonding 7.1 Prior knowledge . . . . . . . . . . . . . . . . . 7.2 Polar covalent bonds . . . . . . . . . . . . . . 7.3 Predicting bonding type using electronegativity 7.4 Polar molecules . . . . . . . . . . . . . . . . . 7.5 The bonding continuum . . . . . . . . . . . . . 7.6 Summary . . . . . . . . . . . . . . . . . . . . . 7.7 Resources . . . . . . . . . . . . . . . . . . . . 7.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 92 102 107 107 108 . . . . . . . . 113 115 116 119 125 128 129 129 130 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 139 140 144 146 150 152 152 153 8 Intermolecular forces 8.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . 8.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 8.3 London dispersion forces . . . . . . . . . . . . . . . . . 8.4 Hydrogen bonding . . . . . . . . . . . . . . . . . . . . . 8.5 Relating properties to bonding . . . . . . . . . . . . . . 8.6 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Predicting solubilities from solute and solvent polarities 8.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Resources . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 160 160 162 166 169 172 174 178 179 180 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 End of unit test 183 Glossary 193 Answers to questions and activities 1 Reaction rates - collision theory . . . . . . . . . 2 Reaction rates - reaction profiles . . . . . . . . 3 Catalysis . . . . . . . . . . . . . . . . . . . . . 4 The Periodic Table . . . . . . . . . . . . . . . . 5 Bonding and structure . . . . . . . . . . . . . . 6 Periodic Table trends . . . . . . . . . . . . . . . 7 Bonding continuum and polar covalent bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 196 199 201 206 209 213 218 © H ERIOT-WATT U NIVERSITY CONTENTS 8 9 Intermolecular forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . End of unit test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © H ERIOT-WATT U NIVERSITY iii 221 224 1 Topic 1 Reaction rates - collision theory Contents 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Prior knowledge Introduction . . Rate of reaction Collision theory Summary . . . Resources . . . End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3 4 6 10 10 10 Prerequisite knowledge Before you begin this topic, you should already know that: • reactions can be monitored by measuring changes in concentration, mass and volume of reactants and products; • reactions can be monitored and graphs drawn and interpreted in relation to rate; • the rates of reactions are affected by changes in concentration, particle size and temperature; • calculations can be carried out to determine the average rate of a chemical reaction from a graph of the change in mass or volume against time. Learning objectives By the end of this topic, you should be able to: • state that reaction rates can be controlled by chemists; • explain that if reaction rates are too low a manufacturing process will not be economically viable; • explain that if reaction rates are too high there is a risk of thermal explosion; • describe, using collision theory, the effects of concentration, pressure, surface area (particle size), temperature and collision geometry on reaction rates; • calculate the relative rate of a reaction using the formula Rate = 1t . 2 TOPIC 1. REACTION RATES - COLLISION THEORY 1.1 Prior knowledge Test your prior knowledge Q1: Go online What three variables can be altered to change the rate of a reaction? .......................................... Q2: Name two variables you could monitor to follow the course of a reaction. .......................................... Q3: A student obtains the following set of results shown in the graph when carrying out a reaction with marble chips and dilute hydrochloric acid. What is the average rate, in g s-1 , of reaction between 60 and 120 seconds? .......................................... .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY 1.2 Introduction Chemical reactions occur at different rates depending on the nature of the reaction and the conditions under which it occurs. Fireworks are fast reactions Rusting is a slow reaction Rate is very important to chemists. In industry, as in the lab, it is important to have knowledge of how to control rates of reaction. If reaction rates are too low, a manufacturing process will not be economically viable. However, if reaction rates are too high, there is a risk of thermal explosion. How can rate be measured and what is meant by rate anyway? Think about this: Imagine yourself having to travel 500 kilometres. There are three methods of travel available: a) walking; b) fast car; c) rocket. In each case, think about it and decide on a rough time it would take to complete the journey, then work out the rate of travel. The rate of travel in a), b) or c) will be measured in units of distance per time interval, possibly in these units: i. km day-1 ii. km hour-1 iii. km second-1 In each case, the rate is 'per time interval'. Rate is measured in units involving the reciprocal of time. If the time taken is low, the rate is high. If the time taken is high, the rate is low. © H ERIOT-WATT U NIVERSITY 3 4 TOPIC 1. REACTION RATES - COLLISION THEORY Rate is proportional 1 t The study of rates of reaction and the factors which influence the rate is known as 'kinetics'. Knowledge of how to control the rate of reaction is very important in the chemical industry. 1.3 Rate of reaction During the course of a chemical reaction, reactants are being converted into products. Measurement of the rate of reaction involves measurement of the change in the amount of a reactant or product in a certain time. The rate of reaction changes as it progresses, being relatively fast at the start and slowing towards the end. What is being measured is the average rate over the time interval chosen. Reactions can be followed by measuring changes in concentration, mass and volume, or by using properties of reactants or products which change in step with concentration such as pressure, conductivity, pH or colour intensity. If the change in concentration is measured then: Average rate = Change in concentration Change in time The units of rate would be moles per second (written as mol s -1 ). The units will, of course, depend on which variable is being measured. In general: Average rate = Change in variable Change in time It is worth mentioning at this stage that the unit of concentration of a solution is measured as moles per litre. In previous courses, this may have been shortened to mol/ or even as mol/cm3 . At this level, the same unit will always be written as mol -1 . This means that a change in concentration will be measured as mol -1 s-1 . © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY 5 Measuring rate Measuring the speed of a car, or even a rocket, can be performed directly using a speedometer. Measuring the rate of a reaction directly is more difficult since there is no such instrument as a reaction rate meter. A change in the amount of reactant or product over time can be used. When zinc reacts with hydrochloric acid, hydrogen gas is released. The volume of gas produced can be recorded at regular time intervals. The illustration shows the apparatus used and the results. Q4: What volume of hydrogen is released in the first 5 seconds? .......................................... Q5: Calculate the rate of hydrogen production in the first 5 seconds (remember units). .......................................... Q6: Calculate the rate of hydrogen production from 5 to 10 seconds in the reaction. .......................................... Q7: Explain the pattern of these two results. .......................................... Q8: Calculate the average rate of the reaction from the start to the end. (Hint: When has it ended?) .......................................... © H ERIOT-WATT U NIVERSITY Go online 6 TOPIC 1. REACTION RATES - COLLISION THEORY 1.4 Collision theory All substances are made up of particles called atoms, ions or molecules, and these particles are constantly moving. The degree of movement depends upon the state of the substance. This is known as the 'kinetic model' of matter. In any sample of solution, liquid or gas there is a range of kinetic energies known as an energy distribution. The collision theory of reactions suggests that, for a chemical reaction to occur, particles must collide. Simple collision is not enough however, as many collisions do not have sufficient kinetic energy to successfully rearrange the reactants to form new products. In many cases, the way the particles line up, sometimes called the collision geometry or orientation, is wrong and successful collision cannot occur. Collision theory, based on the kinetic model of matter, provides an explanation for the effect that various factors have on the rate of chemical reactions in terms of the number of successful collisions which occur. Collision theory can be stated thus: • particles must collide to react; • not all collisions are successful; • sufficient energy is needed; • orientation must be correct. In the illustration the single step formation of products in the reaction is a simplification. The reaction itself actually proceeds in a number of stages. Key point The rates of reactions are affected by changes in concentration, particle size and temperature, and the collision theory can be used to explain these effects. Earlier work in chemistry showed that the rate of a reaction can be influenced by concentration of reactants, particle size and temperature. How can collision theory explain these effects? A suitable reaction which will be used to study this over the next three activities is the reaction of hydrochloric acid with a sample of calcium carbonate (chalk). © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY 7 Collisions and concentration Look at the illustrations showing the result of collisions between two different concentrations of hydrochloric acid and calcium carbonate, both after 10 seconds of reaction. The hydrochloric acid is represented as a large sphere and the calcium carbonate as a small sphere. Products of the reaction are shown as a combination of a small sphere and a large sphere. Q9: How many successful collisions have occurred using the 1mol -1 acid after 10 seconds? .......................................... Q10: What is the rate of reaction with calcium carbonate and 1 mol -1 acid expressed as successful collisions per second? .......................................... Q11: What is the rate of reaction with calcium carbonate and 2 mol -1 acid expressed in the same units? .......................................... Q12: Predict what the rate might be if 4 mol -1 acid were used in the same experiment. .......................................... Key point In many reactions the rate of reaction is directly proportional to the concentration of reactant, but there is no simple way to predict the relationship in advance. .......................................... © H ERIOT-WATT U NIVERSITY Go online 8 TOPIC 1. REACTION RATES - COLLISION THEORY Collisions and particle size Go online Look at the illustrations showing the result of collisions between hydrochloric acid and two particle sizes of the same mass of calcium carbonate, both after 10 seconds of reaction. Q13: Which sample of calcium carbonate has the greater surface area? (Hint: Compare the number of particles on the outside edges of the solids which would have been available before reaction.) .......................................... Q14: How many successful collisions have occurred with the large sample after 10 seconds? .......................................... Q15: What is the rate of reaction with the large sample expressed as successful collisions per second? .......................................... Q16: What is the rate of reaction with the small sample expressed in the same units? .......................................... Key point In many reactions involving solid reactant the rate of reaction is raised if the surface area of the reactant is increased. This is the same as saying the particle size is decreased. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY 9 Collisions and temperature Look at the illustrations showing the result of collisions between hydrochloric acid and calcium carbonate at temperature T ◦ C and then at T + 10 ◦ C, both after 10 seconds of reaction. Q17: How many successful collisions have occurred at T◦ C after 10 seconds? .......................................... Q18: What is the rate of reaction at T ◦ C expressed as successful collisions per second? .......................................... Q19: What is the rate of reaction at T +10 ◦ C expressed in the same units? .......................................... Q20: What happens to the rate when the temperature is increased by 10 ◦ C? a) It stays the same. b) It halves. c) It doubles. d) It quadruples. .......................................... Key point In many reactions a rise in temperature of 10 ◦ C causes the rate of reaction to approximately double. There are more successful collisions in the earlier parts of the reactions, when there are plenty of reactant particles available to react. As some react, there are fewer available for collision and therefore the rate of reaction falls. This is as expected as a consequence of collision theory. .......................................... © H ERIOT-WATT U NIVERSITY Go online 10 TOPIC 1. REACTION RATES - COLLISION THEORY 1.5 Summary Summary 1.6 • 1.7 • Rates of chemical reactions can be controlled by chemists. • If reaction rates are too low, a manufacturing process will not be economically viable. • If reaction rates are too high, there is a risk of thermal explosion. • The rates of reactions are affected by changes in concentration, particle size and temperature, and collision theory can be used to explain these effects. • The relative rate of a reaction can be calculated using the formula Rate = 1t . Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 End of topic test End of Topic 1 test Q21: Go online For any chemical, the temperature is a measure of the: a) average kinetic energy of all the particles. b) minimum kinetic energy required before reaction occurs. c) average kinetic energy of the particles which react. d) activation energy. .......................................... Q22: Which of the following is not a factor which affects the rate of a reaction? a) Time taken for reaction to complete. b) Concentration of reactants. c) Collision geometry. d) Kinetic energy of reactants. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY Q23: When marble pieces and hydrochloric acid are reacted, carbon dioxide is evolved. The curves below, showing the mass of the reaction vessel, were obtained under different conditions. The change in form of the rate of the curve from P to Q would be obtained by: a) increasing the concentration of the acid. b) decreasing the volume of the acid. c) increasing the particle size of the marble. d) decreasing the temperature of the reactants. .......................................... Q24: Calculate the average rate of reaction in g s -1 for curve Q over the first 50 seconds of reaction. .......................................... Q25: Calculate the average rate of reaction in g s -1 for curve Q over the 250 seconds shown on the graph. .......................................... © H ERIOT-WATT U NIVERSITY 11 12 TOPIC 1. REACTION RATES - COLLISION THEORY Q26: Magnesium was added to 1.0 mol -1-1 sulfuric acid. The volume of hydrogen gas released was plotted as curve C. Which curve shows the results of a repeat experiment using the same volume of 0.5 mol -1-1 sulfuric acid? a) Curve A b) Curve B c) Curve D d) Curve E .......................................... Q27: Which curve shows the results of a repeat experiment using double the volume of 0.5 mol -1-1 sulfuric acid? a) Curve A b) Curve B c) Curve D d) Curve E .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY 13 Q28: The same reaction was carried out at four different temperatures. The table shows the times taken. Temperature (◦ C) 20 30 40 50 Time (s) 60 30 14 5 These results show that: a) a small rise in temperature results in a large increase in reaction rate. b) the activation energy increases with increasing temperature. c) the rate of reaction is directly proportional to the temperature. d) the reaction is endothermic. .......................................... Q29: A reaction was carried out under different conditions. The curves shown were obtained when copper carbonate was reacted with acid, carbon dioxide being produced. The change from P to Q could be brought about by: a) decreasing the mass of copper carbonate. b) increasing the particle size of the copper carbonate. c) decreasing the particle size of the copper carbonate. d) increasing the concentration of the acid. © H ERIOT-WATT U NIVERSITY 14 TOPIC 1. REACTION RATES - COLLISION THEORY .......................................... Q30: Using the apparatus shown, a 6.075 g block of magnesium was added to 100 cm 3 of 1.0 mol -1 hydrochloric acid and the change in mass noted at regular time intervals. Which of the following graphs would be drawn from the results? a) b) © H ERIOT-WATT U NIVERSITY TOPIC 1. REACTION RATES - COLLISION THEORY c) d) .......................................... Q31: Calculate the number of moles of magnesium added to the acid. .......................................... Q32: Calculate the number of moles of hydrochloric acid present at the start of the reaction. .......................................... Q33: A vitamin C solution, which is acidic, slowly breaks down any sugar present. The sugar concentration measured over a number of hours was found to be decomposing at an average rate of 0.0002 mol -1-1 min-1 . If the starting concentration in a fresh solution was found to be 0.5 mol -1-1 , what would be its sugar concentration after 5 hours? .......................................... .......................................... © H ERIOT-WATT U NIVERSITY 15 16 TOPIC 1. REACTION RATES - COLLISION THEORY © H ERIOT-WATT U NIVERSITY 17 Topic 2 Reaction rates - reaction profiles Contents 2.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2 Interpreting graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Activation energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 25 2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 30 2.6 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Prerequisite knowledge Before you begin this topic, you should already know that: • reactions can be monitored and graphs plotted and interpreted (National 4, Unit 1); • when chemical reactions occur, they are often accompanied by a change in energy (National 4, Unit 1); • reactions can be exothermic or endothermic - this is dependent on the overall energy change taking place (National 4, Unit 1); • there are four factors that affect the rate of reaction: temperature, concentration, catalyst and surface area (National 3, Unit 1); • the effects of concentration, pressure, surface area (particle size), temperature and collision geometry on reaction rates can be described using collision theory (Higher Unit 1, Topic 1). 18 TOPIC 2. REACTION RATES - REACTION PROFILES Learning objectives By the end of this topic, you should be able to: • interpret rate graphs; • explain that a potential energy diagram can be used to show the energy pathway for a reaction; • state that enthalpy change is the energy difference between products and reactants; • explain that enthalpy change can be calculated from a potential energy diagram; • state that enthalpy change has a negative value for exothermic reactions and a positive value for endothermic reactions; • explain that an activated complex is an unstable arrangement of atoms formed during a reaction at the maximum of the potential energy barrier; • state that activation energy is the energy required by colliding particles to form an activated complex; • explain that activation energy can be calculated from a potential energy diagram; • state that temperature is a measure of the average kinetic energy of the particles of a substance (revision: Higher Unit 1, Topic 1); • state that activation energy is the minimum kinetic energy required by colliding particles before reaction may occur; • describe that energy distribution diagrams can be used to explain the effect of changing temperature on the kinetic energy of particles; • describe that the effect of temperature on reaction rate can be explained in terms of an increase in the number of particles with energy greater than the activation energy. © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES 2.1 19 Prior knowledge Test your prior knowledge Q1: During the course of a combustion reaction, energy in the form of heat is given out. What type of reaction is this? .......................................... Q2: During the course of a decomposition reaction, energy in the form of heat is taken in. What type of reaction is this? .......................................... Q3: Collision theory provides an explanation for the effect that various factors have on the rate of chemical reactions. For successful collisions to occur, what must happen? (Name two things.) .......................................... 2.2 Interpreting graphs Investigations which measure the change in a property, such as concentration, during the progress of a chemical reaction often have results displayed as a graph or a table. In the reaction between zinc and hydrochloric acid, the total volume of hydrogen given off was measured against time and the graph that resulted showed a steep slope which tailed off to a level gradient as the reaction finished and the volume of hydrogen released stayed constant. Revisit the activity in the previous topic if you are unsure about this point. It would also be possible to follow the progress of the same chemical reaction between zinc and hydrochloric acid by measuring the mass of the beaker and its contents against time using the apparatus shown in Figure 2.1. The graph which results is shown in Figure 2.2. Figure 2.1: Zinc and hydrochloric acid mass experiment .......................................... © H ERIOT-WATT U NIVERSITY Go online 20 TOPIC 2. REACTION RATES - REACTION PROFILES Graphs which use the same axes to plot results for change in concentration of both reactant and product on the same graph may also be met. Figure 2.2 shows such a graph. Figure 2.2: Reactant and product on shared axes .......................................... Graphs which use the same axes and place the results for different experiments in which the concentration or temperature are varied from one experiment to the next are common and show up how that variable affects the reaction progress. © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES 21 Measuring rate at different concentrations A chemical reaction between zinc and hydrochloric acid is repeated at three different concentrations of acid. The data collected, involving changes of volume, is graphed against time, giving multiple graphs on the same axes. The three features of each line which give information about the reaction, and are commonly met in examination questions are: • the steepness of the slope; • the point at which the line becomes horizontal; • the final level of the horizontal line. © H ERIOT-WATT U NIVERSITY Go online 22 TOPIC 2. REACTION RATES - REACTION PROFILES Q4: Which graph shows the steepest slope? a) 0.5 mol -1 b) 1.0 mol -1 c) 2.0 mol -1 .......................................... Q5: Which graph shows the fastest rate of reaction? a) 0.5 mol -1 b) 1.0 mol -1 c) 2.0 mol -1 .......................................... Q6: How long did it take for the 2.0 mol -1 reaction to finish? .......................................... Q7: What is the volume of hydrogen in the 2.0 mol -1 reaction after 30 seconds? .......................................... Q8: Compare the 1.0 mol -1 graph with the 2.0 mol -1 graph in three areas: • the steepness of the slope; • the point at which the line becomes horizontal; • the final level of the horizontal line. Explain the differences. .......................................... Q9: Predict the final level (in cm3 to 1 decimal place) of the horizontal line in the 0.5 mol -1 graph. .......................................... In each of the graphs showing how a variable affects the reaction progress, it is the slope of the graph which gives an indication of the average rate of reaction. The rate of reaction changes as it progresses, being relatively fast at the start and slowing towards the end. Average rate is measured as: Average rate = Change in variable Change in time In the graphs developed previously, a comparison of rate of reaction of 0.5 mol -1 , 1.0 mol -1 and 2.0 mol -1 acid can be determined by looking at how much hydrogen has been given off in the same time span. If the volume of hydrogen given off after 5 seconds is measured, the question "How does the rate of reaction depend on the concentration of acid?" can be answered. © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES 23 The table of results (Table 2.1) taken from the graphs is shown below. The resulting graph of rate of reaction is plotted against concentration of hydrochloric acid (Figure 2.3). It is clearly a straight line showing that, in this case, the rate is directly proportional to the concentration of hydrochloric acid. (Remember, however, that there is no simple way to predict the relationship in advance and each concentration/rate relationship must be investigated experimentally to determine any relationship.) Table 2.1: Rate versus concentration of hydrochloric acid Hydrochloric acid concentration/mol -1 Volume of hydrogen/cm3 Rate/(cm3 of H2 s-1 ) 0.5 6.5 1.3 1.0 13.0 2.6 2.0 26.0 5.2 .......................................... Figure 2.3 .......................................... Q10: Predict the volume of hydrogen in cm 3 given off in 5 seconds in a similar experiment using 1.5 mol -1 hydrochloric acid. .......................................... Q11: Predict the average rate in cm 3 s-1 in a similar experiment using 4.0 mol -1 hydrochloric acid. .......................................... Many investigations into how concentration of solutions or temperature changes affect © H ERIOT-WATT U NIVERSITY 24 TOPIC 2. REACTION RATES - REACTION PROFILES the rate of a reaction involve 'clock reactions'. The time taken for a colour change to appear may be an experiment you have seen. Sometimes a cross drawn on paper is placed below the reaction vessel. The time taken for it be obscured is called the end point. In each case, the time taken to reach the same stage in the reaction is being measured. The time is converted into rate by calculating the reciprocal of time: Rate = 1 t Conversions between time, rate and concentration or temperature are common. Example Problem The graph shows a rate of reaction plotted against concentration of a reactant. Figure 2.4: Rate versus concentration graph .......................................... a) How long would it take to complete the reaction if the reactant concentration is 0.4 mol -1 ? b) At what concentration was the reaction completed in 10 seconds? © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES Solution a) From the graph, at 0.4 mol -1 the rate = 0.2 s-1 . 1 t 1 0.2 = t 1 t= 0.2 t = 5 seconds Rate = b) Since the rate is given by: 1 t 1 Rate = 10 Rate = 0.1 Rate = From the graph, concentration = 0.2 mol -1 .......................................... 2.3 Activation energy Collision theory states that colliding particles must have 'sufficient energy' to react. Some reactions start as soon as the reactants are mixed. A neutralisation reaction between hydrochloric acid and sodium hydroxide solutions would react immediately on mixing. The colliding particles, in this case, must have sufficient energy to react. Some reactions require an input of energy to start. For example, a firework burns well in air only after lighting. The particles in the firework had insufficient energy to react at room temperature. The activation energy is the minimum kinetic energy required by colliding particles before reaction will occur. This is often thought of as an 'energy barrier' which has to be overcome. The kinetic model of matter shows the energy distribution of particles in a gas, liquid or solution. The average kinetic energy of the particles is measured as 'temperature'. Only those molecules with a kinetic energy greater than the activation energy (shown as EA or as Ea ) are capable of successful collision. © H ERIOT-WATT U NIVERSITY 25 26 TOPIC 2. REACTION RATES - REACTION PROFILES Figure 2.5: Kinetic energy distribution graph .......................................... These questions refer to the four points on the distribution graph. Q12: Which point represents particles with the lowest kinetic energy? a) b) c) d) A B C D .......................................... Q13: Which point represents the least number of particles? a) b) c) d) A B C D .......................................... Q14: The average kinetic energy of particles determines the 'temperature'. Which point represents particles nearest to the average kinetic energy? a) b) c) d) A B C D .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES 27 Q15: Which point represents particles with sufficient kinetic energy to react successfully? a) b) c) d) A B C D .......................................... The effect of temperature on kinetic energy Temperature is a measure of the average kinetic energy of the particles of a substance. An increase in temperature changes the energy distribution and increases the average kinetic energy. Controlling the rate - temperature and kinetic energy The following graphs show the kinetic energy distribution of a group of particles involved in a chemical reaction at T◦ C and T + 10 ◦ C. Compare the graphs and consider the effect of increasing the temperature by © H ERIOT-WATT U NIVERSITY 10 ◦ C. Go online 28 TOPIC 2. REACTION RATES - REACTION PROFILES Q16: How has the increased temperature affected the value of the activation energy? a) Increased b) Decreased c) Not changed .......................................... Q17: How has the increased temperature affected the average kinetic energy of the particles? a) Increased b) Decreased c) Not changed .......................................... Q18: At the increased temperature, which point or points represent particles with sufficient kinetic energy to react successfully? a) b) c) d) D C and D B,C and D A,B,C and D .......................................... Q19: What does the shaded area to the right of E A represent? a) b) c) d) Particles with sufficient energy to react. Particles with insufficient energy to react. Particles with the average kinetic energy. Particles with the same kinetic energy. .......................................... Q20: How has the 10 ◦ C increase in temperature affected the number of particles with energy greater than the activation energy? a) Roughly the same. b) Roughly halved. c) Roughly doubled. .......................................... Remember, the Chemical Industry is not in existence to manufacture chemicals: like any other industry it exists to create wealth and wealth can only be created if it can make profits. For this reason, an understanding of rate is very important to chemists. In industry, as in the lab, it is important to have knowledge of how to control rates of reaction. If reaction rates are too low, a manufacturing process will not be economically viable. However, if reaction rates are too high, there is a risk of thermal explosion. In recent years, a number of thermal explosions at chemical plants have made the news, including West Thurrock, Essex and The Imperial Sugar Company, USA. © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES Reactions increase their rate at higher temperatures because a higher proportion of the molecules involved have energy in excess of the activation energy and more successful collisions can occur. It is observed that a 10◦ C rise is responsible for an approximate doubling of rate in many reactions. .......................................... The effect of light energy on kinetic energy With some chemical reactions, light energy is absorbed and can provide particles with energy sufficient to raise their kinetic energy above the value needed to cross the activation energy barrier. Photosynthesis and photography provide two common examples of photochemical reactions. (This subject will be considered further in a later topic on skin care products.) There are many examples of this type of reaction taking place in the atmosphere. High energy ultraviolet radiation which would be harmful to humans is absorbed by ozone (O3 ) molecules in the upper atmosphere. The ozone layer which protects us is fragile, however, as the ozone can be broken down by certain chemicals. Moves have taken place to reduce the use of these chemicals and protect the ozone layer. Key point • Temperature is a measure of the average kinetic energy of the particles of a substance and activation energy is the minimum kinetic energy required by colliding particles before reaction can occur. • Energy distribution diagrams can be used to explain how an increase in temperature or, in some chemical reactions the energy from light, increases the number of particles with energy greater than the activation energy. © H ERIOT-WATT U NIVERSITY 29 30 TOPIC 2. REACTION RATES - REACTION PROFILES 2.4 Summary Summary 2.5 • • Graphs which use the same axes and place the results for different experiments in which the concentration or temperature are varied from one experiment to the next are common and show us how that variable affects the reaction progress. • Temperature is a measure of the average kinetic energy of the particles of a substance. • Activation energy is the minimum kinetic energy required by colliding particles before reaction can occur. • Energy distribution diagrams can be used to explain how an increase in temperature or, in some chemical reactions, the energy from light increases the number of particles with energy greater than the activation energy (Ea /EA ). • Reactions increase their rate at higher temperatures because a higher proportion of the molecules involved have energy in excess of the activation energy so more successful collisions can occur. • It is observed that a 10◦ C rise is responsible for an approximate doubling of rate in many reactions. • The effect of temperature on reaction rate can be explained in terms of an increase in the number of particles with energy greater than the activation energy. Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES 2.6 31 End of topic test End of Topic 2 test Q21: A piece of phosphorus ignites when touched with a hot wire, whereas magnesium ribbon needs strong heating before it will burn. What can you deduce about the activation energies of the two reactions? a) b) c) d) The activation energy is less for magnesium than for phosphorus. The activation energy is less for phosphorus than for magnesium. The activation energies are the same for the reactions. No information about activation energy can be deduced. .......................................... Q22: A student carries out a reaction and then repeats the experiment at a temperature ten degrees higher. What is the likely effect on rate of reaction? a) b) c) d) No effect. The reaction rate halves. The reaction rate doubles. The reaction rate triples. .......................................... Q23: Why is this? a) b) c) d) The activation energy has been decreased. The activation energy has increased. Fewer particles have energy greater than the activation energy. More particles have energy greater than the activation energy. .......................................... Q24: Activation energy is: a) b) c) d) the minimum kinetic energy required by colliding molecules for a reaction to occur. the maximum kinetic energy colliding molecules can reach for a reaction to occur. the temperature at which a reaction will take place. the temperature at which rate of reaction will be greatest. .......................................... Q25: The activated complex is: a) b) c) d) the initial stage of reaction when the reactants are first put together. an intermediate stage at the top of the activation energy barrier. the maximum kinetic energy colliding molecules can reach for a reaction to occur. the final stage of a reaction when the products have been formed. .......................................... © H ERIOT-WATT U NIVERSITY Go online 32 TOPIC 2. REACTION RATES - REACTION PROFILES Q26: The activation energy can be calculated using: a) b) c) d) time taken for reaction to complete. temperature the reaction occurs at. potential energy diagrams. the activated complex. .......................................... Q27: The graph below shows how the concentrations of both a reactant and a product change during the course of a reaction. What is the average rate of reaction over the first 15 seconds? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 2. REACTION RATES - REACTION PROFILES Q28: This graph shows the effect of temperature on the rate of a reaction. How long, in seconds, did the reaction take to complete at 40 ◦ C? .......................................... Q29: Explain, in terms of the collision theory, why increasing the temperature increases the reaction rate. .......................................... Q30: A reaction between A and B forms product C. At a temperature of 40 ◦ C the reaction is complete after 20 seconds. At what temperature would the rate be expected to be four times as fast? a) 50◦ C b) 60◦ C c) 80◦ C d) 160◦ C .......................................... © H ERIOT-WATT U NIVERSITY 33 34 TOPIC 2. REACTION RATES - REACTION PROFILES Q31: A reaction was carried out at four different temperatures. The table shows the time taken for the reaction to occur. Temp (◦ C) 20 30 40 50 Time (seconds) 60 30 14 5 The results show that: a) b) c) d) the activation energy increases with increasing temperature. a small rise in temperature results in a large increase in reaction rate. the rate of the reaction is directly proportional to the temperature. the reaction is endothermic. .......................................... .......................................... © H ERIOT-WATT U NIVERSITY 35 Topic 3 Catalysis Contents 3.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.2 Introduction to catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Catalyst mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 38 3.4 Catalysts in industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Examples of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 40 3.5 Potential energy diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 What produces this energy change? . . . . . . . . . . . . . . . . . . . . 43 47 3.5.2 The thermochemical equation . . . . . . . . . . . . . . . . . . . . . . . 3.6 Catalysts and energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 52 3.6.1 The activated complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 How catalysts work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 56 3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 61 3.9 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 36 TOPIC 3. CATALYSIS Prerequisite knowledge Before you begin this topic, you should already know that: • there are four factors that affect the rate of reaction: temperature, concentration, catalyst and surface area (National 3, Unit 1); • adding a catalyst will speed up a reaction (National 3, Unit 1); • reactions can be exothermic or endothermic - this is dependent on the overall energy change taking place (National 4, Unit 1); • the effects of concentration, pressure, surface area (particle size), temperature and collision geometry on reaction rates can be understood using collision theory (Higher, Unit 1, Topic 1); • a potential energy diagram can be used to show the energy pathway for a reaction (Higher, Unit 1, Topic 1); • enthalpy change is the energy difference between products and reactants - it can be calculated from a potential energy diagram (Higher, Unit 1, Topic 1); • enthalpy change has a negative value for exothermic reactions and a positive value for endothermic reactions (Higher, Unit 1, Topic 1); • activation energy can be calculated from potential energy diagrams (Higher, Unit 1, Topic 1); Learning objectives By the end of this topic, you should be able to: • state that the enthalpy change for a reaction has a negative value for exothermic reactions and a positive value for endothermic reactions; • explain that the activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction; • state that the activation energy is the energy required by colliding particles to form an activated complex; • explain that activation energy can be calculated from potential energy diagrams; • state that a catalyst provides an alternative reaction pathway with a lower activation energy; • explain that a potential energy diagram can be used to show the effect of a catalyst on activation energy; • explain how catalysis works and learn about adsorption; • define the terms 'activated complex' and 'activation energy' and hence explain how catalysts work in terms of potential energy diagrams. © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 3.1 37 Prior knowledge Test your prior knowledge Q1: For any chemical, the temperature is a measure of the: a) b) c) d) average kinetic energy of all the particles; minimum kinetic energy required before reaction occurs; average kinetic energy of the particles which react; activation energy. .......................................... Q2: During the course of a decomposition reaction, energy in the form of heat is taken in. What type of reaction is this? a) Exothermic b) Endothermic .......................................... Q3: What is activation energy? a) b) c) d) The minimum kinetic energy required by colliding molecules for a reaction to occur. The maximum kinetic energy colliding molecules can reach for a reaction to occur. The temperature at which a reaction will take place. The temperature at which rate of reaction will be greatest. .......................................... 3.2 Introduction to catalysis Catalysts are amongst the most important chemicals in the world around us. Catalysts are essential in: • the production of materials such as plastics; • the production of fuels to power our transport; • the removal of pollutants produced by transport; • the production of medicines and food; • the functioning and health of our bodies. It is said that 90% of all manufactured items use catalysts at some stage of their production. Catalysts are substances which alter the rates of reactions, but are chemically unchanged at the end of the reactions. They can be recovered at the end of reactions chemically unchanged. They are neither reactants nor products and do not appear in the chemical equation. Often, the catalyst used in a reaction is shown above the arrow in the equation, as in the Haber Process. © H ERIOT-WATT U NIVERSITY Go online 38 TOPIC 3. CATALYSIS Figure 3.1: Haber process .......................................... Catalysts allow chemical reactions to occur more quickly at lower temperatures and so reduce energy costs. 3.3 Catalyst mechanism Hydrogenation of ethene Often, the catalyst is in a different state from the reactants. An example of this is the addition of hydrogen gas to ethene gas using a solid nickel catalyst. Addition of hydrogen to ethene Go online The atoms within the metal are fully bonded to their neighbours whereas the atoms on the surface have 'spare' bonds and are capable of forming weak bonds with the reactant molecules. This is known as adsorption. (This should not be confused with absorption, which involves a substance penetrating inside a solid - like water being soaked up by a sponge.) When the molecules leave the surface of the catalyst, the process is known a desorption. Q4: a) b) c) d) Reactant molecules bond to the surface of the metal. This is known as: absorption. desorption. adsorption. activation. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 39 Q5: What happens to the bonds within the molecules when they attach to the surface? a) b) c) d) They break. They get weaker. They get stronger. They stay the same. .......................................... Q6: When the product molecules leave the surface of the catalyst, what change has taken place to the surface of the catalyst? .......................................... Q7: Which of these statements is false? a) The catalyst speeds up the reaction by weakening the bonds in the reactant molecules. b) The larger the surface area of the catalyst the more effective it will be. c) The catalyst speeds up the reaction without taking part. d) The catalyst speeds up the reaction without being used up. .......................................... Enzymes Enzymes are biological catalysts that are responsible for the chemical processes that occur in all living organisms, from the simplest bacteria to the most complex mammals. They are proteins and are the target for many modern pharmaceutical products used to control diseases. Enzymes are studied in more detail in the topic on Proteins later in the course. Key point Many catalysts work by adsorbing reactant molecules onto their surface. 3.4 Catalysts in industry Many important industrial processes involve catalysis. Figure 3.2: Examples of industrial catalysis Process Use Catalyst Haber Production of ammonia Iron Contact Production of sulfuric acid Vanadium (V) oxide Oswald Production of nitric acid Platinum Hardening of oils Production of margarine Nickel Catalytic cracking Production of alkenes Aluminium oxide © H ERIOT-WATT U NIVERSITY 40 TOPIC 3. CATALYSIS .......................................... 3.4.1 Examples of catalysts Catalysis by cobalt(II) chloride solution The oxidation of aqueous tartrate ions by hydrogen peroxide solution is slow, even when the solution is heated to 60 ◦ C. The reaction can be catalysed by adding a solution containing cobalt(II) ions. Figure 3.3: Oxidation of tartrate ions 5H2 O2 (aq) + C4 H4 O6 2- (aq) → 4CO2 (g) + 6H2 O(l) + 2OH- (aq) .......................................... Catalysis by cobalt(II) chloride solution Go online Study the following diagram, looking closely for any evidence that the cobalt(II) chloride is acting as a catalyst, and then answer the questions which follow. Catalysis by cobalt(II) chloride Q8: a) b) c) d) Which species is responsible for the pink colour at the beginning? Cl- (aq) ions Co2+ (aq) ions Co3+ (aq) ions Tartrate ions .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 41 Q9: Suggest a species that could be responsible for the green colour which appears? a) b) c) d) Cl- (aq) ions Co2+ (aq) ions Co3+ (aq) ions Tartrate ions .......................................... Q10: Which species acts as the catalyst? a) b) c) d) Cl- (aq) ions Co2+ (aq) ions Co3+ (aq) ions Tartrate ions .......................................... Q11: A student defines a catalyst as a substance which speeds up a reaction without taking part. Explain whether or not this is correct, with reference to the above reaction. .......................................... Key point Cobalt(II) chloride speeds up the reaction between tartrate ions and hydrogen peroxide. Colour changes clearly show that the catalyst takes part in the reaction and is chemically unchanged at the end of the reaction. Ozone layer destruction Another example of catalysis is found in the destruction of the ozone layer by CFCs (chlorofluorocarbons). Ozone, O3 , is constantly being formed in the upper atmosphere where it provides a protective layer by absorbing a lot of the harmful ultraviolet radiation which could cause serious skin damage, including skin cancer. Ozone slowly breaks down by reacting with oxygen atoms to form oxygen molecules. The breakdown of ozone This breakdown is accelerated by chlorine atoms, which are produced when sunlight in the upper atmosphere causes the CFCs to break down. The catalysed breakdown of ozone © H ERIOT-WATT U NIVERSITY 42 TOPIC 3. CATALYSIS Q12: Which species is acting as a catalyst? a) b) c) d) O3 O2 ClO Cl .......................................... The Monsanto process A very important industrial example of catalysis is the Monsanto process for the synthesis of ethanoic acid from methanol. The Monsanto process The reaction takes place in solution using a soluble compound containing rhodium. Industrial uses of catalysts Go online Throughout this topic, examples have been given of the use of catalysts in industry. You do not need to know them all, but should be able to give some examples. Without the use of catalysts, most industrial processes would not be feasible. You may need to refer back to the earlier sections of this topic or use the Internet or some of the text books listed in the section on Resources. Q13: Use the terms from the 'word bank' to complete the table. Some of the terms may be required more than once. Process Catalyst Type Purpose Haber Production of ammonia Washing powders Breakdown of protein stains Catalytic converters Pt, Pd, Rb Brewing Production of ethanol Catalytic cracking Production of alkenes Food processing Rennin Ostwald Pt gauze Monsanto Rhodium complex Word bank: Al2 O3 , Enzyme, Ethanoic acid from methanol and CO, Iron, Production of cheese, Production of nitric acid, Protease, Reduction of pollution, Solid, Solution, Zymase © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 43 .......................................... 3.5 Potential energy diagrams Consider the exothermic reaction between zinc and sulfuric acid. Zn(s) + H2 SO4 (aq) → ZnSO4 (aq) + H2 (g) After the reaction, the energy stored in the products is less than the energy originally stored in the reactants - that is, some of the chemical potential energy stored in reactants has been transferred to the surroundings. This transferred heat energy is defined as the enthalpy change for this reaction. Strictly, the change in energy which occurs during a chemical reaction consists of two components: heat and work. We live on the surface of the Earth, at the bottom of a 'sea' of atmosphere which exerts pressure. In the reaction above, the gaseous hydrogen produced will have to 'work' to displace the atmosphere. This will require use of some energy, so the heat energy released to the surroundings will be less than the total energy change. In this reaction, for 1 mole of zinc, the heat (enthalpy) change is 152.4 kJ and the work against the atmosphere 2.5 kJ, the overall energy change being 154.9 kJ. Chemists often carry out reactions at constant pressure, hence the use of 'enthalpy change' which is the heat change at constant pressure. Let's consider another familiar exothermic reaction, the burning (oxidation) of methane in a flame. You see this every time you use natural gas. CH4 + 2O2 → CO2 + 2H2 O These changes are often shown as a potential energy diagram. An example is shown for methane oxidation. The first diagram shows the potential energy (the enthalpy) of the reactants at the start of the reaction © H ERIOT-WATT U NIVERSITY 44 TOPIC 3. CATALYSIS Next, the potential energy (enthalpy) of the products at the end of the reaction is shown. The arrow shows the energy (enthalpy) change during the reaction. In the case of methane oxidation, energy is given out to the surroundings. Figure 3.4: Methane oxidation .......................................... This diagram shows the potential energy of the reactants, at the start of the reaction. It also shows the potential energy of the products, at the end of the reaction. The arrow shows the energy given to the surroundings. © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 45 The easiest measurement to make is the enthalpy change, ΔH, when the reaction occurs. This is defined as the difference in enthalpy of the products and reactants by the equation below. The units of energy and enthalpy are joules (J). Enthalpy change ΔH is negative for an exothermic reaction. The energy of the products is less than that of the reactants. Exothermic reaction The potential energy diagram for an endothermic reaction, the decomposition of calcium carbonate is shown below. Calcium carbonate decomposition In this case, the energy of the products is greater than that of the reactants. © H ERIOT-WATT U NIVERSITY 46 TOPIC 3. CATALYSIS Q14: What will happen to the temperature of the surroundings during an endothermic reaction? a) It will increase. b) It will not change. c) It will decrease. .......................................... Q15: What will the value of ΔH be for an endothermic reaction? a) Negative b) Zero c) Positive .......................................... Endothermic reaction In order to distinguish endothermic and exothermic reactions, the + and - sign are always used when giving ΔH values. Q16: The equation for the 'water gas' reaction is: C(s) + H2 O(g) → CO(g) + H2 (g) Note that this equation states that the carbon is in the solid state (s) and the other materials are gases (g). This reaction absorbs energy from its surroundings. What type of reaction is this? a) b) c) d) Fast Exothermic Explosive Endothermic .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 47 Q17: What will be the sign of ΔH? a) b) c) d) Positive Has no sign. Negative Insufficient information to know. .......................................... Q18: Draw a potential energy diagram for this reaction. .......................................... But how much energy is involved in this reaction? In the case of the water gas reaction, 121 kJ is absorbed for each mole of carbon and steam used. In other words, ΔH = +121 kJ mol-1 . The full potential energy diagram for the water gas reaction is as follows. ΔH = +121 kJ mol-1 Products CO(g) + H2(g) H/kJ -111 -232 Reactants C(s) + H2O(g) Reaction progress Water gas potential energy diagram Key point A potential energy diagram can be used to show the energy changes for a reaction. The enthalpy change is the energy difference between products and reactants. The enthalpy change can be calculated from a potential energy diagram. The enthalpy change has a negative value for exothermic reactions and a positive value for endothermic reactions. 3.5.1 What produces this energy change? The chemical energy in molecules is stored in the bonds. When a reaction occurs, the bonds in the reactant molecules are broken and the atoms rearrange to form new bonds with different energies in the products. For example, think of making ammonia from nitrogen and hydrogen. © H ERIOT-WATT U NIVERSITY 48 TOPIC 3. CATALYSIS The balanced chemical equation is N2 + 3H2 2NH3 You can think of this reaction as first breaking the bonds in the nitrogen and hydrogen molecules. This stage requires an input of energy. Then these atoms rearrange and combine to form new N - H bonds in ammonia. This stage releases energy. The enthalpy change will be given by the total energy in new bonds, minus the energy present in the original bonds. It is important to remember that reactions do not normally proceed by such a route, but the enthalpy change can be calculated as if they did. © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 3.5.2 49 The thermochemical equation It is usual to express this information in a thermochemical equation such as that for the water gas reaction (Figure 3.5). Figure 3.5: Thermochemical equation for the water gas reaction .......................................... Three points should be noted. 1. The enthalpy value quoted in the balanced equation is measured in kilojoules per mole (kJ mol-1 ). Look at Figure 3.5 again. The 121 kJ of energy is absorbed when one mole of solid carbon and one mole of steam react; two moles of each reacting would absorb 242 kJ, and so on. (This makes sense when you think that burning 2 kg of carbon would give out twice the energy from burning 1 kg.) 2. The equation always contains the state symbols of reactants and products. The enthalpy of reactions will change if the states are different. Look carefully at these two equations for the reaction of hydrogen and oxygen. (N.B. the equation has been written for forming 1 mole of water. The " 1 /2 " before the oxygen means 1 /2 mole of reactant, not 1 /2 a molecule of oxygen.) The extra enthalpy in the second case is due to the heat released when the water vapour condenses to liquid water. 3. Since the enthalpy change is independent of the route of the reaction, the enthalpy change for the reverse reaction will be equal in value, but of opposite sign. © H ERIOT-WATT U NIVERSITY 50 TOPIC 3. CATALYSIS Q19: For the reaction below: What is the enthalpy change for the reverse reaction? CO2 (g) → CO(g) + 1 /2 O2 (g) a) b) c) d) ΔH = -566 kJ mol-1 ΔH = -283 kJ mol-1 ΔH = 0 kJ mol-1 ΔH =+283 kJ mol-1 .......................................... Q20: The equation for calculating the enthalpy change (ΔH) for a reaction is: a) b) c) d) ΔH = Hproducts + Hreactants ΔH = Hproducts - Hreactants ΔH = Hreactants + Hproducts ΔH = Hreactants - Hproducts .......................................... Q21: A ΔH value of -500 kJ mol -1 would indicate that the reaction is: a) b) c) d) Fast Reversible Exothermic Endothermic .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 51 Q22: The potential energy diagram for the combustion of one mole of ethene is: What is the enthalpy change for this reaction? a) b) c) d) ΔH = +52 kJ mol-1 ΔH = -1362 kJ mol-1 ΔH = -1310 kJ mol-1 ΔH = -1414 kJ mol-1 .......................................... Q23: The equation for the combustion of ethanol is: How many kJ of energy will be released to the surroundings if 5 moles of ethanol are burned? .......................................... Q24: Given that: what would be the enthalpy change for the reaction below? (Hint: don't forget the sign.) .......................................... © H ERIOT-WATT U NIVERSITY 52 TOPIC 3. CATALYSIS Q25: The Ostwald process for producing nitric acid involves the oxidation of ammonia (NH3 ) to form nitric oxide (NO) and water. Write a balanced equation for this reaction. .......................................... Q26: Given that the enthalpy of the reactants (as written in the balanced equation) is -184 kJ and the enthalpy of the products is -1090 kJ, calculate the enthalpy change for this reaction in kJ. Remember the sign. .......................................... Q27: What would be the enthalpy change per mole of ammonia oxidised? Give your answer in kJ mol-1 to 1 decimal place. .......................................... Q28: When 102 g of ammonia is oxidised, how many kJ of heat have to be removed to maintain a constant temperature? .......................................... 3.6 Catalysts and energy The effect of a catalyst on a reaction rate can be explained in terms of energy. Potential energy diagrams are used to show the energy pathway of a chemical reaction. You will study the following diagrams in much more detail in the next topic in this unit. Q29: Which diagram shows an exothermic reaction, one in which the products have less energy than the reactants so heat is released to the surroundings? a) A b) B .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 53 Q30: Enthalpy change is the difference in energy between products and reactant. What is the approximate enthalpy change in diagram A? .......................................... Q31: Enthalpy change is the difference in energy between products and reactant. Which of these could be the enthalpy change in B? a) b) c) d) +1040 kJ -1040 kJ +890 kJ -890 kJ .......................................... 3.6.1 The activated complex When considering energy changes in reactions, only the starting point and finishing point are considered. Energy has to be put in to break the old bonds before energy can be released when the new bonds form. As a result, the potential energy diagram for an exothermic reaction has the shape shown below. Figure 3.6: Potential energy diagram for an exothermic reaction .......................................... There is an energy barrier between the reactants and products. Reactant molecules must have enough energy when they collide to get over this barrier. The maximum point, X, in the graph above corresponds to the activated complex. This is a very unstable arrangement of atoms in which the old bonds are half broken and the new bonds are half formed. This species can either break down to reform the reactants ('fall' back down the same side of the 'hill') or break down to form the products ('fall' down the other side of the 'hill'). © H ERIOT-WATT U NIVERSITY 54 TOPIC 3. CATALYSIS Reaction profile Go online Reaction profile with activated complex .......................................... The activation energy (EA ), is the energy required by the colliding molecules to form the activated complex, i.e. it is the height of the barrier. The activation energy can be calculated from potential energy diagrams. Q32: Which of these could be the activation energy of the reaction shown in Figure 3.6? a) b) c) d) -180 kJ +180 kJ -110 kJ +110 kJ .......................................... Q33: Which of these could be the activation energy of the reaction shown in the Reaction profile activity? a) b) c) d) -22 kJ -55 kJ +55 kJ +90 kJ .......................................... Q34: Explain what will happen to the rate of the reaction if the activation energy is decreased. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 55 Earlier, the activation energy was introduced as the minimum kinetic energy required by colliding molecules before reaction can occur, as shown in the following energy distribution diagram. Figure 3.7: Energy distribution graph .......................................... Q35: Which letter shows the molecules with least energy? a) b) c) d) A B C D .......................................... Q36: Which letter shows molecules with enough energy to react? a) b) c) d) A B C D .......................................... Q37: What will happen to the number of molecules with enough energy to react if the activation energy is decreased? a) It will decrease. b) It will stay the same. c) It will increase. .......................................... © H ERIOT-WATT U NIVERSITY 56 TOPIC 3. CATALYSIS The two definitions of activation energy are quite compatible. As colliding molecules get closer together, they begin to repel (potential energy increases) and slow down (kinetic energy decreases) as kinetic energy is converted into potential energy. If the colliding molecules have enough kinetic energy, then enough potential energy will be produced to form the activated complex and so cause a reaction. A successful collision will have taken place. Key point The activated complex is an unstable arrangement of atoms formed during a reaction at the maximum of a potential energy barrier. The activation energy is energy required by colliding molecules to form the activated complex. The activation energy can be calculated from potential energy diagrams. 3.6.2 How catalysts work How catalysts work Go online The following activity uses kinetic energy distribution graphs and a potential energy diagrams to show the effect of a catalyst on the reaction rate. Without a catalyst © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 57 With a catalyst Q38: Without a catalyst, which of the following is true? a) b) c) d) High activation energy, lots of successful collisions. High activation energy, few successful collisions. Low activation energy, lots of successful collisions. Low activation energy, few successful collisions. .......................................... Q39: With a catalyst, which of the following is true? a) b) c) d) High activation energy, lots of successful collisions. High activation energy, few successful collisions. Low activation energy, lots of successful collisions. Low activation energy, few successful collisions. .......................................... Q40: What effect does a catalyst have on the reaction rate? a) b) c) d) It speeds up the reaction by adding extra energy. It speeds up the reaction by making the reaction exothermic. It speeds up the reaction by lowering the activation energy. It speeds up the reaction by raising the activation energy. .......................................... Q41: What effect does a catalyst have on the enthalpy change? a) Increases it. b) No effect. c) Decreases it. .......................................... © H ERIOT-WATT U NIVERSITY 58 TOPIC 3. CATALYSIS The catalyst provides an alternative pathway for the reaction. Often, the catalyst provides a surface on which the reaction occurs. The bonds in the reactant molecules are weakened when the molecules are adsorbed onto the surface. This pathway has a lower activation energy and so the reaction is faster. Mechanism of catalysis Go online This activity shows how a catalyst might speed up the gas phase reaction between two reactants A and B, as discussed earlier. The equations show the possible steps. In the uncatalysed reaction, A2 molecules and B2 molecules must collide with sufficient energy to form the activated complex. In the catalysed reaction, step 1 involves a lower energy collision between the catalyst (Y) and an A2 molecule. In step 2, the intermediate, A 2 Y, then undergoes another low energy collision with B2 to form the products. Y is regenerated in step 2 and is free to repeat the process. A potential energy diagram can be drawn for the process. Figure 3.8: PE diagram with catalyst © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 59 .......................................... Q42: What indicates the activation energy of the catalysed formation of AB? a) X b) W .......................................... Q43: What indicates the activation energy of the uncatalysed decomposition of AB? a) b) c) d) 1 2 3 4 .......................................... Q44: Which arrow shows the enthalpy change for the catalysed formation of AB? a) b) c) d) 1 2 3 4 .......................................... Q45: What effect has the catalyst had on the activation energy of the reverse reaction? a) Increases it b) Decreases it c) No effect .......................................... Q46: By considering the equations, which letter shows the stage in the uncatalysed reaction at which you are most likely to find A2 Y + B2 ? a) X b) W .......................................... Q47: By considering the equations which letter in Figure 3.8 shows the stage in the catalysed reaction at which you are most likely to find the following species? a) X b) W © H ERIOT-WATT U NIVERSITY 60 TOPIC 3. CATALYSIS .......................................... .......................................... Key point Catalysts speed up reactions by providing an alternative pathway which has a lower activation energy. A potential energy diagram can be used to show the effect of a catalyst on activation energy. 3.7 Summary Summary • In general, the lower the activation energy the faster the reaction. • Catalysts are substances which speed up chemical reactions without being used up in the process. They are widely used in industrial processes. • Catalysts allow chemical reactions to occur more quickly at lower temperatures and so reduce energy costs. • Catalysts work by the adsorption of reactant molecules onto the surface of the catalyst with consequent weakening of the bonds in the reactant molecules. • Catalytic converters are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen. Cars with catalytic converters can only use 'lead-free' petrol to prevent poisoning of the catalyst. • Enzymes catalyse the chemical reactions which take place in the living cells of plants and animals. They are also widely used in industrial processes. • Cobalt(II) chloride speeds up the reaction between tartrate ions and hydrogen peroxide. Colour changes clearly show that the catalyst takes part in the reaction and is chemically unchanged at the end of the reaction. • The activated complex is an unstable arrangement of atoms formed during a reaction at the maximum of a potential energy barrier. • A potential energy diagram can be used to show the energy pathway for a reaction. • The enthalpy change, which can be calculated from the potential energy diagram, is the energy difference between products and reactants. • The enthalpy change has a negative value for exothermic reactions, which causes heat energy to be released to the surroundings. © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS Summary continued 3.8 • • The enthalpy change has a positive value for endothermic reactions, which causes absorption of heat energy from the surroundings. • The activation energy is the energy required by colliding molecules to form the activated complex. • The activation energy can be calculated from potential energy diagrams. • Catalysts speed up reactions by providing an alternative pathway which has a lower activation energy. • A potential energy diagram can be used to show the effect of a catalyst on activation energy. Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY 61 62 TOPIC 3. CATALYSIS 3.9 End of topic test End of Topic 3 test Q48: Go online In the shaded area on the graph: a) molecules always have energy above the activation energy. b) there are no molecules with kinetic energy above the average. c) there are no molecules with energy above the activation energy. d) collisions between molecules are always successful in forming products. .......................................... Q49: Activation energy is: a) b) c) d) the minimum kinetic energy required by colliding molecules for a reaction to occur. the maximum kinetic energy colliding molecules can reach for a reaction to occur. the temperature at which a reaction will take place. the temperature at which rate of reaction will be greatest. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS The progress of reactions can be followed by energy diagrams. © H ERIOT-WATT U NIVERSITY 63 64 TOPIC 3. CATALYSIS Q50: Which graph represents the catalysed version of the reaction in diagram F? a) A b) B c) C d) D e) E .......................................... Q51: Which graph represents the reaction with the highest activation energy? a) A b) B c) C d) D e) E f) F .......................................... Q52: Which diagram represents the reaction with an enthalpy change of -200 kJ mol -1 ? a) A b) B c) C d) D e) E f) F .......................................... Q53: What is an activated complex? a) A very unstable intermediate is formed when bonds within the reactant molecules begin to break and new bonds begin to form. b) The energy required for a reaction to take place. c) When the surface of a catalyst forms weak bonds with reacting molecules. d) A compound containing a transition metal. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 3. CATALYSIS 65 Use the diagram to answer the following questions. Q54: What type of reaction does this graph represent? .......................................... Q55: What is the activation energy value for the forward reaction? .......................................... Q56: What is the activation energy value for the reverse reaction? .......................................... Q57: What is the value of the enthalpy change for the forward reaction? .......................................... Q58: What is the value of the enthalpy change for the reverse reaction? .......................................... © H ERIOT-WATT U NIVERSITY 66 TOPIC 3. CATALYSIS Q59: a) Name the energy changes represented by the numbered intervals on the energy diagram shown. b) The reaction shown is exothermic, how can you tell this from the shape of the graph? .......................................... .......................................... © H ERIOT-WATT U NIVERSITY 67 Topic 4 The Periodic Table Contents 4.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2 Arrangement of elements in the Periodic Table: Introduction . . . . . . . . . . . 4.3 History of the Periodic Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 70 4.4 Trends and patterns (periodicity) . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Melting and boiling points . . . . . . . . . . . . . . . . . . . . . . . . . . 75 75 4.4.2 Atomic size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 79 4.6 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 80 Prerequisite knowledge Before you begin this topic, you should already know that: • atoms contain protons, neutrons and electrons, each with a specific charge, mass and position within the atom - the number of protons defines an element and is known as the atomic number (National 4, Unit 1); • or have knowledge of: sub-atomic particles, their charge, mass and position within the atom, the structure of the Periodic Table, groups, periods and atomic number (National 5, Unit 1); • all matter is made of atoms - when a substance contains only one kind of atom it is known as an element (National 4, Unit 1); • elements are arranged in the Periodic Table in order of increasing atomic number - elements with similar chemical properties are grouped together (National 4, Unit 1); • or be familiar with the seven diatomic elements (National 5, Unit 1); • elements can be categorised as metals and non-metals (National 4, Unit 1); 68 TOPIC 4. THE PERIODIC TABLE Learning objectives By the end of this topic, you should be able to: • state that elements are arranged in the Periodic Table in order of increasing atomic number; • explain that the Periodic Table allows chemists to make accurate predictions of physical properties and chemical behaviour for any element based on its position; • state that there are periodic variations in the densities, melting points and boiling points of the elements across a Period and down a Group. © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE 4.1 69 Prior knowledge Test your prior knowledge Q1: What does the number of protons in an atom determine? a) b) c) d) Go online Atomic number Mass number Element name Atom charge .......................................... Q2: Name the seven diatomic elements. .......................................... Q3: Complete the following table; Particle Charge Mass Location Proton Neutron Electron .......................................... Q4: What is a row in the Periodic Table is called? a) b) c) d) Group Period Row Set .......................................... Q5: What is a column in the Periodic Table is called? a) b) c) d) Column Group Period Set .......................................... 4.2 Arrangement of elements in the Periodic Table: Introduction As more and more elements were discovered and their properties investigated, chemists showed a natural desire to simplify the study of the elements by organising them according to similarities in their chemical behaviour. Eventually this resulted in the modern Periodic Table. If you understand how the Periodic Table is constructed, you will realise that it contains a huge amount of information stored in a very compact form. This makes it a vital resource for any chemist. © H ERIOT-WATT U NIVERSITY 70 TOPIC 4. THE PERIODIC TABLE 4.3 History of the Periodic Table Background Information In 1800, about 33 elements were known but there was no obvious pattern or relationship between them. By 1830, a further 20 or so elements had been discovered and some similarities in properties within small groups of elements was recognised. The German chemist, Johan Wolfgang Dobereiner, made a tentative connection between chemical behaviour and the atomic masses of certain groups of elements, each containing three elements, which he called 'triads'. Figure 4.1: Triads .......................................... In each case, the atomic mass of the central element was approximately the mean of the other two. The next significant development occurred in 1866 when the English chemist, John Newlands, published a paper on the 'Law of Octaves'. Newlands' octaves Go online Figure 4.2 shows how Newlands organised the first 14 elements. Study the information carefully and then answer the questions which follow. Figure 4.2 .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE 71 Q6: What property did Newlands use to put the elements in order? a) b) c) d) Name (i.e. alphabetical order) Colour Atomic mass Atomic number .......................................... Q7: What property is shared by the elements in the first column (H and F)? a) b) c) d) Both are gases. Both are green. Both are reactive metals. None - they are completely different. .......................................... Q8: What property is shared by the elements in the second column (Li and Na)? a) b) c) d) Both are gases. Both are purple. Both are reactive metals. None - they are completely different. .......................................... Q9: Why do you think Newlands referred to these as 'octaves'? (Hint: Think about musical scales.) .......................................... 'Soh-fah' so good! Now look at the second arrangement in which the next seven elements have been added and then answer the questions which follow. Figure 4.3 .......................................... © H ERIOT-WATT U NIVERSITY 72 TOPIC 4. THE PERIODIC TABLE Q10: At first sight, the pattern seems to continue. For how many of the further elements does the pattern work? .......................................... Q11: Can you think of a reason why chromium, manganese and iron do not fit with the elements immediately above them? .......................................... .......................................... Key point When Newlands arranged the elements in order of increasing atomic mass, similar chemical properties were repeated with every eighth element, but this only worked for the first 17 elements. Newlands' ideas were subjected to ridicule and it was even suggested unkindly that he would get better agreement if he arranged the elements in alphabetical order. However, he was on the right lines. The Modern Periodic Table The major breakthrough, indeed one of the most important advances in all chemistry, was provided by Dmitri Ivanovitch Mendeleev in 1869. Like Newlands, he organised the elements in order of increasing atomic mass, but there were other important differences. Mendeleev's Periodic Table Go online Mendeleev organised the first 45 of the 62 elements known at that time. Figure 4.4 shows his final arrangement. Figure 4.4: Constructing Mendeleev's table © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE .......................................... Q12: Unlike Newlands, Mendeleev left spaces (shown as *) in his table. Why? .......................................... © H ERIOT-WATT U NIVERSITY 73 74 TOPIC 4. THE PERIODIC TABLE Q13: Explain why Mendeleev swapped the positions of the elements iodine and tellurium. .......................................... Q14: Name the Group of elements which is found in the modern Periodic Table but is absent from both Newlands' and Mendeleev's tables. .......................................... Key point Mendeleev organised the elements in order of increasing atomic mass (mostly) in conjunction with similar chemical properties, leaving gaps for elements yet to be discovered. .......................................... The modern Periodic Table The relationship between the modern Periodic Table and that of Mendeleev can be seen. Go online Figure 4.5 .......................................... Mendeleev was so confident that his table was correct that he predicted the properties of some of the undiscovered elements. His predictions proved to be very accurate when these elements were finally isolated, providing startling vindication of his theory. The modern Periodic Table can be shown in a variety of different ways. The usual form is the one used in the recommended data booklet. The resources at the end of this topic contain addresses for a number of web-sites which contain interactive Periodic Tables. One of the most useful is http://www.webelements.com/. © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE 75 Q15: In the modern Periodic Table, the elements are not arranged in order of increasing atomic mass. What is used instead? .......................................... .......................................... 4.4 Trends and patterns (periodicity) When the elements are arranged in order of increasing atomic number, many properties vary in a regular way. As you move across a Period from left to right, a pattern emerges. A similar pattern appears on crossing the next Period. Properties which behave in this way are said to be periodic. is the regular recurrence of similar element properties. Throughout this topic, we will concentrate on the properties of the main group elements and in particular on the elements in Periods 2 and 3. Much of this work involves the collection of data, presentation of data in graphical form and the interpretation of such graphs. 4.4.1 Melting and boiling points Melting and boiling points across a Period Figure 4.6: Period 2 and 3 (melting point and boiling point) Go online .......................................... © H ERIOT-WATT U NIVERSITY 76 TOPIC 4. THE PERIODIC TABLE Melting points and boiling points are also periodic properties and depend on the strength of the forces that exist between the particles which make up a substance. Q16: Which element in Period 2 which has the strongest forces between its atoms in the solid state? .......................................... Q17: Which of the Period 3 elements is the easiest to boil? .......................................... Q18: Of all the elements shown in the data booklet, which element which has the weakest forces between its atoms? .......................................... .......................................... Melting and boiling points down a Group A more obvious trend can be seen on descending a Group. Go online Figure 4.7: Alkali metals (melting point and boiling point) .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE 77 Q19: Which alkali metal has the strongest forces between its atoms? .......................................... Q20: Which alkali metal has the weakest forces between its atoms? .......................................... Q21: What happens to the forces between the atoms on descending Group 1? .......................................... .......................................... The variation in melting point and boiling point will be explained when the bonding and structure of the first 20 elements is considered later in this topic. Key point There are periodic variations in the densities, melting points and boiling points of the elements across a Period and down a Group. 4.4.2 Atomic size The size of an atom is determined by the amount of space taken up by the electrons and so must be connected to the electron arrangement of the atom. The electron arrangement is itself a periodic property. Use the following activity (Figure 4.8) to explain the trends in covalent radius. Atomic size Figure 4.8: Trends in nuclear charge and electron arrangement Go online © H ERIOT-WATT U NIVERSITY 78 TOPIC 4. THE PERIODIC TABLE .......................................... Q22: Which of the following provides the best reason for the increase in covalent radius on going down a Group? a) b) c) d) The number of protons increases. The number of electrons increases. The number of electron shells increases. The number of neutrons increases. .......................................... Q23: Which of the following provides the best reason for the decrease in covalent radius on going from left to right across a Period? a) b) c) d) The number of electrons increases. The number of electron shells increases. The number of neutrons increases. The number of protons increases. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE .......................................... Key point • The covalent radius decreases across a Period because the increase in nuclear charge attracts the electrons more strongly. • The covalent radius increases on going down a Group as the number of occupied electron shells increases. 4.5 Summary Summary 4.6 • • Elements are arranged in the Periodic Table in order of increasing atomic number. • The Periodic Table allows chemists to make accurate predictions of physical properties and chemical behaviour for any element based on its position. • There are periodic variations in the densities, melting points and boiling points of the elements across a Period and down a Group. Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY 79 80 TOPIC 4. THE PERIODIC TABLE 4.7 End of topic test End of Topic 4 test Q24: Go online Menedeleev is famous for producing the Periodic Table on which the modern version is based. Which of the following statements is true? a) b) c) d) Mendeleev organised the elements in order of their atomic number. Mendeleev left gaps because some elements did not fit the pattern of reactivity. Mendeleev left gaps for elements which had not yet been discovered. Mendeleev swapped some elements round so that their atomic masses fitted the pattern. .......................................... Q25: Which of the following statements about the Periodic Table are true? a) There is a steady increase in melting point across a period from left to right. b) There is a steady decrease in density on going down Group 1. c) There is a steady decrease in atomic size across a period from left to right. d) There is a decrease in first ionisation energy on going down Group 0. e) There is a decrease and then an increase in boiling point on crossing a period from left to right. f) There is an increase in electronegativity on going down Group 7. .......................................... Q26: What does the number of protons in an atom determine? a) b) c) d) Atomic number Mass number Element name Atom charge .......................................... Q27: Name the seven diatomic elements. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 4. THE PERIODIC TABLE Q28: What is Group 1 in the Periodic Table called? a) b) c) d) The halogens. The alkali metals. The noble gases. Transition metals. .......................................... Q29: What is Group 7 in the Periodic Table called? a) b) c) d) The halogens. The alkali metals. The noble gases. Transition metals. .......................................... Q30: What is Group 0 in the Periodic Table called? a) b) c) d) The halogens. The alkali metals. The noble gases. Transition metals. .......................................... Q31: Elements in the same group in the Periodic Table have the same: a) b) c) d) number of occupied energy shells. density. number of outer electrons. number of protons. .......................................... Q32: Where in the Periodic Table are non-metals found? a) b) c) d) The top. The bottom. The right hand side. The left hand side. .......................................... Periodicity © H ERIOT-WATT U NIVERSITY 81 82 TOPIC 4. THE PERIODIC TABLE © H ERIOT-WATT U NIVERSITY 83 Topic 5 Bonding and structure Contents 5.1 Prior knowledge . . . . . . . . . . . . . 5.2 Introduction . . . . . . . . . . . . . . . 5.3 Metallic bonds . . . . . . . . . . . . . . 5.3.1 Metallic structures . . . . . . . 5.4 Covalent bonds . . . . . . . . . . . . . 5.4.1 Covalent molecular structures 5.4.2 Covalent network structures . 5.5 Ionic bonds . . . . . . . . . . . . . . . 5.5.1 Ionic lattice structures . . . . . 5.6 Summary . . . . . . . . . . . . . . . . 5.7 Resources . . . . . . . . . . . . . . . . 5.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 86 90 91 92 94 98 102 105 107 107 108 Prerequisite knowledge Before you begin this topic, you should already know that: • elements can be categorised as metals and non-metals (National 4, Unit 1); • experimental procedures are required to confirm the type of bonding present in a substance (National 5, Unit 1); • metallic bonding can explain the conductivity of metals (National 5, Unit 3); • covalent compounds form when non-metal atoms form covalent bonds by sharing their outer electrons (National 4, Unit 1); • covalent molecular compounds have low melting and boiling points - as a result, they can be found in any state at room temperature (National 4, Unit 1); • in a covalent bond, the shared pair of electrons is attracted to the nuclei of the two bonded atoms (National 5, Unit 1); • more than one bond can be formed between atoms leading to double and triple covalent bonds (National 5, Unit 1); • covalent substances can form either discrete molecular or giant network structures (National 5, Unit 1); 84 TOPIC 5. BONDING AND STRUCTURE • diagrams show how outer electrons are shared to form the covalent bond(s) in a molecule and the shape of simple two-element compounds (National 5, Unit 1); • covalent molecular substances have low melting and boiling points due to only weak forces of attraction between molecules being broken (National 5, Unit 1); • giant covalent network structures have very high melting and boiling points because the network of strong covalent bonds must be broken (National 5, Unit 1); • when there is an imbalance in the number of positive protons and electrons, the particle is known as an ion (National 5, Unit 1); • ionic bonds are the electrostatic attraction between positive and negative ions ionic compounds form lattice structures of oppositely charged ions (National 5, Unit 1); • ionic compounds have high melting and boiling points because strong ionic bonds must be broken in order to break down the lattice - dissolving also breaks down the lattice structure (National 5, Unit 1); • ionic compounds have high melting and boiling points - as a result, they are found in the solid state at room temperature (National 4, Unit 1); • ionic compounds form when metal atoms join to non-metal atoms by transferring electron(s) from the metal to the non-metal - the resulting charged particles are called ions and an ionic bond is the attraction of the oppositely charged ions (National 4, Unit 1); • ionic compounds conduct electricity, only when molten or in solution due to the breakdown of the lattice resulting in the ions being free to move (National 5, Unit 1). Learning objectives By the end of this topic, you should be able to: • state that the first 20 elements in the Periodic Table can be categorised according to bonding and structure: ◦ metallic Li, Be, Na, Mg, Al, K, Ca; ◦ covalent molecular H2 , N2 , O2 , F2 , Cl2 , P4 , S8 and fullerenes (eg C 60 ); ◦ covalent network B, C, Si (diamond, graphite and the element silicon and silicon dioxide) monatomic (noble gases); explain each of these types of substance in terms of bonding and structure. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 5.1 85 Prior knowledge Test your prior knowledge Q1: Covalent bonding involves: a) b) c) d) a shared pair of electrons. transfer of electrons. delocalised electrons. gaining electrons. .......................................... Q2: Metals can conduct electricity in their solid state because of their: a) b) c) d) ions. positive cores. lattice structure. delocalised electrons. .......................................... Q3: Ionic bonding involves: a) b) c) d) a shared pair of electrons. transfer of electrons. delocalised electrons. protons only. .......................................... Q4: One property of covalent networks is that they: a) b) c) d) have high melting and boiling points. have low melting and boiling points. are soluble in water. conduct electricity when molten or in solution. .......................................... Q5: One property of ionic substances is that they: a) b) c) d) are all white in colour. have low melting and boiling points. are insoluble in water. conduct electricity when molten or in solution. .......................................... © H ERIOT-WATT U NIVERSITY Go online 86 TOPIC 5. BONDING AND STRUCTURE 5.2 Introduction Most elements are found on Earth as compounds. Q6: Can you think of any elements that are found 'free', not as compounds? .......................................... Comparing the elements iron and tantalum Sometimes ores can be simple compounds (e.g. haematite, an oxide of iron, Fe 2 O3 ); some are complex minerals (e.g. columbite, containing tantalum, (Fe,Mn)(Ta,Nb) 2O6 ). Haematite (http:/ / bit.ly/ 2aK AD0W by http:/ / commons.w ikimed ia.or g/ w iki/ User : Sailko, is licensed under http:/ / cr eativecommons.or g/ licenses/ by/ 2.0/ d eed .en) Columbite is a black mineral group that is an ore of niobium (http:/ / commons.w ikime d ia.or g/ w iki/ F ile:Columbite-7 5444.jpg, by http:/ / w w w .ir ocks.com, is licensed under http:/ / cr eativecommons.or g/ licenses/ by/ 2.0/ d eed .en) © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE Iron is also abundant in the Earth's crust (5%); tantalum is rare (1 - 2 parts per million). The extraction of iron from its ores is an established, well-understood process; the extraction of tantalum is complex, mainly because it occurs with the very similar metal, niobium. Q7: Niobium is used for a very special purpose in space rockets. Can you find out what it is for? .......................................... Iron is used for a vast variety of purposes; tantalum is almost exclusively used to make high-performance capacitors in electronic equipment, for example, mobile phones. Despite these contrasts, you should always remember that all the 'chemicals' we humans use have to be extracted from the finite resources present in the Earth. A variety of chemical processes are used to extract elements from their compounds, but the choice depends on the bonding and structure of the materials containing the element. Bonding and structure in elements and simple compounds Everything we see around us is made from fewer than 100 different types of atoms chemically bonded in various ways to produce a multitude of different molecules. The different types of chemical bonding determine the structure that elements and compounds adopt. In turn the structure, together with the size of intermolecular forces mainly determines the physical and chemical properties possessed by these materials. Bonding and structure will be studied in this topic. Intermolecular forces (the forces that exist between molecules) and properties are studied in a later topic. Examples of some substances with different properties, depending on different bonding and structures are shown in the images below. © H ERIOT-WATT U NIVERSITY 87 88 TOPIC 5. BONDING AND STRUCTURE Iron screws - a typical metal Water - a volatile liquid Nitrogen dioxide - a brown gas Sodium chloride - a white crystalline solid No atoms, except those of the noble gases, exist in isolation under normal conditions. They all interact in one way or another to form more stable structures. Atoms consist of a tiny positively charged nucleus, with different numbers of electrons, in shells of increasing energy levels, around it. The following activity shows the final electronic structure of a sodium atom. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 89 Construction of a sodium atom Go online Construction of a sodium atom In the case of the atoms of noble gases, all shells are completely filled with electrons. This arrangement is particularly stable, which makes noble gases unreactive. Atoms of other elements combine in ways which try to achieve this stable noble gas arrangement of electrons, in other words, to become isoelectronic with a noble gas. Q8: Look at the Periodic Table in the SQA data booklet (page 2). Which noble gas has an electronic structure closest to chlorine? .......................................... Q9: What must happen to a chlorine atom to get the noble gas electronic arrangement? .......................................... Q10: What must happen to the sodium atom for it to have a noble gas electronic structure? .......................................... Q11: What will the sodium atom become? .......................................... Q12: Which noble gas has the same electronic structure as a sodium ion? .......................................... © H ERIOT-WATT U NIVERSITY 90 TOPIC 5. BONDING AND STRUCTURE 5.3 Metallic bonds Metals occur to the left and centre of the Periodic Table (see Topic 4). When atoms of metals come together, the most stable condition is for them to 'pool' their outer electrons and become, in effect, a regular arrangement of fixed metal ions in a 'sea' of delocalised electrons shared by all the ions. For example, in metallic sodium each atom will donate an electron to the delocalised pool, and will achieve the stable sodium ion structure (isoelectronic with neon). Metallic bonds The following shows the structure of metallic sodium. Go online Such an array of positively charged ions would normally split apart by mutual repulsions, but the influence of the negative electrons holds the particles in the structure so well that most metals are hard solids with high melting and boiling points. Q13: There are exceptions. Can you think of soft metals, and one that is liquid under normal conditions? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 5.3.1 91 Metallic structures In metallic bonding, the delocalised electrons are able to migrate freely throughout the metal (making them good conductors of electricity), but the positive ions have a regular 3D structure known as a lattice. Part of the structure of the giant lattice for copper is shown below. Copper lattice Go online Q14: The electrons in a metallic bond are said to be: a) b) c) d) loose. ionised. delocalised. hard. .......................................... Q15: What do the ions in a metallic bond form into? a) b) c) d) Gel Grid Bar Lattice .......................................... Q16: Suggest a reason why aluminium is a better conductor of electricity than sodium. .......................................... Key point Metallic bonding is the electrostatic force of attraction between positively charged ions and delocalised outer electrons. A metallic structure consists of a giant lattice of positively charged ions and delocalised outer electrons. © H ERIOT-WATT U NIVERSITY 92 TOPIC 5. BONDING AND STRUCTURE 5.4 Covalent bonds You should already have studied covalent bonds, which are the most common type of bond. This section revises your knowledge. Covalent bonds form when atoms share two electrons, enabling both atoms to complete their valency shells. The activity below illustrates two separate chlorine atoms and the sharing of electrons to form a chlorine molecule. Covalent bond formation Go online .......................................... This sharing of two electrons (one from each atom) to complete an outer shell of electrons is called a covalent bond. In most cases the outer shell will contain 8 electrons, but for hydrogen only two are present. The following diagram shows the electrostatic forces in a hydrogen molecule. The positively charged nuclei will repel each other, as will the negatively charged electrons; but these forces are more than balanced by the attraction between the nuclei and electrons. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 93 Attractive and repulsive forces in a hydrogen molecule When atoms require more than one electron to complete their outer shell, they can share two or three electrons to make a double or triple covalent bond, or share with more than one other atom. The diagrams show the shared electrons in a molecule of oxygen (O 2 ) and ammonia (NH3 ). Notice the easier way to show a covalent bond with a line for each bonding (shared) pair of electrons. Covalent bonding in oxygen and ammonia The dark pairs of electrons in above diagram are the bonding electrons, shared between the atoms. The light pairs of electrons, completing the shells around the atoms, are called lone pairs. Q17: How many electrons are involved in the double bond in an oxygen molecule? .......................................... Q18: How many lone pairs are there in an ammonia molecule? .......................................... Q19: Which two words complete this sentence? A covalent bond is formed when two atoms configuration. achieve a noble gas © H ERIOT-WATT U NIVERSITY a pair of electrons, so that each can 94 TOPIC 5. BONDING AND STRUCTURE a) b) c) d) share, proton share, electron donate, atomic donate, ionic .......................................... Q20: The dominant attractive force in a covalent bond is between: a) b) c) d) the positively charged nuclei. the negatively charged shared electrons. negatively charged nuclei and positively charged shared electrons. positively charged nuclei and negatively charged shared electrons. .......................................... Q21: Draw the structure of iodine chloride. Look at the SQA data booklet page 2 to get the electron structures of the atoms. .......................................... Key point Atoms in a covalent bond are held together by electrostatic forces of attraction between positively charged nuclei and negatively charged shared electrons. 5.4.1 Covalent molecular structures Most covalent substances (for example all those mentioned in the previous section) exist as discrete molecules. There are strong covalent bonds binding the atoms together in the molecule, but much weaker forces between these molecules. You will study these forces in detail later (refer to the 'Intermolecular Forces' topic). Consequently, many small covalent molecules are gases (e.g. fluorine, oxygen, nitrogen, carbon dioxide, sulfur dioxide etc). Larger molecules make structures which are liquids or low melting point solids (think of candle wax, with molecular mass around 500, but a low melting point about 70 ◦ C). The molecules of nitrogen, oxygen, and the halogens consist of two atoms, they are diatomic. (You always write them N2 , O2 etc.) The non-metals phosphorus and sulfur form larger molecules as described in the following activities. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 95 White phosphorus Go online White phosphorus Phosphorus (in the common form of 'white' phosphorus) forms P 4 molecules, with each phosphorus atom at the corner of a tetrahedron. Tetrahedron of four atoms in white phosphorus .......................................... © H ERIOT-WATT U NIVERSITY 96 TOPIC 5. BONDING AND STRUCTURE Red phosphorus Go online Red phosphorus Phosphorus can also form a much less reactive (and less toxic) form, 'red' phosphorus, which is used in making matches. Its structure consists of chains of these tetrahedra. The structure is shown below. These P4 tetrahedra are quite stable, so that it is usual to write "P 4 " in equations where phosphorus is involved. It is even retained in some compounds - the oxides are molecules of formula P4 O6 and P4 O10 . Red phosphorus structure .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 97 Sulfur Go online Sulfur Molecules of sulfur (in the normal solid state as rhombic sulfur) consist of puckered rings of eight sulfur atoms, written S8 . Rings of eight sulfur atoms In this case, using S8 in equations would be rather cumbersome so sulfur in equations is written 'S'. Allotropes of sulfur In addition to the eight-membered rings in the rhombohedral, α-sulfur found in common 'flowers of sulfur', there are many other allotropes of sulfur. At temperatures above 95◦ C, the S8 rings pack to form monoclinic crystals of β-sulfur. Sulfur can also form rings and chains with any number of S atoms from 2 to 20. S2 is a gaseous form, an analogue of O 2 ; the blue colour of burning sulfur is due to the emission of light by S2 molecules produced in the flame. S3 is cherry red and has a structure like ozone, O3 . N.B. The recently discovered molecular structures of carbon (the fullerenes) are discussed in the next section, after the other common forms of carbon. © H ERIOT-WATT U NIVERSITY 98 TOPIC 5. BONDING AND STRUCTURE .......................................... 5.4.2 Covalent network structures In some elements and compounds the covalent bonds are not limited only to those within the molecules, but all the atoms are held to others by strong covalent bonds. An example of this is the structure of diamond. It consists solely of a network of carbon atoms held together by covalent bonds. Each carbon atom makes covalent bonds to four different carbon atoms, which each bond to three more carbons, and so on, binding the whole structure together, as shown in the following activity. This covalent network structure has the characteristic properties of hardness and a high melting point, both due to the strong bonding throughout the structure. Diamond Go online Arrangement of carbon atoms in diamond Q22: How many pairs of electrons are around each carbon atom? .......................................... Q23: What type of bond holds the carbon atoms together in diamond? a) Single covalent. b) Double covalent. c) Triple covalent. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 99 The following activities allow you explore other covalent network structures. Silicon dioxide Look at the following model, which compares the structure of silica (quartz, SiO 2 ) with diamond. Again, this consists of a network of covalent bonds, with each silicon atom bonded to four oxygen atoms which in turn bond to another silicon atom, to form a strongly-bound network. Quartz is hard and has a high melting point, like diamond. Go online Silica and diamond .......................................... Graphite structure Carbon has another interesting structure, graphite. In this case, carbon atoms covalently bond with only three other atoms to form a sheet of hexagonal rings. © H ERIOT-WATT U NIVERSITY Go online 100 TOPIC 5. BONDING AND STRUCTURE The distance between carbon atoms within these sheets is 142 pm. Q24: This is less than the bond length in diamond (154 pm) and suggests that the bond strength is: a) weaker. b) the same. c) stronger. .......................................... Q25: In these sheets, each carbon atom is bonded to only three others. How many bonding electrons surround each carbon atom in this case? .......................................... Graphene It is only recently that single layers of carbon atoms as found in graphite have been isolated in sufficient quantities to study their properties. The Nobel Prize for physics for 2010 was awarded to Andre Geim and Konstantin Novoselov for work on graphene, as it was called, which was performed at Manchester University in 2004. They found that these isolated single layers have very unusual electrical and physical properties, which have started research into new electronic applications. Graphite The carbon atoms have four electrons in their outer shell. Only three are used to bond with three other atoms, leaving a fourth electron. These sheets are layered together and the carbon atoms donate their extra electron to a delocalised pool which holds the sheets in a metallic type bonding. Layers of carbon atoms in graphite The distance between these sheets is 335 pm which indicates a weak bonding. These weaker forces produce a material which is greasy (as the layers slip over each other) and can be used as a lubricant, and the delocalised electron pool makes graphite a good conductor of electricity. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 101 Fullerenes Another form of carbon (discovered in the 1980s) exists as covalent molecular structures. These are the fullerenes. The most stable of these has 60 carbon atoms arranged in 5- and 6-membered rings forming a large sphere, looking like a football. It is called buckminsterfullerene after the architect (Robert Buckminster Fuller) who creates geodesic domes resembling the structure found in these carbon molecules. Buckminsterfullerene These fullerenes have interesting properties, which are currently being researched. For example, the 'buckyball', as illustrated, will dissolve in benzene to form a red solution. Q26: Why do you think buckminsterfullerene is soluble when diamond and graphite are not? .......................................... © H ERIOT-WATT U NIVERSITY Go online 102 TOPIC 5. BONDING AND STRUCTURE Long 'nanotubes' of carbon have tensile strengths 50 to 100 times that of steel! Why are carbon nanotubes so strong? The diagram shows only a small part of a tube. When very many of these long molecules are combined, the result consists of very large molecules of carbon atoms bonded to each other with strong bonds, similar to those in sheets of graphite. These bonds are much stronger than metallic bonds in steel. .......................................... Key point A covalent molecular structure consists of discrete molecules held together by intermolecular forces (see next topic). A covalent network structure consists of a giant lattice of covalently bonded atoms. 5.5 Ionic bonds Polar covalent bonds (you will study 'Polar covalent bonding' in a later topic) are formed when the atoms involved in a bond have different attractions for the bonding electrons. When two atoms have a large difference in their attraction for electrons (e.g. sodium and chlorine), it is most energetically favourable for an electron in the metal to be donated completely to the non-metal. This is the concept of electronegativity and will be fully explained in the next topic. Both then achieve a noble gas structure as shown in the following activity. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 103 Ionic bond formation Go online Q27: How many outer electrons does a sodium atom have? .......................................... Q28: How many outer electrons does a chlorine atom have? .......................................... Q29: When a sodium atom loses its outer electron to a chlorine atom, how many outer electrons do both ions then have? .......................................... Q30: Why is this special? .......................................... The original atoms change to ions (the metal forms a positive ion, the non-metal a negative ion) which are held together by electrostatic attraction. You can think of covalent, polar covalent and ionic bonds as forming a spectrum: bonds become more polar, then ionic as the difference in electronegativity increases. © H ERIOT-WATT U NIVERSITY 104 TOPIC 5. BONDING AND STRUCTURE The extent of covalent or ionic character depends mainly on the difference in electronegativity. The properties of the different bonds are summarised in the following table. Bond Covalent Polar covalent Ionic Electrons Equally shared Unequally shared Totally transferred Charge Distribution None Partial +ve and -ve Fully charged +ve and -ve ions Other ionic compounds Metals in Group 2, for example calcium, need to lose two electrons from their atoms to achieve a stable noble gas structure. This is reflected in the relatively low first and second ionisation energies. These elements will require two chlorine atoms to receive these two electrons and maintain electrical neutrality so the ratio of metal to non metal is 1:2 (e.g. [Ca2+ ][ Cl- ]2 ). Metals from other groups in the Periodic Table can form ions with 3+ charge and other non-metal ions with 2- and 3- charges. Higher charges than these are rare. When elements form ionic bonds, the ratio of ions is such that positive and negative charges always balance. Further consideration of the nature of the ions produced by different elements will be discussed in a later section. Q31: Ionic bonds are formed between atoms which have a electronegativity. a) b) c) d) difference in zero small large 0.2 .......................................... Q32: Caesium chloride is an ionic solid consisting of: a) b) c) d) negative caesium ions and negative chloride ions. negative caesium ions and positive chloride ions. positive caesium ions and negative chloride ions. positive caesium ions and positive chloride ions. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 105 .......................................... Q33: An ionic crystal consists of M2+ and X3- ions. What will be the ratio of ions (M 2+ : X3- ) in the solid? a) b) c) d) 1: 2: 3: 3: 1 3 2 3 .......................................... 5.5.1 Ionic lattice structures The mutual electrostatic attraction of positive and negative ions is completely non directional (unlike covalent bonds which are highly directed). When solid ionic compounds form, a positive ion will be surrounded by several negative ions which in turn will attract more positive ions. This process results in the formation of a lattice of regularly arranged ions all held together by electrostatic forces. These forces act equally in all directions , so that no one ion is attracted particularly to any other specific ion. For example, in sodium chloride there are no molecules formed. The ionic lattice for sodium chloride is shown in the following activity. Sodium chloride ionic lattice Go online Sodium chloride lattice This extensively bonded ionic solid is relatively hard, though not as hard as the covalent network solids, but is brittle. (Think of how easily salt (sodium chloride) crystals can be crushed.) © H ERIOT-WATT U NIVERSITY 106 TOPIC 5. BONDING AND STRUCTURE Rock salt cellar Q34: What are caesium chloride (CsCl) crystals formed from? a) b) c) d) Covalent molecules. A covalent lattice. An ionic lattice. Ionic molecules. .......................................... Now that we have looked at the three types of bonding in elements and compounds, see the table below roughly compares their strengths. The table also gives a figure for the weaker bonds between molecules, which will be studied in detail in the next topic. Bond type Strength/kJ mol-1 Metallic 80 - 600 Covalent 100 - 500 Ionic 100 - 450 Between covalent molecules 1 - 30 Strengths of bonds You can see that metallic bonds have the greatest range of strengths. (This is sensible if you think of hard metals like tungsten and chromium, and soft metals like sodium.) The strengths of covalent and ionic bonds are similar, and much greater than the bonds which hold covalent molecules together. (These are the intermolecular forces, which are studied in the next topic.) Key point Ionic bonding is the electrostatic force of attraction between positively and negatively charged ions. An ionic structure consists of a giant lattice of oppositely charged ions. © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 5.6 Summary Summary • Metallic bonding is the electrostatic force of attraction between positively charged ions and delocalised outer electrons. • A metallic structure consists of a giant lattice of positively charged ions and delocalised outer electrons. • Atoms in a covalent bond are held together by electrostatic forces of attraction between positively charged nuclei and negatively charged shared electrons. • A covalent molecular structure consists of discrete molecules held together by intermolecular forces. • A covalent network structure consists of a giant lattice of covalently bonded atoms. • Ionic bonding is the electrostatic force of attraction between positively and negatively charged ions. • An ionic structure consists of a giant lattice of oppositely charged ions. • Elements can be categorised into four classes according to their bonding and structure: 1. Metallic 2. Covalent molecular 3. Covalent network 4. Monatomic 5.7 • Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY 107 108 TOPIC 5. BONDING AND STRUCTURE 5.8 End of topic test End of Topic 5 test Go online Q35: In which of the following compounds do both ions have the same electron arrangement as argon? a) b) c) d) Calcium sulfide Magnesium oxide Sodium sulfide Calcium bromide .......................................... Q36: Which element is a solid at room temperature and consists of discrete molecules? a) b) c) d) Sulfur Neon Silicon Boron .......................................... Q37: Graphite, a form of carbon, conducts electricity because it has: a) b) c) d) pure covalent bonding. delocalised electrons. metallic bonding. van der Waals' bonding. .......................................... Q38: Which of the following can be applied to lithium but not to carbon? a) b) c) d) e) f) Covalent Metallic Made up of discrete molecules. Made up of diatomic molecules. Gas Solid .......................................... Q39: Which of the following can be applied to both fluorine and phosphorus? a) Covalent b) Metallic c) Made up of discrete molecules. d) Made up of diatomic molecules. e) Gas f) Solid .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE 109 Q40: Identify the element which exists as a covalent network solid. a) b) c) d) e) f) Boron Chlorine Nitrogen Phosphorus Sodium Sulfur .......................................... Q41: Identify the two elements which exist as discrete covalent molecular solids. a) Boron b) Chlorine c) Nitrogen d) Phosphorus e) Sodium f) Sulfur .......................................... Q42: Identify the two elements which react to form a compound with the most ionic character. a) Boron b) Chlorine c) Nitrogen d) Phosphorus e) Sodium f) Sulfur .......................................... Q43: Which of the elements is most likely to have a covalent network structure? Element Melting Point (K) Boiling Point (K) Density (g cm-3 ) Conduction when solids A 317 553 1.82 No B 933 2740 2.7 Yes C 1683 2628 2.32 No D 387 457 4.93 No a) b) c) d) A B C D .......................................... © H ERIOT-WATT U NIVERSITY 110 TOPIC 5. BONDING AND STRUCTURE Q44: Which type of structure is found in a substance melting at 771 K which conducts electricity when molten, but not when solid? a) b) c) d) Ionic Covalent (network structure) Metallic Covalent (discrete molecules) .......................................... Q45: Identify the covalent network substance. a) b) c) d) e) f) NH4 Cl (s) CH3 OH (l) C6 H14 (l) SO2 (g) Na2 CO3 (s) SiO2 (s) .......................................... Q46: Identify the two substances which are ionic. a) NH4 Cl (s) b) CH3 OH (l) c) C6 H14 (l) d) SO2 (g) e) Na2 CO3 (s) f) SiO2 (s) .......................................... Q47: Diamond and graphite are forms of carbon with very different properties. Graphite can mark paper, is a lubricant and is a conductor of electricity. Diamond has none of these properties. Draw a diagram to show the structure of diamond. .......................................... Q48: Diamond and graphite are forms of carbon with very different properties. Graphite can mark paper, is a lubricant and is a conductor of electricity. Diamond has none of these properties. Why is graphite an effective lubricant? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 5. BONDING AND STRUCTURE Q49: Boron nitride can form a similar structure to graphite. The boron and nitrogen atoms alternate throughout the structure as shown. Why is this substance a non-conductor, while graphite conducts electricity? .......................................... Q50: Suggest why the bonds between the layers in boron nitride are stronger than the bonds between the layers in graphite. .......................................... Q51: Which of the following structures is never found in compounds? a) b) c) d) Ionic Monatomic Covalent Molecular Covalent Network .......................................... © H ERIOT-WATT U NIVERSITY 111 112 TOPIC 5. BONDING AND STRUCTURE © H ERIOT-WATT U NIVERSITY 113 Topic 6 Periodic Table trends Contents 6.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Covalent radius . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 The trends in covalent radius . . . . . . . . . . . . 6.3 Ionisation energies . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Explanation of the trends in first ionisation energy 6.4 Electronegativity . . . . . . . . . . . . . . . . . . . . . . . 6.5 Summary of trends in the Periodic Table . . . . . . . . . . 6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 116 117 119 122 125 128 129 129 130 Prerequisite knowledge Before you begin this topic, you should already know that: • atoms contain protons, neutrons and electrons each with a specific charge, mass and position within the atom - the number of protons defines an element and is known as the atomic number (National 4, Unit 1); • or have knowledge of: sub-atomic particles, their charge, mass and position within the atom, the structure of the Periodic Table, groups, periods and atomic number (National 5, Unit 1); • all matter is made of atoms - when a substance contains only one kind of atom it is known as an element (National 4, Unit 1); • elements are arranged in the Periodic Table in order of increasing atomic number - elements with similar chemical properties are grouped together (National 4, Unit 1); • or be familiar with the seven diatomic elements (National 5, Unit 1); • covalent compounds form when non-metal atoms form covalent bonds by sharing their outer electrons (National 4, Unit 1); • covalent molecular compounds have low melting and boiling points - as a result, they can be found in any state at room temperature (National 4, Unit 1); 114 TOPIC 6. PERIODIC TABLE TRENDS • in a covalent bond, the shared pair of electrons is attracted to the nuclei of the two bonded atoms (National 5, Unit 1); • more than one bond can be formed between atoms leading to double and triple covalent bonds (National 5, Unit 1); • covalent substances can form either discrete molecular or giant network structures (National 5, Unit 1); • diagrams show how outer electrons are shared to form the covalent bond(s) in a molecule and the shape of simple two-element compounds (National 5, Unit 1); • when there is an imbalance in the number of positive protons and electrons the particle is known as an ion (National 5, Unit 1). Learning objectives By the end of this topic, you should be able to: • state that covalent radius is a measure of the size of an atom; • state that trends in covalent radius across periods and down groups can be explained in terms of the number of occupied shells, and the nuclear charge; • state that trends in ionisation energies across periods and down groups can be explained in terms of the atomic size, nuclear charge and the screening effect due to inner shell electrons; • explain that atoms of different elements have different attractions for bonding electrons; • state that electronegativity is a measure of the attraction an atom involved in a bond has for the electrons of the bond; • state that electronegativity values increase across a period and decrease down a group; • state that electronegativity trends can be rationalised in terms of nuclear charge, covalent radius and the presence of 'screening' inner electrons. © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 6.1 115 Prior knowledge Test your prior knowledge Q1: Complete the following table: Particle Charge Mass Proton Neutron Electron .......................................... Q2: Elements in the same group in the Periodic Table have the same: a) b) c) d) number of occupied energy shells. density. number of outer electrons. number of protons. .......................................... Q3: Elements in the same period in the Periodic Table have the same: a) b) c) d) number of occupied energy shells. density. number of outer electrons. number of protons. .......................................... Q4: What determines the number of protons in an atom? a) b) c) d) Atomic number Mass number Name of element Charge of atom .......................................... Q5: Covalent bonding involves: a) b) c) d) a shared pair of electrons. transfer of electrons. delocalised electrons. gaining electrons. .......................................... .......................................... © H ERIOT-WATT U NIVERSITY Location Go online 116 TOPIC 6. PERIODIC TABLE TRENDS 6.2 Covalent radius The size of individual atoms is difficult to measure since the size is determined by the space taken up by the constantly moving electrons. However, the distance between the nuclei of atoms in the solid state can be measured by a technique called X-ray diffraction. Consequently, the size of an atom is usually described in terms of its covalent radius. This is defined as half the distance between the nuclei of two bonded atoms of the element (see Figure 6.1). Figure 6.1: Definition of covalent radius .......................................... In Figure 6.1, the distance between the nuclei is shown as 2r and so the covalent radius in each case is r. Covalent radius - relative sizes The covalent radius is another periodic property. Go online Figure 6.2: The covalent radius .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 117 Q6: What happens to the size of the atoms on crossing a Period from left to right? a) An increase b) A decrease c) Nothing .......................................... Q7: What happens to the size of the atoms on descending a group? a) An increase b) A decrease c) Nothing .......................................... Q8: Can you suggest why are there no covalent radii quoted for the noble gases (Group 0)? .......................................... .......................................... 6.2.1 The trends in covalent radius The size of an atom is determined by the amount of space taken up by the electrons and so must be connected to the electron arrangement of the atom. The electron arrangement is itself a periodic property. Use the following activity to explain the trends in covalent radius. The trends in covalent radius Figure 6.3: The trends in covalent radius Go online © H ERIOT-WATT U NIVERSITY 118 TOPIC 6. PERIODIC TABLE TRENDS .......................................... Q9: Which of the following provides the best reason for the increase in covalent radius on going down a group? a) b) c) d) The number of protons increases. The number of electrons increases. The number of electron shells increases. The number of neutrons increases. .......................................... Q10: Which of the following provides the best reason for the decrease in covalent radius on going from left to right across a Period? a) b) c) d) The number of electrons increases. The number of electron shells increases. The number of neutrons increases. The number of protons increases. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS .......................................... Key point The covalent radius decreases across a Period because the increase in nuclear charge attracts the electrons more strongly. The covalent radius increases on going down a group as the number of occupied electron shells increases. 6.3 Ionisation energies The electrons orbiting the nucleus of an atom are held by the electrostatic attraction between the negative electrons and the positive nucleus. Some atoms lose electrons relatively easily, whereas some lose electrons only with great difficulty. No atoms simply give away electrons; to remove an electron always requires energy to overcome the force of attraction between the electron and the nucleus. The electron lost always comes from the outer shell of electrons. The energy required to remove an electron from an atom can be measured. It is normal to quote values not for an individual atom but for one mole of atoms. Moles At National 5 level, the mole was introduced as the gram formula mass of a substance (the formula mass expressed in grams). For an element, one mole is normally the gram formula mass, e.g. • 12 g of carbon is 1 mole • 4 g of helium is 1 mole • 40 g of calcium is 1 mole These three quantities are different masses but they have one very important feature in common: they all contain the same number of atoms. This arises because calcium atoms are ten times as heavy as helium atoms while carbon atoms are three times as heavy as helium atoms. This number is also known as one mole which refers to a particular number of particles (this will be investigated more fully in Unit 3). In fact, one mole of a substance contains a huge number of particles. Ionisation energy The first ionisation energy (IE) is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms (one electron from each atom). The units used are kilojoules per mole (kJ mol-1 ). Consider the element carbon. The value quoted for the 1 st ionisation energy of carbon is 1090 kJ mol-1 . In other words, 1090 kJ of energy is required to remove one electron from each atom in one mole (12 g) of gaseous carbon. © H ERIOT-WATT U NIVERSITY 119 120 TOPIC 6. PERIODIC TABLE TRENDS This can be represented in the following way: • the first ionisation energy for an element E refers to the reaction E(g) → E+ (g) + e- ; • the second ionisation energy refers to the reaction E+ (g) → E2+ (g) + e- . Figure 6.4: First ionisation energy of carbon .......................................... The C+ ion is smaller than the C atom because the remaining five electrons in the ion still feel the full attractive force from six protons and so they are more tightly held. As a result, it is even more difficult to remove the next electron. Figure 6.5: Second ionisation energy of carbon .......................................... The removal of one mole of electrons from one mole of gaseous C + ions is known as the second ionisation energy of carbon. This has a value of 2360 kJ mole -1 . The figure below shows equations representing the first four ionisation energies of carbon. © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 121 Figure 6.6: Successive ionisation energies .......................................... Ionisation energies Q11: Using a data booklet, plot a graph of first ionisation energy against atomic number for the first 20 elements. .......................................... Q12: Is the first ionisation energy a Periodic property? .......................................... Q13: From the shape of the graph, explain your answer to the previous question. .......................................... Q14: Name the group of elements which appears at the peaks in the graph. .......................................... Q15: Name the group of elements which have the lowest ionisation energies. .......................................... Q16: What is the general trend in first ionisation energy, going across a Period from left to right? a) b) c) d) A steady increase. A steady decrease. An increase followed by a decrease. A decrease followed by an increase. .......................................... Q17: What is the general trend in first ionisation energy, going down a group? a) b) c) d) A steady increase. A steady decrease. An increase followed by a decrease. A decrease followed by an increase. .......................................... Q18: Confirm your answer to the previous question by listing the first ionisation energies of the alkali metals in order of atomic number. .......................................... .......................................... © H ERIOT-WATT U NIVERSITY Go online 122 TOPIC 6. PERIODIC TABLE TRENDS 6.3.1 Explanation of the trends in first ionisation energy Going down a group In general, the first ionisation energy decreases on going down a group. For this exercise, we will concentrate on Group 1 since there is only one outer electron. The higher the ionisation energy, the more difficult it is to remove the electron, i.e. the stronger are the attractive forces between the electron and the nucleus. The strength of this force of attraction will depend on: 1. the size of the nuclear charge; 2. the distance between the electron and the nucleus; 3. the number of other electrons between the electron and the nucleus (i.e. the number of inner-shell electrons). The inner electrons cause a screening effect which prevents the outer electron from feeling the full effect of the nuclear charge. First ionisation energy - Going down a group Consider the electron arrangements of lithium, sodium and potassium (see Figure 6.7). Figure 6.7: Electron arrangements of alkali metals .......................................... Now discuss the following points with a partner, a group or even your tutor. • What effect will an increase in nuclear charge have on the ionisation energy? • What effect will an increase in covalent radius have on the ionisation energy? • What will happen to the number of inner shell electrons on going down a group? • What effect will this have on the ionisation energy? After discussion, you should be able to answer the following question. © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 123 Q19: Explain fully why the first ionisation energy decreases on going down Group 1. You should mention nuclear charge, covalent radius and screening effect. (There would be about 3 marks allocated to this type of 'explain' question.) .......................................... .......................................... Going across a Period The first ionisation energy increases on going from left to right across a Period. The same factors, as described previously, will be involved in explaining this trend. Concentrate on Period 2 for this part. First ionisation energy - Going across a Period Discuss the same points with a partner as before. Go online • What effect will an increase in nuclear charge have on the ionisation energy? • What effect will an increase in covalent radius have on the ionisation energy? • What will happen to the number of inner shell electrons on going down a group? • What effect will this have on the ionisation energy? Now answer the following questions. Q20: Explain fully why the first ionisation energy increases on going across Period 2 from left to right. (There would be about three marks allocated to this type of 'explain' question.) .......................................... Q21: What happens to the nuclear charge? a) It decreases. b) It increases. c) It stays the same. .......................................... Q22: What effect will this have on the first ionisation energy? a) It increases. b) It decreases. c) It stays the same .......................................... © H ERIOT-WATT U NIVERSITY 124 TOPIC 6. PERIODIC TABLE TRENDS Q23: What happens to the number of inner shell electrons? a) It decreases. b) It increases. c) It stays the same. .......................................... Q24: What happens to the screening effect? a) It decreases. b) It increases. c) It stays the same. .......................................... Q25: As a result, what happens to the size of the atoms? a) It decreases. b) It increases. c) It stays the same. .......................................... • Use your data book to calculate the energy required to carry out the following reactions: Al(g) → Al+ (g) + e- Mg(g) → Mg2+ (g) + 2e- - • Why is there such a large increase in the energy required to remove a fourth electron from aluminium compared to removing the first, second or third electrons? .......................................... Key point The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms. The second and subsequent ionisation energies refer to the energies required to remove further moles of electrons. First ionisation energies increase across a Period and decrease down a group. This can be explained in terms of atomic size, nuclear charge and the screening effect due to inner shell electrons. © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 6.4 125 Electronegativity The first ionisation energy involves the removal of electrons from gaseous atoms and so is a measure of how strongly an isolated atom holds on to its outermost electrons. In the world around us, isolated atoms are very rare and the vast majority of atoms are found bonded to one or more other atoms. How and why atoms bond together is the basis of chemistry and electrons are the fundamental particles involved in bonding. In all theories of bonding, different types of bond arise because atoms of different elements have different attractions for electrons. Atoms which tend to attract the electrons within a bond are said to be electronegative. An atom with a high electronegativity will tend to attract bonded electrons towards it whereas an atom with a low electronegativity will have a very weak attraction for electrons. There will be a 'tug of war' between the different atoms for the electrons. Electronegativity In Figure 6.8, the darker shading shows the electrons being pulled towards the more electronegative atom. Figure 6.8: Electronegativity .......................................... © H ERIOT-WATT U NIVERSITY Go online 126 TOPIC 6. PERIODIC TABLE TRENDS Q26: If X is hydrogen, Y could be: a) Beryllium b) Bromine c) Boron .......................................... Q27: The bond X-Y could be: a) C-Cl b) H-S c) C-S .......................................... Q28: If X is in Group 5, Y could be in: a) Group 3 b) Group 6 c) Group 7 .......................................... .......................................... There are several different methods for estimating electronegativity values. The one most commonly used is that devised by double Nobel Prize winner, Linus Pauling. He assigned a value to each of the elements most commonly found in bonds. Lithium was assigned a value of 1.0 while fluorine was assigned the highest value of 4.0. The electronegativity values produced by Pauling are quoted in the SQA data booklet (page 11). Study these values carefully and look for any patterns. © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 127 Q29: What is the Group number of the elements which have the lowest electronegativities? .......................................... Q30: What is the Group number of the elements with the highest electronegativities? .......................................... Q31: What is the name of the Group for which no values are quoted? .......................................... Q32: Can you suggest a reason why no values are quoted for these elements? .......................................... In general, the electronegativity increases from left to right across a Period. This is because as you move from left to right, there are more protons and so the nuclear charge is increased. As you move down a Group, atomic size is increasing so outer electrons are further from the positively charged nucleus. As a result, the electronegativity decreases. Q33: Which of the following statements describes the trends in electronegativity values in the Periodic Table? a) Electronegativity values increase on going from left to right increase on going down a Group. b) Electronegativity values decrease on going from left to right decrease on going down a Group. c) Electronegativity values increase on going from left to right decrease on going down a Group. d) Electronegativity values decrease on going from left to right increase on going down a Group. across a Period and across a Period and across a Period and across a Period and .......................................... Key point Electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond. Electronegativity values increase across a Period and decrease down a Group. © H ERIOT-WATT U NIVERSITY 128 TOPIC 6. PERIODIC TABLE TRENDS 6.5 Summary of trends in the Periodic Table Summary: Periodic Table trends Go online Q34: Complete the table by selecting the appropriate arrow symbol to indicate the increasing size of each property as you move across a Period and up/down Group 1 and 7. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 6.6 Summary Summary 6.7 • • The covalent radius decreases across a Period because the increase in nuclear charge attracts the electrons more strongly. • The covalent radius increases on going down a Group as the number of occupied electron shells increases. • The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms. • The second and subsequent ionisation energies refer to the energies required to remove further moles of electrons. • First ionisation energies increase across a Period and decrease down a Group. • This can be explained in terms of atomic size, nuclear charge and the screening effect due to inner shell electrons. • Electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond. • Electronegativity values increase across a Period and decrease down a Group. • This can be explained in terms of atomic size, nuclear charge and the screening effect due to inner shell electrons. Resources Higher Chemistry for CfE, J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY 129 130 TOPIC 6. PERIODIC TABLE TRENDS 6.8 End of topic test End of Topic 6 test Go online Q35: Menedeleev is famous for producing the Periodic Table on which the modern version is based. Which of the following statements is true? a) Mendeleev organised the elements in order of their atomic number. b) Mendeleev swapped some elements round so that their atomic masses fitted the pattern. c) Mendeleev left gaps for elements which had not yet been discovered. d) Mendeleev left gaps because some elements did not fit the pattern of reactivity. .......................................... Q36: Which of the following statements about the Periodic Table are true? 1. There is a steady decrease in density on going down Group 1. 2. There is an increase in electronegativity on going down Group 7. 3. There is a decrease in first ionisation energy on going down Group 0. 4. There is a decrease and then an increase in boiling point on crossing a Period from left to right. 5. There is a steady increase in melting point across a Period from left to right. 6. There is a steady decrease in atomic size across a Period from left to right. .......................................... Q37: Which property of the Group 1 elements could be represented by the following graph? Arbitrary Scale Li a) b) c) d) Na K Rb Electronegativity Atomic size Melting point First ionisation energy .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 131 Q38: Some information about four atoms A, B, C and D is presented below. D C Covalent Radius B A Number of occupied electron shells A 3 B 4 C 3 D 4 Which atom will have the largest nuclear charge? a) b) c) d) A B C D .......................................... Q39: Calcium has a larger covalent radius than magnesium because calcium has: a) b) c) d) a smaller first ionisation energy? a larger nucleus? a larger nuclear charge? more occupied electron shells? .......................................... Q40: The difference in atomic size between sodium and chlorine is mainly due to the number of: a) b) c) d) electron shells? neutrons? protons? electrons? .......................................... © H ERIOT-WATT U NIVERSITY 132 TOPIC 6. PERIODIC TABLE TRENDS Q41: Which of the following equations represents the first ionisation energy of magnesium? a) Mg(s) → Mg+ (g) + eb) Mg(g) → Mg2+ (g) + 2ec) Mg(g) → Mg+ (g) + 2ed) 12 Mg(g) → 12 Mg2+ (g) + e− .......................................... Q42: Which of the following equations represents the first ionisation energy of fluorine? a) F- (g) → F(g) + eb) 12 F2 (g) → F + (g) + e− c) F(g) + e- → F- (g) d) F(g) → F+ (g) + e.......................................... Q43: Identify the statements which correctly describe a trend in the Periodic Table. 1. The covalent radius increases from Li to F. 2. The boiling point increases from Li to F. 3. The first ionisation energy decreases from Na to Cl. 4. The first ionisation energy decreases from Li to Cs. 5. Electronegativity decreases from Li to Cs. 6. The number of electrons in the outer shell increases from Li to Cs. .......................................... Q44: The change in first ionisation energy from Li to F is mainly due to: a) b) c) d) increasing number of electron shells? increased screening effect? increasing number of electrons? increasing nuclear charge? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS 133 Q45: The bar graph shows the variation in the first ionisation energy with atomic number for sixteen consecutive elements in the Periodic Table. The element at which the bar graph starts is not specified. First Ionisation Energy (kJ mol-1) Z Atomic Number Element Z is identified in the bar graph. In which Group of the table is element Z? a) b) c) d) 1 3 5 6 .......................................... Q46: Which of the following graphs shows the variation in first ionisation energy on going from left to right across period 2? a) Atomic Number b) Atomic Number © H ERIOT-WATT U NIVERSITY 134 TOPIC 6. PERIODIC TABLE TRENDS c) Atomic Number d) Atomic Number .......................................... Q47: Which of the following will not affect the first ionisation energy of an element? a) b) c) d) Screening effect Atomic size Atomic number Atomic mass .......................................... Q48: bonding electrons. is a measure of the ability of an atom in a bond to attract the .......................................... Q49: has the highest attraction for bonding electrons. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 6. PERIODIC TABLE TRENDS Q50: Atoms of different elements have different attractions for the electrons in a bond. This property shows Periodic variation. Which of the following shows the correct trends within the Periodic Table? a) b) c) d) .......................................... Q51: Explain fully why the first ionisation energy increases on going across Period 2 from left to right. (3 marks) .......................................... Q52: Explain fully why the first ionisation energy decreases on going down Group 1. You should mention nuclear charge, covalent radius and screening effect. (3 marks) .......................................... Q53: Why are no electronegativity values are quoted for the Group 0 elements? a) b) c) d) They have a value of zero. They generally do not form bonds with other elements. They are unreactive non-metals. Their electronegativities are too high for the scale. .......................................... © H ERIOT-WATT U NIVERSITY 135 136 TOPIC 6. PERIODIC TABLE TRENDS © H ERIOT-WATT U NIVERSITY 137 Topic 7 Bonding continuum and polar covalent bonding Contents 7.1 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 7.2 Polar covalent bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Predicting bonding type using electronegativity . . . . . . . . . . . . . . . . . . 140 144 7.4 Polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Polar molecules and molecular polarity . . . . . . . . . . . . . . . . . . 146 147 7.5 The bonding continuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 152 7.7 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 153 Prerequisite knowledge Before you begin this topic, you should already know that: • covalent compounds form when non-metal atoms form covalent bonds by sharing their outer electrons (National 4, Unit 1); • covalent molecular compounds have low melting and boiling points - as a result, they can be found in any state at room temperature (National 4, Unit 1); • in a covalent bond, the shared pair of electrons is attracted to the nuclei of the two bonded atoms (National 5, Unit 1); • covalent substances can form either discrete molecular or giant network structures (National 5, Unit 1); • diagrams show how outer electrons are shared to form the covalent bond(s) in a molecule and the shape of simple two-element compounds (National 5, Unit 1); • the first 20 elements in the Periodic Table can be categorised according to bonding and structure: ◦ metallic (Li, Be, Na, Mg, Al, K, Ca); ◦ covalent molecular (H2 , N2 , O2 , F2 , Cl2 , P4 , S8 and fullerenes (eg C 60 )); 138 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING ◦ covalent network (B, C (diamond, graphite), Si) monatomic (noble gases); • electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond (Higher, Unit 1); • electronegativity values increase across a Period and decrease down a Group (Higher, Unit 1); • trends in electronegativity can be explained in terms of atomic size, nuclear charge and the screening effect due to inner shell electrons (Higher, Unit 1). Learning objectives By the end of this topic, you should be able to: • state that, in a covalent bond, atoms share pairs of electrons; • explain that a covalent bond is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons; • explain that polar covalent bonds are formed when the attraction of the atoms for the pair of bonding electrons is different; • explain that delta positive and delta negative notation can be used to indicate the partial charges on atoms, which give rise to a dipole; • state that pure covalent bonding and ionic bonding can be considered as being at opposite ends of a bonding continuum with polar covalent bonding lying between these two extremes; • state that the larger the difference in electronegativities between bonded atoms, the more polar the bond will be; • state that, if the difference is large, then the movement of bonding electrons from the element of lower electronegativity to the element of higher electronegativity is complete, resulting in the formation of ions; • state that compounds formed between metals and non-metals are often, but not always ionic. © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.1 139 Prior knowledge Test your prior knowledge Q1: Covalent bonding involves: a) b) c) d) transfer of electrons. a shared pair of electrons. delocalised electrons. gaining electrons. .......................................... Q2: Why are no electronegativity values quoted for the Group 0 elements? a) b) c) d) They have a value of zero. They are unreactive non-metals. They generally do not form bonds with other elements. Their electronegativities are too high for the scale. .......................................... Q3: Which of the following elements has the greatest attraction for the shared pair of electrons in a bond? a) b) c) d) Fluorine Carbon Hydrogen Chlorine .......................................... Q4: Which of the following elements has the least attraction for the shared pair of electrons in a bond? a) b) c) d) Fluorine Carbon Hydrogen Chlorine .......................................... © H ERIOT-WATT U NIVERSITY Go online 140 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.2 Polar covalent bonds Previously, you saw how the electronegativity of atoms is a measure of their attraction for electrons in a bond. Only when the two atoms bonded by a covalent bond are the same (e.g. Cl2 in chlorine gas, C-C in diamond) will the electrons be exactly shared equally. In other cases there will be an unequal distribution of the electrons and the atom with the higher electronegativity will have a greater share of the electrons. Since electrons carry a negative charge, an unequal distribution will result in the bond having a partial negative charge (called 'delta negative' and drawn δ-) where there is an excess of electrons around the most electronegative atom, and a partial positive charge (δ+) where there is a deficiency. This is not to be confused with a full charge as found on ions. Figure 7.1: Polar bond .......................................... In the case of HCl (Figure 7.1), chlorine, with electronegativity 3.0, becomes negative and hydrogen, at 2.2, positive. The δ partial charge is about 0.17 of a full charge. This type of bond is called a polar covalent bond (sometimes abbreviated to a polar bond). The greater the difference between the electronegativities of the atoms, the greater the distortion of the electrons in the bond, and the greater the charge distribution. This effect is shown in the following activity. Electronegativity Go online In Figure 7.2, the darker shading shows the electrons being pulled towards the more electronegative atom. © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Figure 7.2: Electronegativity and charge .......................................... Q5: How much ionic character could this bond have? a) 0% b) 50% c) 100% .......................................... Q6: This bond has almost zero ionic character. It could be: a) Li-F b) Mg-O c) C-S © H ERIOT-WATT U NIVERSITY 141 142 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING .......................................... Q7: This bond could be: a) X is Na and Y is O b) X is S and Y is K c) X is N and Y is Cl .......................................... Use the table of electronegativities, where necessary, (SQA data booklet, page 11) to answer the following questions. Q8: a) b) c) d) Which molecule contains polar bonds? Hydrogen (H2 ) Chlorine (Cl2 ) Hydrogen chloride (HCl) Nitrogen (N2 ) .......................................... Q9: a) b) c) d) Which molecule contains the most polar bonds? Hydrogen fluoride (HF) Hydrogen chloride (HCl) Hydrogen bromide (HBr) Hydrogen iodide (HI) .......................................... Q10: Which molecule has the bonds that are most polar? a) b) c) d) Hydrogen sulfide (H2 S) Phosphine (PH 3 ) Methane (CH4 ) Ammonia (NH 3 ) .......................................... can be used to show the direction of the dipole, the arrow The symbol pointing to the negative side. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 143 Direction of charge Use the electronegativity values (page 11 of the data booklet) to help show the direction of partial charge (or no charge) in some bonds. Q11: Complete the table by selecting the correct sign for the bond shown. .......................................... Key point Polar covalent bonds occur when the atoms of the bond attract the bonding electrons unequally causing the atoms to have partial positive and negative charges. The polarity of a covalent bond depends on the difference in electronegativity between the bonded atoms, the most electronegative becoming more negative. © H ERIOT-WATT U NIVERSITY Go online 144 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.3 Predicting bonding type using electronegativity One of the main factors determining the type of bond formed between two elements in a compound is the difference in electronegativity. As previously discussed, electronegativity is a Periodic property, with increasing electronegativity as you move across a Period from left to right, and decreasing electronegativity as you move down a Group. The relationship between electronegativity and bond type is summarised below. Figure 7.3: Relationship between electronegativity and bond type .......................................... Electronegativity difference is only one predictor of bond type. For a more complete description, the properties of the substances (as described in another topic) need to be considered. Most covalent bonded compounds generally exist as discrete molecular structures. However, two important covalent network compounds are silicon dioxide (SiO 2 , ) and silicon carbide (SiC, carborundum), which has a similar structure to diamond. Both of these are used as abrasives on account of their hardness. Use the electronegativity values in the SQA data booklet to help you answer the following questions. © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Q12: Hydrogen, bromine and potassium have electronegativities of 2.2, 2.8 and 0.8 respectively. What type of bonding would you expect: • between two bromine atoms in a Br 2 molecule? • between hydrogen and bromine in HBr? • between potassium and bromine in KBr? .......................................... Q13: What type of bonding would you expect between the Group 1 metal caesium and the Group 6 element sulfur? a) b) c) d) Pure covalent Polar covalent Ionic Metallic .......................................... Q14: What type of bonding would you expect between phosphorus and hydrogen in phosphine (PH 3 ) ? a) b) c) d) Pure covalent Polar covalent Ionic Metallic .......................................... Q15: Which of the elements below will bond with oxygen, to produce a polar bond with a partial positive charge on the oxygen atom? a) b) c) d) Carbon Fluorine Hydrogen Lithium .......................................... Key point The type of bonding between two atoms depends mainly on the difference in electronegativity between the atoms. When the difference is zero, the bond will be covalent. With a small difference, a polar covalent bond is likely. When the difference is large, fully charged ions are produced and ionic bonding will be predicted. © H ERIOT-WATT U NIVERSITY 145 146 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.4 Polar molecules When two atoms of different electronegativity are bonded together by sharing electrons, one atom attracts the electrons more than the other and a polar bond results. In simple molecules like hydrogen chloride and iodine chloride this polar bond has a permanent dipole. Polar molecules are attracted to one another by forces called permanent dipole-permanent dipole interactions (see Figure 7.4) as well as the London dispersion attractions caused by the movement of electrons observed in the last section. Permanent dipole interactions Figure 7.4: Permanent dipoles in iodine chloride Go online .......................................... How do permanent dipole-permanent dipole interactions compare to London dispersion forces of attraction in terms of strength? A comparison of strengths can be made by looking at the table below. Figure 7.5: Boiling points of halogen containing compounds A B C Cl - Cl I - Cl Br - Br -35o C 97o C 59o C .......................................... Any comparison of boiling points has to be between molecules of similar size and shape so that the London dispersion forces are similar. © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Q16: Which molecule has permanent dipole-permanent dipole interactions? a) A b) B c) C .......................................... Q17: Which other molecule would have almost the same London dispersion forces? a) A b) B c) C .......................................... Q18: Which of the molecules has the strongest intermolecular forces? a) A b) B c) C .......................................... .......................................... Key point Permanent dipole-permanent dipole interactions act in addition to London dispersion electrostatic attractions between polar molecules and are stronger than these attractions for molecules of equivalent size. 7.4.1 Polar molecules and molecular polarity Not all covalent molecules with polar bonds result in polar molecules. Those molecules which are highly symmetrical prove to be non-polar (see Figure 7.6). Figure 7.6: Non-polar molecules .......................................... In each of the two molecules shown, although there is a difference in electronegativity in the polar bonds, the charge is distributed around the central carbon atoms with the positive and negative charges balancing out. The molecules have no overall dipole. © H ERIOT-WATT U NIVERSITY 147 148 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Notice that the central carbon on each of the molecules is shown with only one (δ + ) sign for clarity, even though each atom has sufficient charge to balance out the negative charge. In molecules with polar bonds which are not symmetrical (see Figure 7.7), the dipoles cannot cancel each other out. If the molecule has a permanent slight positive charge at one side and a negative charge at the other, then it is a polar molecule. Bond polarity can thus be predicted from electronegativity differences and molecular polarity can be predicted from electronegativity differences if we also take into account the shape of the molecule. can be used to show the direction of the dipole, the arrow The symbol pointing to the negative side. The molecules shown in the figure below are both polar molecules. Figure 7.7: Polarity of molecules .......................................... Predicting molecular polarity Go online Electronegativity values are used to predict the polarity of bonds, and the shapes of molecules are used to predict molecular polarity. Page 11 in the data booklet may be helpful. Q19: Complete the table by using the items below it. Dipole symbols may be repeated. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 149 Detecting polar molecules An experiment is performed to show the attractions between polar molecules and electrostatically charged rods. Figure 7.8 Paraffin Water Charged Rods .......................................... Q20: Which liquid is affected by the charged rod? a) Water b) Paraffin .......................................... Q21: Which liquid is polar? a) Water b) Paraffin .......................................... Q22: Which of these statements is correct when a jet of polar molecules passes a charged rod? a) b) c) d) It is not affected by the rod. It is attracted by the rod. It is repelled by the rod. It is sped up by the rod. .......................................... Q23: Which of these statements is correct when a jet of non-polar molecules passes a charged rod? a) b) c) d) It is not affected by the rod. It is attracted by the rod. It is repelled by the rod. It is sped up by the rod. © H ERIOT-WATT U NIVERSITY Go online 150 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING .......................................... .......................................... Key point An electrostatically charged rod can be used to detect the presence of polar molecules in a liquid. Polar molecules are attracted to both a negative and positive rod. 7.5 The bonding continuum Covalent bonding usually involves elements close to each other in the Periodic Table. Ionic bonding generally involves metals from the left side and non-metals from the opposite side of the table. Most chemical bonds are somewhere between the two extremes and it is best to think of ionic and covalent bonding as being at opposite ends of a bonding continuum with varying degrees of polar covalent bonding lying between the extremes (see Figure 7.9). Figure 7.9: Bonding spectrum .......................................... In most cases, the bigger the difference in electronegativity between the atoms, the more polar the bond and the greater the ionic character. However, other factors make a contribution and care needs to be taken before jumping to conclusions. Identical atoms share the electrons in a covalent bond equally. In the hydrogen molecule there is no charge distribution. Non-identical atoms like hydrogen and chlorine attract the bonding electrons unequally, since chlorine attracts electrons more strongly. A polar bond results (see Figure 7.10). © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Figure 7.10: Polar bond .......................................... In some cases, the bond polarity results in a polar molecule. In certain symmetrical molecules the polar nature of the bonds tends to cancel out (see Figure 7.11). Figure 7.11: Polar and non-polar molecules .......................................... In water, the molecule is asymmetrical, so the δ+ on the hydrogen atoms will add and the molecule will have a resultant charge distribution; it will be a polar molecule. The tetrachloromethane molecule is highly symmetrical, so the charge distribution on the C-Cl bonds will cancel out and the molecule will be non-polar, despite having polar bonds. Q24: Will trichloromethane (CHCl 3 , chloroform) be a polar or non-polar molecule? a) Polar b) Non-polar .......................................... © H ERIOT-WATT U NIVERSITY 151 152 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.6 Summary Summary 7.7 • • Polar covalent bonds occur when the atoms of the bond attract the bonding electrons unequally, causing the atoms to have partial positive and negative charges. • The polarity of a covalent bond depends on the difference in electronegativity between the bonded atoms, the most electronegative becoming more negative. • There are polar covalent bonds between pure covalent and pure ionic bonds. • The type of bonding between two atoms depends mainly on the difference in electronegativity between the atoms: ◦ when the difference is zero, the bond will be covalent; ◦ with a small difference, a polar covalent bond is likely; ◦ when the difference is large, fully charged ions are produced and ionic bonding will be predicted. • Permanent dipole-permanent dipole interactions act in addition to London dispersion electrostatic attractions between polar molecules and are stronger than these attractions for molecules of equivalent size. • Not all covalent molecules with polar bonds result in polar molecules. • Molecules which are highly symmetrical tend to be non-polar. • An electrostatically charged rod can be used to detect the presence of polar molecules in a liquid. Polar molecules are attracted to both a negative and positive rod. • There is a complete range of bond types leading to a bonding spectrum mainly based on electronegativity. Resources Higher Chemistry for CfE, J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 7.8 153 End of topic test End of Topic 7 test Q25: In which of the following compounds do both ions have the same electron arrangement as argon? a) b) c) d) Calcium bromide Magnesium oxide Sodium sulfide Calcium sulfide .......................................... Q26: Which element is a solid at room temperature and consists of discrete molecules? a) b) c) d) Sulfur Carbon Silicon Boron .......................................... Q27: Graphite, a form of carbon, conducts electricity because it has: a) b) c) d) metallic bonding. pure covalent bonding. delocalised electrons. London dispersion forces. .......................................... Q28: Which of the following is true of lithium but not of carbon? a) b) c) d) e) f) Covalent Metallic Made up of discrete molecules. Made up of diatomic molecules. Gas Solid .......................................... Q29: Which of the following is true of both fluorine and phosphorus? a) Covalent b) Metallic c) Made up of discrete molecules. d) Made up of diatomic molecules. e) Gas f) Solid .......................................... © H ERIOT-WATT U NIVERSITY Go online 154 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING Q30: Identify the element which exists as a covalent network solid. a) b) c) d) e) f) Boron Chlorine Nitrogen Phosphorus Sodium Sulfur .......................................... Q31: Identify the two elements which exist as discrete covalent molecular solids. a) Boron b) Chlorine c) Nitrogen d) Phosphorus e) Sodium f) Sulfur .......................................... Q32: Identify the element which exists as a covalent network solid. a) Boron b) Chlorine c) Nitrogen d) Phosphorus e) Sodium f) Sulfur .......................................... Q33: In which molecule will the chlorine atom carry a partial positive charge (δ + )? a) b) c) d) Cl-F Cl-Br Cl-Cl Cl-I .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING 155 Q34: Which of the following elements is most likely to have a covalent network structure? Element Melting point (K) Boiling point (K) Density ( g cm-3 ) Conduction when solid A 317 553 1.82 No B 933 2740 2.7 Yes C 1683 2628 2.32 No D 387 457 4.93 No .......................................... Q35: Which of the following chlorides is likely to have least ionic character? a) b) c) d) LiCl CsCl BeCl2 CaCl2 .......................................... Q36: Which type of structure is found in a substance melting at 771 K which conducts electricity when molten, but not when solid? a) b) c) d) Covalent (network structure) Covalent (discrete molecules) Ionic Metallic .......................................... Q37: Identify the covalent network substance. a) b) c) d) e) f) NH4 Cl (s) CH3 OH (l) C6 H14 (l) SO2 (g) Na2 CO3 (s) SiO2 (s) .......................................... Q38: Identify the two substances which are ionic. a) NH4 Cl (s) b) CH3 OH (l) c) C6 H14 (l) d) SO2 (g) e) Na2 CO3 (s) f) SiO2 (s) © H ERIOT-WATT U NIVERSITY 156 TOPIC 7. BONDING CONTINUUM AND POLAR COVALENT BONDING .......................................... Q39: Identify the covalent network substance. a) b) c) d) e) f) NH4 Cl (s) CH3 OH (l) C6 H14 (l) SO2 (g) Na2 CO3 (s) SiO2 (s) .......................................... © H ERIOT-WATT U NIVERSITY 157 Topic 8 Intermolecular forces Contents 8.1 8.2 8.3 8.4 8.5 Prior knowledge . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . London dispersion forces . . . . . . . . . . . . . . . . . Hydrogen bonding . . . . . . . . . . . . . . . . . . . . . Relating properties to bonding . . . . . . . . . . . . . . 8.5.1 Boiling point . . . . . . . . . . . . . . . . . . . . 8.5.2 Density . . . . . . . . . . . . . . . . . . . . . . . 8.6 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Predicting solubilities from solute and solvent polarities 8.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Resources . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 160 162 166 169 169 170 172 174 178 179 180 Prerequisite knowledge Before you begin this topic, you should already know that: • the first 20 elements in the Periodic Table can be categorised according to bonding and structure: ◦ metallic Li, Be, Na, Mg, Al, K, Ca; ◦ covalent molecular H2 , N2 , O2 , F2 , Cl2 , P4 , S8 and fullerenes (eg C 60 ); ◦ covalent network B, C (diamond, graphite), S i ; ◦ monatomic (noble gases). • electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond (Higher, Unit 1); • electronegativity values increase across a Period and decrease down a Group (Higher, Unit 1); • this can be explained in terms of atomic size, nuclear charge and the screening effect due to inner shell electrons (Higher, Unit 1); • in a covalent bond, atoms share pairs of electrons (National 4, Unit 1); 158 TOPIC 8. INTERMOLECULAR FORCES • the covalent bond is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons (National 4, Unit 1); • polar covalent bonds are formed when the attraction of the atoms for the pair of bonding electrons is different (Higher, Unit 1); • delta positive and delta negative notation can be used to indicate the partial charges on atoms, which give rise to a dipole (Higher, Unit 1); • pure covalent bonding and ionic bonding can be considered as being at opposite ends of a bonding continuum with polar covalent bonding lying between these two extremes (Higher, Unit 1); • the larger the difference in electronegativities between bonded atoms, the more polar the bond will be (Higher, Unit 1). Learning objectives By the end of this topic, you should be able to: • state that all molecular elements and compounds and monatomic elements condense and freeze at sufficiently low temperatures - for this to occur, some attractive forces must exist between the molecules or discrete atoms; • state that any ‘intermolecular’ forces acting between molecules are known as van der Waals’ forces; • state that there are several different types of van der Waals’ forces such as London dispersion forces and permanent dipole-permanent dipole interactions, which include hydrogen bonding; • state that London dispersion forces are forces of attraction that can operate between all atoms and molecules; • state that these forces are much weaker than all other types of bonding; • explain that London dispersion forces are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles, caused by movement of electrons in atoms and molecules; • state that the strength of London dispersion forces is related to the number of electrons within an atom or molecule; • state that a molecule is described as polar if it has a permanent dipole - the spatial arrangement of polar covalent bonds can result in a molecule being polar; • state that permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction between polar molecules; • state that permanent dipole-permanent dipole interactions are stronger than London dispersion forces for molecules with similar numbers of electrons; • state that bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such fluorine, oxygen or nitrogen are highly polar; © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES • state that hydrogen bonds are electrostatic forces of attraction between molecules which contain these highly polar bonds; • state that a hydrogen bond is stronger than other forms of permanent dipolepermanent dipole interaction, but weaker than a covalent bond; • explain that melting points, boiling points and viscosity can all be rationalised in terms of the nature and strength of the intermolecular forces which exist between molecules; • explain that, by considering the polarity and number of electrons present in molecules, it is possible to make qualitative predictions of the strength of the intermolecular forces; • state that the melting and boiling points of polar substances are higher than the melting and boiling points of non-polar substances with similar numbers of electrons; • state that the anomalous boiling points of ammonia, water and hydrogen fluoride are a result of hydrogen bonding; • explain that boiling points, melting points, viscosity and solubility/miscibility in water are properties of substances which are affected by hydrogen bonding; • explain that hydrogen bonding between molecules in ice results in an expanded structure which causes the density of ice to be less than that of water at low temperatures. © H ERIOT-WATT U NIVERSITY 159 160 TOPIC 8. INTERMOLECULAR FORCES 8.1 Prior knowledge Test your prior knowledge Go online Q1: Which of these is a measure of the ability of an atom in a bond to attract the bonding electrons? a) b) c) d) Ionisation Energy Periodicity Electronegativity Polarity .......................................... Q2: Which of the following elements has the greatest attraction for the shared pair of electrons in a bond? a) b) c) d) Fluorine Nitrogen Phosphorus Lithium .......................................... Q3: a) b) c) d) As you move across the Periodic Table, the electronegativity: stays the same. decreases. increases. decreases then increases. .......................................... 8.2 Introduction Simple molecules like nitrogen N 2 , methane CH4 and water H2 O, have their atoms held together by covalent bonds within the molecule. Bonds within a molecule are called intramolecular forces (intra: on the inside, as in intramuscular, intravenous). Nitrogen and methane are both gases at room temperature, but can be made liquid by cooling to a very low temperature. Water is a liquid at room temperature and other covalent molecules like candle wax are solid. Forces of attraction must exist between all these covalent molecules or it would never be possible to have covalent molecular solids at all. The molecules would fly apart and all discrete covalent molecules would be gases even at the lowest temperatures possible. The forces between molecules are called intermolecular forces (inter: between, as in inter-city, international). © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES Figure 8.1 shows both intermolecular and intramolecular forces in a sample of water. Figure 8.1 .......................................... The way that the charge is distributed in a water molecule is not symmetrical. There is more negative charge on one side than the other. It is said to have a dipole. The Greek letter delta, δ, is used to show a 'small amount'. Van der Waals' forces Johannes Diderik van der Waals recognised that relatively weak forces were responsible for the change of state from gas to liquid. He was awarded the Nobel prize for his work in 1910. There are several different forms of these forces and these will be discussed in the following sections. • London dispersion forces. • Permanent dipole-permanent dipole interactions. • Hydrogen bonding. In this topic the forces of attraction caused by dipoles will be explored. © H ERIOT-WATT U NIVERSITY 161 162 TOPIC 8. INTERMOLECULAR FORCES 8.3 London dispersion forces The strength of the intermolecular forces between covalent substances can be estimated by looking at the boiling points. The lower the boiling point, the weaker the forces must be, since it is the intermolecular forces which have to be overcome to change the liquid into gas. Steam Water Heat energy added Liquid to gas The covalent diatomic elements like hydrogen, H 2 gas, and bromine, Br 2 , which is a liquid at room temperature, are completely non polar and would seem to have no dipole at all. As well as these, even monatomic gases like helium and neon can be liquified. How can intermolecular forces exist in these substances? The work of Fritz London (in his 1930 paper) led to suggestions that an atom or molecule could have a temporary dipole at a particular instant in time and that if there were other atoms or molecules nearby; this temporary dipole might affect them and induce a dipole in the nearby atom or molecule. The resultant electrostatic attraction between the temporary dipole and the induced dipole might, although very weak, be able to hold the substance together. These forces of attraction are known as London dispersion forces. © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES 163 Induced dipoles Figure 8.2 shows how the temporary dipole in a helium atom can exist at a single brief instant in time. This can induce a dipole in its neighbour. Hydrogen molecules are shown forming the same temporary dipole-induced dipole pair. The attractions between the dipole pairs form the same type of bonds; London dispersion forces. Figure 8.2: Temporary dipoles and induced dipoles .......................................... Q4: When the electron movement is paused, as summarised in Figure 8.2, the electron distribution is: a) evenly spread. b) unevenly spread. .......................................... Q5: This distribution of electrons causes: a) a permanent dipole. b) a temporary dipole. .......................................... Q6: What effect does this have on the neighbouring particle? a) b) c) d) The permanent dipole causes an induced dipole. The temporary dipole causes an induced dipole. The permanent dipole induces a temporary dipole. The temporary dipole induces a permanent dipole. .......................................... Q7: The London dispersion forces of attraction formed can be said to be: a) intramolecular. b) intermolecular. .......................................... © H ERIOT-WATT U NIVERSITY Go online 164 TOPIC 8. INTERMOLECULAR FORCES .......................................... London dispersion forces of attraction can operate between all atoms and molecules. They are much weaker than other types of bonding, at around 2 kJ mol -1 compared to covalent bonds of between 150-500 kJ mol -1 . In many substances, London dispersion forces are not large enough to influence the behaviour of substances. If they are the only type of intermolecular force present, however, as in the noble gases, they must be considered. One example of this relates the London dispersion forces to the size of atoms or molecules. This diagram shows the relative sizes of atoms and boiling points ( ◦ C) of the first four noble gases. Noble gas boiling points Q8: Name the noble gas shown which has the largest atom. .......................................... Q9: Which noble gas shown has the highest boiling point? .......................................... Q10: Which noble gas shown must have the strongest London dispersion forces holding the atoms together? .......................................... Q11: Look at the relationship between the London dispersion forces and size. Which of these statements is true? a) b) c) d) The size of the atom doesn't affect the London dispersion forces. The larger the atom the weaker the London dispersion forces. The larger the atom the stronger the London dispersion forces. The smaller the atom the stronger the London dispersion forces. .......................................... The lowest temperature it is possible to reach is called Absolute Zero. This occurs at -273◦ C. Solid helium is changed into liquid helium at approximately -272 ◦ C and liquid helium to gas at -269◦ C, both by breaking London dispersion forces of attraction. Q12: What does this suggest about the strength of London dispersion forces? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES 165 Molecular substances which have only London dispersion forces between the molecules, i.e. intermolecular, also show a relationship between size and strength of the London dispersion forces. The following table shows the boiling points of the first four alkane molecules. Name Formula Boiling point (◦ C) Methane CH4 -164 Ethane C2 H6 -89 Propane C3 H8 -42 Butane C4 H10 -1 Alkane boiling points Q13: Describe the relationship between size and boiling point and explain why it occurs. (There would be about three or four marks allocated to this type of 'describe' and 'explain' question and these require practice. If you are in any way unsure about being able to explain your answer fully, work through the next four questions before attempting the complete answer in the repeated question. If you feel confident, check your answer.) .......................................... Q14: Name the alkane shown which has the largest molecules. .......................................... Q15: Name the alkane shown which has the highest boiling point. .......................................... Q16: Name the alkane shown which must have the strongest London dispersion forces. .......................................... Q17: Which relationship between molecule size and London dispersion forces is true? a) b) c) d) The size of the molecule doesn't affect the London dispersion forces. The larger the molecule the weaker the London dispersion forces. The larger the molecule the stronger the London dispersion forces. The smaller the molecule the stronger the London dispersion forces. .......................................... Key point London dispersion forces of attraction operate between all atoms and molecules and are weaker than all other types of bonding. The strength of London dispersion forces is related to the size of the atoms or molecules. © H ERIOT-WATT U NIVERSITY 166 TOPIC 8. INTERMOLECULAR FORCES 8.4 Hydrogen bonding Hydrogen bonding is the name given to an intermolecular force which is actually a type of permanent dipole-permanent dipole attraction. These hydrogen bonds are separated from other examples of this type of interaction because they are unusually strong (about 15-25 kJ mol -1 ) as shown below. For hydrogen bonding to occur, the hydrogen atom involved needs to: • be the positive end of a strong dipole (estimated from the difference in electronegativity). • have a small, highly electronegative atom on a neighbouring molecule. Only the three elements fluorine, oxygen and nitrogen are considered able to satisfy these conditions. Hydrogen bonding elements Notice that the bonds called hydrogen bonds are the forces of electrostatic attraction between the molecules which contain these highly polar bonds. i.e. the hydrogen bonds are the intermolecular forces. Key point Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are highly polar and the electrostatic attractions between molecules which contain these highly polar bonds are called hydrogen bonds. © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES 167 Strength of hydrogen bonds This activity shows the formation of hydrogen bonds in a sample of water, and the following diagram shows the boiling points and relative sizes of the first four hydrides of the Group 6 elements. Strength of hydrogen bonds A comparison of the strength of hydrogen bonds with other permanent dipole-permanent dipole interactions and London dispersion forces can be made by considering the data in the following diagram. Boiling points of the Group 6 hydrides Remember that London dispersion forces of attraction will operate between all atoms and molecules, and that some of the hydride molecules will have permanent dipole-permanent dipole attractions since they are polar molecules. Q18: Use the boiling point data to compare the intermolecular forces present in water with those present in the other Group 6 hydrides and explain fully how hydrogen bonds compare in strength to others present. (There would be about three or four marks allocated to this type of 'describe' and 'explain' question and these require practice. If you are in any way unsure about being able to explain your answer fully, work through the next four questions before attempting the complete answer in the repeated question. If you feel confident, check your answer.) © H ERIOT-WATT U NIVERSITY Go online 168 TOPIC 8. INTERMOLECULAR FORCES .......................................... Q19: Name the type of weak intermolecular force of attraction which increases as size increases. .......................................... Q20: Name the substance which shows boiling point evidence which goes against this trend. .......................................... Q21: Name the substance which has the strongest intermolecular force of attraction holding the molecules together. .......................................... Q22: How do the hydrogen bonds in water compare in strength to the London dispersion forces present and the other permanent dipole-permanent dipole interactions present in some of the other molecules? a) The hydrogen bonds are weaker than London dispersion forces and the other permanent dipole-permanent dipole interactions. b) The hydrogen bonds are stronger than London dispersion forces and the other permanent dipole-permanent dipole interactions. c) The hydrogen bonds are stronger than London dispersion forces but weaker than the other permanent dipole-permanent dipole interactions. .......................................... A comparison of the strength of hydrogen bonds with intramolecular bonds like the covalent bond between hydrogen and oxygen can be made by remembering that the hydrogen bond has a value of about 15-25 kJ mol -1 and the strength of the covalent O-H bond can be found in the data booklet (page 9). The table of 'Mean Bond Enthalpies' gives the value. Q23: Which statement is true about hydrogen bonds in comparison to covalent bonds? a) b) c) d) Covalent bonds are much stronger than hydrogen bonds. Hydrogen bonds are much stronger than covalent bonds. Covalent bonds are the same strength as hydrogen bonds. Hydrogen bonds are twice as strong as covalent bonds. .......................................... Key point A hydrogen bond is stronger than other forms of permanent dipole-dipole interaction but weaker than a covalent bond. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES 8.5 Relating properties to bonding Intermolecular bonds are the relatively weak bonds which attract molecules to each other. They are not as strong as the bonds, for example covalent bonds, that bind the atoms together into a molecule. Q24: Which of the following properties of a substance are a reflection of the intermolecular forces between the molecules of the substance? • Melting point • Boiling point • Density • Viscosity .......................................... Water is extremely important for life on Earth; in fact, the search for extraterrestrial life involves looking for water or evidence that water was present at some time. Water is the commonest liquid on Earth and also one of the most unusual! The next sections explore some properties of water from the point of view of hydrogen bonding. 8.5.1 Boiling point The boiling points of the hydrides along the Periods for Groups 4 to 7 elements are: Boiling points of several hydrides © H ERIOT-WATT U NIVERSITY 169 170 TOPIC 8. INTERMOLECULAR FORCES Hydrides of non-metals Go online Q25: The trend in boiling points in Group 4 from CH 4 to SnH4 shows a regular increase with formula mass. Is there any evidence of polar attractions between molecules? a) Yes b) No .......................................... Q26: Three of the second period hydrides have higher boiling points than expected by comparison with the trends seen for other elements. (Remember that higher formula mass usually means higher boiling point.) Which are the three hydrides? .......................................... Q27: What is the main force between molecules in group 4 hydrides, from CH 4 to SnH4 ? a) b) c) d) e) Polar-polar attractions Van der Waals attractions Covalent bonds Ionic Hydrogen bonds .......................................... Key point The higher-than-expected boiling points of ammonia, water and hydrogen fluoride result from additional intermolecular forces, in this case, hydrogen bonds. 8.5.2 Density Most substances contract when they are cooled and solids are normally denser than their own liquids. This causes the majority of solids to sink when placed in their own liquid. The structure of ice, however, is very unusual because the solid is less dense then the liquid water. As water freezes, the intermolecular hydrogen bonding spreads out the water molecules into a strong 'open' structure with large spaces in it (see the following diagram). This makes ice less dense and able to float on water. © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES The arrangement of water molecules in ice maximises the hydrogen bonding between them and leads to an open structure. As it melts the structure collapses into the spaces and the liquid becomes less dense. The fact that ice floats on water is good news for fish living in ponds. The ice forms on the surface and the fish and other aquatic life can exist below this layer. The layer of ice thermally insulates the water beneath. A disadvantage of water expanding as it forms ice is that water trapped in pipes over a cold spell causes the pipes to crack open as it freezes, resulting in leaks when it thaws. Key point Hydrogen bonding between molecules in ice results in an expanded structure which causes the density of ice to be less than that of water at low temperatures. © H ERIOT-WATT U NIVERSITY 171 172 TOPIC 8. INTERMOLECULAR FORCES 8.6 Viscosity The stronger the intermolecular forces are between molecules in a liquid, the more viscous the liquid. The viscosity is a measure of how easily it flows (how thick or syrupy a liquid is). The molecules have to be able to move past each other to flow. Testing viscosity Figure 8.3: Testing the viscosity of three different liquids Go online .......................................... Q28: In which liquid does the marble fall fastest? .......................................... Q29: Which chemical structure has the least hydrogen bonding? .......................................... Q30: Which chemical structure has the most hydrogen bonding? .......................................... Q31: Given that the marble in water took 2 seconds to reach the bottom, calculate its rate of descent in m s-1 . .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES Q32: Predict the rate of descent of a marble in an identical experiment using ethoxyethane. a) b) c) d) 0.8 m s-1 0.6 m s-1 0.4 m s-1 0.1 m s-1 .......................................... Q33: Assuming your answer to the last question is correct, how long would it take a marble to travel through the ethoxyethane? .......................................... .......................................... Viscosity is a term which applies to fluids (both liquids and gases), and since these are covalent substances, being able to move past each other involves breaking the London dispersion forces, dipole-dipole interactions or hydrogen bonds. Of these, the hydrogen bonds are the strongest. Key point The viscosity of a liquid is related to the strength of the intermolecular bonding. © H ERIOT-WATT U NIVERSITY 173 174 TOPIC 8. INTERMOLECULAR FORCES 8.7 Predicting solubilities from solute and solvent polarities When you add a spoon of sugar to your cup of coffee and stir, the sugar dissolves. In fact, sugar will dissolve in water to a considerable extent to produce treacle or 'golden syrup'. However, if you added a spoon of sugar to a cup of petrol, no amount of stirring would make it dissolve. A polar solvent is one whose molecules exhibit strong permanent dipoles. Because of this, polar solvents like water can generally dissolve polar substances, like sucrose, and ionic solids, for example sodium chloride. The expression often used to describe this is: Key point Like dissolves like. In the same way, because the intermolecular attractions are of a similar type, non-polar solvents are more likely to dissolve non-polar substances: like dissolves like. For example, wax dissolved in white spirit in a furniture polish. If, however, a mixture of polar and non-polar substances is used, no dissolving occurs. Opposites do not dissolve. Polar and ionic substances tend to be insoluble in non-polar solvents. Non-polar substances tend to be insoluble in polar solvents. The attractions are not sufficiently strong to allow dissolving. When an ionic compound dissolves in water, the ions need to be separated from the lattice. The polar water molecules can sometimes pull the ions into solution and surround them. © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES Dissolving solid sodium chloride The way the water molecules direct themselves depends on the charge the ion carries. The ions are said to be hydrated. Dissolving of sodium chloride can be shown in an equation. Dissolving sodium chloride © H ERIOT-WATT U NIVERSITY 175 176 TOPIC 8. INTERMOLECULAR FORCES A polar molecule, like hydrogen chloride, dissolves in water as the water 'interacts' with the molecules. Hydrogen chloride dissolving Hydrogen chloride dissolving can also be shown in an equation. Hydrogen chloride dissolving (equation). In a similar way, a polar substance, such as sucrose (C12 H22 O11 ), containing several -OH groups, can bond with polar water molecules. Q34: Ammonia (NH 3 ) is a gas which is very soluble in water, whereas nitrogen (N 2 ) is almost insoluble. Can you explain why? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES Key point Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as water and insoluble in non-polar solvents. Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents. Miscibility When two liquids mix thoroughly with no visible boundary between them, they are said to be miscible, e.g. water and methanol. Both of these molecules have hydrogen bonds shown in Figure 8.4 (a). Miscibility arises when the intermolecular attractions between two types of substances are fairly similar. They are then able to mix easily. If one substance has different intermolecular attractions from the other, its molecules stay grouped around each other and form a separate layer, e.g. water (which is hydrogen bonded), and tetrachloromethane (which is non-polar) shown in Figure 8.4 (b). Figure 8.4: Miscible and immiscible liquids .......................................... Whether two liquids are miscible or immiscible can only be determined by experiment, but the general rule of like dissolves like can be useful in trying to make a rough prediction. © H ERIOT-WATT U NIVERSITY 177 178 TOPIC 8. INTERMOLECULAR FORCES 8.8 Summary Summary • All molecular elements, compounds and monatomic elements condense and freeze at sufficiently low temperatures. For this to occur, some attractive forces must exist between the molecules or discrete atoms. • Intermolecular forces acting between molecules are known as van der Waals' forces. • There are several different types of van der Waals' forces such as London dispersion forces and permanent dipole-permanent dipole interactions, which include hydrogen bonding. • London dispersion forces are forces of attraction that can operate between all atoms and molecules. • These forces are much weaker than all other types of bonding. • They are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules. • The strength of London dispersion forces is related to the number of electrons within an atom or molecule. • Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element, such fluorine, oxygen or nitrogen, are highly polar. • Hydrogen bonds are electrostatic forces of attraction between molecules which contain these highly polar bonds. • A hydrogen bond is stronger than other forms of permanent dipolepermanent dipole interaction, but weaker than a covalent bond. • Melting points, boiling points and viscosity can all be rationalised in terms of the nature and strength of the intermolecular forces which exist between molecules. • By considering the polarity and number of electrons present in molecules, it is possible to make qualitative predictions of the strength of the intermolecular forces. • The melting and boiling points of polar substances are higher than the melting and boiling points of non-polar substances with similar numbers of electrons. • The anomalous boiling points of ammonia, water and hydrogen fluoride are a result of hydrogen bonding. • Boiling points, melting points, viscosity and solubility/miscibility in water are properties of substances which are affected by hydrogen bonding. © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES Summary continued 8.9 • • Hydrogen bonding between molecules in ice results in an expanded structure which causes the density of ice to be less than that of water at low temperatures. • Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as water and insoluble in non-polar solvents. • Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents. Resources Higher Chemistry for CfE: J Anderson, E Allan and J Harris, Hodder Gibson, ISBN 978-1444167528 © H ERIOT-WATT U NIVERSITY 179 180 TOPIC 8. INTERMOLECULAR FORCES 8.10 End of topic test End of Topic 8 test Q35: London dispersion forces of attraction are a result of: Go online a) b) c) d) an induced dipole causing an temporary dipole. a permanent dipole causing an permanent dipole. an induced dipole causing an permanent dipole. a temporary dipole causing an induced dipole. .......................................... Q36: Which of these is the strongest? a) b) c) d) Hydrogen bonds Covalent bonds London dispersion forces Permanent dipole-permanent dipole interactions .......................................... Q37: What is the connection between the size of molecules and the London dispersion forces of attraction? a) b) c) d) The size makes no difference. The smaller the molecule the weaker the force. The smaller the molecule the stronger the force. The weakest force is between the largest molecules. .......................................... Q38: Which of these molecules has the largest permanent dipole? a) b) c) d) HF HCl HBr HI .......................................... Q39: Identify the trends which would occur in descending the elements of the halogen group. a) The electronegativity increases. b) The London dispersion forces become stronger. c) The boiling point decreases. d) The covalent radius decreases. e) The relative atomic mass increases. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 8. INTERMOLECULAR FORCES 181 Q40: Look at the following table which shows the boiling points of some substances. Substance Boiling Point (K) Bonds (or forces) broken at the boiling point Sodium 610 Metallic Neon 27 A Water 373 B Which word(s) should be inserted at A? .......................................... Q41: Which word(s) should be inserted at B? .......................................... Q42: Identify the substance which has neither covalent or metallic bonding. a) b) c) d) e) f) NH3 (l) CCl4 (l) He (g) Hg (l) H2 (g) HBr (l) .......................................... Q43: Identify the substance which has hydrogen bonding between the molecules. a) b) c) d) e) f) NH3 (l) CCl4 (l) He (g) Hg (l) H2 (g) HBr (l) .......................................... Q44: Identify the substances which are polar molecules. a) NH3 (l) b) CCl4 (l) c) He (g) d) Hg (l) e) H2 (g) f) HBr (l) .......................................... © H ERIOT-WATT U NIVERSITY 182 TOPIC 8. INTERMOLECULAR FORCES Q45: Identify the substance which has polar bonds but is a non-polar molecule. a) b) c) d) e) f) NH3 (l) CCl4 (l) He (g) Hg (l) H2 (g) HBr (l) .......................................... Q46: The following diagram shows the structure of kevlar, which is strong because of the intermolecular bonding between neighbouring molecules. Name the type of intermolecular bonding involved. .......................................... Q47: Which area of the diagram shows the position of one such bond? .......................................... © H ERIOT-WATT U NIVERSITY 183 Topic 9 End of unit test 184 TOPIC 9. END OF UNIT TEST End of Unit 1 test Q1: Go online Ethanol is formed industrially by of ethene. a) increases. b) decreases. .......................................... Q2: a) b) c) d) When the enthalpy change has a positive sign, the reaction is: exothermic. endothermic. large. small. .......................................... Q3: Ionic bonding is most likely when the electronegativity difference between the elements is: a) b) c) d) exothermic. endothermic. large. small. .......................................... Q4: a) b) c) d) Which of the following exist as diatomic molecules? Ethane Potassium bromide Carbon monoxide Neon .......................................... Q5: a) b) c) d) When an atom X of an element in Group 2 reacts to become X 2+ : the mass number of X increases. the charge on the nucleus increases. the atomic number of X decreases. the number of occupied energy levels decreases. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 9. END OF UNIT TEST 185 Q6: The same reaction was carried out at four different temperatures. The following table shows the time taken for the reaction to occur. Temp (◦ C) 20 30 40 50 Time (seconds) 60 30 14 5 The results show that: a) b) c) d) the rate of the reaction is directly proportional to the temperature. a small rise in temperature results in a large increase in reaction rate. the reaction is endothermic. the activation energy increases with increasing temperature. .......................................... The potential energy diagram below refers to the reversible reaction involving reactants R and products P. Q7: What is the value of the activation energy for the forward reaction (reactants to products)? a) b) c) d) 10 kJ mol-1 20 kJ mol-1 40 kJ mol-1 60 kJ mol-1 .......................................... Q8: What is the enthalpy change for the forward reaction (reactants to products)? a) b) c) d) -30 kJ mol-1 -20 kJ mol-1 +20 kJ mol-1 +30 kJ mol-1 .......................................... © H ERIOT-WATT U NIVERSITY 186 TOPIC 9. END OF UNIT TEST Q9: a) b) c) Which of the following molecules may be described as polar? C1 Be H C1 C1 d) C1 C1 C1 C C1 C1 C1 .......................................... Q10: The melting points of Group 7 elements increase on descending the group increase: because the a) b) c) d) mean bond energies nuclear charges covalent bond lengths London dispersion forces .......................................... Q11: The difference between the covalent radii of lithium and carbon is mainly due to the difference in the: a) b) c) d) number of electrons. number of protons. mass of each atom. number of neurons. .......................................... Q12: In general, covalent substances have lower melting points than ionic substances because: a) b) c) d) covalent bonds have no electrostatic attractive forces. bonds between molecules are weaker than bonds between ions. covalent compounds are composed of non-metals which have low melting points. ionic bonds are stronger than covalent bonds. .......................................... Q13: Carbon dioxide is a gas at room temperature while silicon dioxide is a solid because: a) London dispersion forces are much weaker than covalent bonds. b) carbon dioxide contains double covalent bonds and silicon dioxide contains single covalent bonds. c) carbon-oxygen bonds are less polar than silicon-oxygen bonds. d) the relative formula mass of carbon dioxide is less than that of silicon dioxide. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 9. END OF UNIT TEST 187 The apparatus shown can be used to prepare iron(III) chloride. Q14: Which substance in the diagram contains metallic bonding? .......................................... Q15: By considering the method of production, predict the type of bonding in iron(III) chloride. .......................................... Q16: Which of the following describes the bonding in the chlorides across Period 3 from NaCl to SCl2 ? a) b) c) d) Ionic ⇒ polar covalent Ionic ⇒ polar covalent ⇒ pure covalent Polar covalent ⇒ ionic Ionic ⇒ pure covalent ⇒ polar covalent .......................................... Q17: The elements in the third row of the Periodic Table are shown below. Na Mg Al Si P S Cl Ar Why does the atomic size decrease crossing the period from sodium to argon? .......................................... Q18: Atoms of different elements have different attractions for bonded electrons. What term is used as a measure of these differing attractions? .......................................... Q19: Use a table of these values from the SQA data booklet to explain why nitrogen chloride contains pure covalent bonds. .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 9. END OF UNIT TEST Q20: Atoms of different elements have different ionisation energies. Explain clearly why the first ionisation energy of sodium is less than the first ionisation energy of lithium. .......................................... Q21: The ability of an atom to form a negative ion is measured by its electron affinity. The electron affinity is defined as the energy change when one mole of gaseous atoms of an element combines with one mole of electrons to form gaseous negative ions. Write the equation, showing state symbols, that represents the electron affinity of chlorine. .......................................... Graph A shows the volume of hydrogen gas produced against time when an excess of magnesium is added to 50 cm3 of hydrochloric acid of concentration 1 mol l-1 at 20 ◦ C. Volume of Hydrogen 188 B A Time (s) Graph B was obtained when the reaction was repeated with excess magnesium and hydrochloric acid of the same concentration but at a different temperature. Q22: Which reaction was at the higher temperature? a) A b) B .......................................... Q23: How many cm3 of acid was required to produce graph B? .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 9. END OF UNIT TEST 189 The potential energy diagram below represents the decomposition of hydrogen iodide: 2HI (g) → H2 (g) + I2 (g) Q24: What is the value for the activation energy (E A ) for the above reaction? .......................................... Q25: What would be the enthalpy change (ΔH) for the reaction H 2 (g) + I2 (g) → 2HI (g)? .......................................... Q26: The use of a catalyst would the activation energy in part 1. a) increase b) decrease c) have no effect on .......................................... Q27: The use of a catalyst would the enthalpy change in part 2. a) increase b) decrease c) have no effect on .......................................... © H ERIOT-WATT U NIVERSITY 190 TOPIC 9. END OF UNIT TEST Q28: The enthalpy change for the reaction enthalpy of formation of hydrogen iodide. 1 2 H2 (g) + 12 I2 (s) → HI (g) is known as the It differs from the above equation in two respects: • it involves the formation of one mole of HI; • the reactants are in their standard states, so iodine is a solid. Given that the enthalpy of sublimation of iodine I 2 (s) → I2 (g) has ΔH = +62 kJ, calculate the enthalpy of formation of hydrogen iodide. .......................................... Q29: If the enthalpy of formation of hydrogen chloride is -92 kJ mol -1 , predict a value for the enthalpy of formation of hydrogen bromide. .......................................... Q30: Match the following chemical terms to their correct definitions: Definition Chemical term (randomised) For a chemical reaction to occur, particles must collide. Enthalpy change The minimum kinetic energy required by colliding particles before a reaction will occur. Activated complex Intermediate stage at the top of the energy barrier when reactants change to products. Catalyst The difference in potential energy between products and reactants. Collision theory A substance that alters the rate of a reaction without being used up. Activation energy .......................................... © H ERIOT-WATT U NIVERSITY TOPIC 9. END OF UNIT TEST 191 Q31: Match the following chemical terms to their correct definitions: Chemical term (randomised) Definition Regular arrangement of positively charged ions surrounded by delocalised electrons. Covalent bonding The electrostatic force of attraction of a shared pair of electrons for two positive nuclei. Metallic bonding The electrostatic attraction between negative and positive ions. Intramolecular forces Dipole Weak bonds between molecules. A slight negative or positive charge caused by the uneven distribution of electrons. Intermolecular forces Bonds holding atoms together within molecules. Ionic bonding .......................................... Q32: Match the following chemical terms to their correct definitions: Chemical term (randomised) Definition Half the distance between the nuclei of two covalently bonded atoms of an element. Ionisation energy The strength of the attraction of an element for the electrons of its bonding electrons. Covalent radius The energy required to remove an electron from a gaseous atom to form an ion with a single positive charge. Electronegativity The temperature at which a substance changes from a solid to a liquid. Boiling point The temperature at which a substance changes from a liquid to a gas. Melting point .......................................... Q33: Sort the forces of attraction into order of strength from strongest to weakest: • Hydrogen bonding • Permanent dipole - permanent dipole interactions • London dispersion forces • Covalent bonding .......................................... © H ERIOT-WATT U NIVERSITY 192 TOPIC 9. END OF UNIT TEST Q34: Complete each statement using 'increases' or 'decreases' as appropriate. • Going across a period, atomic size . . . increases / decreases. • Going down a group, atomic size . . . increases / decreases. • Going down a group, electronegativity . . . increases / decreases. • Going across a period, electronegativity . . . increases / decreases. • Going across a period, boiling point . . . increases / decreases. • Going down a group, boiling point . . . increases / decreases. .......................................... A student writes the following two statements. Both are incorrect. In each case, explain the mistake in the student's reasoning. Q35: All ionic compounds are solids at room temperature. Many covalent compounds are gases at room temperature. This proves that ionic bonds are stronger than covalent bonds. .......................................... Q36: The formula for magnesium chloride is MgCl2 because, in solid magnesium chloride, each magnesium ion is bonded to chloride ions. .......................................... Q37: Although propane and ethanol have similar molecular masses, the alkane is a gas at room temperature while the alcohol is a liquid. Explain why propane is a gas at room temperature whereas ethanol is a liquid. (3 marks) .......................................... .......................................... © H ERIOT-WATT U NIVERSITY GLOSSARY Glossary Activated complex a very unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a chemical reaction Activation energy the minimum kinetic energy required by colliding particles before reaction will occur, since a high energy activated complex must be formed Adsorption occurs when molecules become bonded to the surface of a catalyst Allotropes one of two or more existing forms of an element, for example, graphite and diamond are allotropes of carbon Bonding electrons shared pairs of electrons from both atoms forming the covalent bond Chemical bonding describes the mechanism by which atoms are held together Chemical structure describes the way in which atoms, ions or molecules are arranged Collision theory suggests that particles must collide for a chemical reaction to occur Covalent bond formed when two atoms share electrons in their outer shell to complete the filling of that shell Covalent radius half the distance between the nuclei of two bonded atoms of an element Delocalised in metallic bonding, delocalised electrons are free from attachment to any one metal ion and are shared amongst the entire structure Desorption occurs when the bonds between the molecules and the surface break and the molecules leave the surface of the catalyst Diatomic molecules with only two atoms (e.g. oxygen, O 2 , and carbon monoxide, CO) Dipole an atom or molecule in which a concentration of positive charges is separated from a concentration of negative charge © H ERIOT-WATT U NIVERSITY 193 194 GLOSSARY Electronegativity a measure of the attraction that an atom involved in a bond has for the electrons of the bond Enthalpy change the name given to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. It is given the symbol ΔH, read as "delta H" Fullerenes molecules of pure carbon constructed from 5- and 6-membered rings combined into hollow structures; the most stable contains 60 carbon atoms in a shape resembling a football Hydrogen bonds electrostatic forces of attraction between molecules containing a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen, and a highly electronegative atom on a neighbouring molecule Intermolecular forces forces of attraction which exist between molecules; they are weaker than chemical bonds Intramolecular forces forces of attraction which exist within a molecule Ionisation energy the energy required to remove one mole of electrons from one mole of atoms in the gaseous state Isoelectronic molecules which have the same arrangement of electrons. For example, the noble gas neon, a sodium ion (Na + ) and a magnesium ion (Mg 2+ ) are isoelectronic Lattice a regular 3D arrangement of particles in space; the term is applied to metal ions in a solid, and to positive and negative ions in an ionic solid London dispersion forces the forces of attraction which result from the electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules Lone pairs pairs of electrons in the outer shell of an atom which take no part in bonding Miscible fluids which mix with or dissolve in each other in all proportions Periodicity the regular recurrence of similar properties when the elements are arranged in order of increasing atomic number © H ERIOT-WATT U NIVERSITY GLOSSARY Polar covalent bond a covalent bond between atoms of different electronegativity, which results in an uneven distribution of electrons and a partial charge along the bond Potential energy diagram shows the enthalpy of reactants and products, and the enthalpy change during a chemical reaction Properties physical and chemical characteristics of a substance; these are often a reflection of the chemical bonding and structure of the material Thermochemical equation states the enthalpy change for the reaction defined, with reactants and products in the states shown Viscosity the resistance to flow that is exhibited by all liquids © H ERIOT-WATT U NIVERSITY 195 196 ANSWERS: TOPIC 1 Answers to questions and activities 1 Reaction rates - collision theory Test your prior knowledge (page 2) Q1: • Concentration; • particle size / surface area; • temperature. Q2: Any two from: • mass of products; • mass of reactants; • mass of gas given off; • concentration of reactants; • concentration of products; • volume of reactants; • volume of products; • pressure; • conductivity; • pH; • colour Intensity. Q3: 0.01 g s-1 (to 2 decimal places) Working: Change in mass Rate = Change in time 0.7 = 60 = 0.01 g s - 1 Measuring rate (page 5) Q4: 7.0 cm3 Q5: 1.4 cm3 s-1 Q6: 0.7 cm3 s-1 Q7: The rate of reaction is dropping as it progresses, being relatively fast at the start and slowing towards the end. This is because the reactants are being used up. Q8: 0.52 cm3 s-1 © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 1 Collisions and concentration (page 7) Q9: 3 Q10: 0.3 Q11: 0.6 Q12: The rate would be 1.2 collisions per second. It would seem that doubling the concentration of acid has doubled the rate, and in some reactions this is correct; the rate can be directly proportional to the concentration of a reactant. On the other hand, sometimes doubling the concentration of a reactant only increases rate a little, or even not at all. There is no simple way to predict the relationship in advance and each concentration/rate relationship must be investigated experimentally to determine any relationship. It can be said however that for many reactions increase in concentration increases rate. Collisions and particle size (page 8) Q13: Small Q14: 6 Q15: 0.6 Q16: 0.8 Collisions and temperature (page 9) Q17: 4 Q18: 0.4 Q19: 0.8 Q20: c) It doubles. © H ERIOT-WATT U NIVERSITY 197 198 ANSWERS: TOPIC 1 End of Topic 1 test (page 10) Q21: a) average kinetic energy of all the particles. Q22: a) Time taken for reaction to complete. Q23: a) increasing the concentration of the acid. Q24: 0.2 g s-1 Q25: 0.08 g s-1 Q26: d) Curve E Q27: c) Curve D Q28: a) a small rise in temperature results in a large increase in reaction rate. Q29: a) decreasing the mass of copper carbonate. Q30: a) Q31: 0.25 moles Q32: 0.1 moles Q33: 0.44 mol -1-1 © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 2 2 Reaction rates - reaction profiles Test your prior knowledge (page 19) Q1: Exothermic Q2: Endothermic Q3: • Particles must collide to react (although not all collisions are successful). • Sufficient energy is needed and orientation must be correct. Measuring rate at different concentrations (page 21) Q4: c) 2.0 mol -1 Q5: c) 2.0 mol -1 Q6: 20 seconds Q7: 26 cm3 Q8: • The steepness of the slope for the 1.0 mol -1 is less because the reaction rate is lower (fewer acid particles). • The point at which the 1.0 mol -1 line becomes horizontal takes longer to reach (25 seconds rather than 20 seconds) because the reaction is slower (fewer acid particles). • The final level of the 1.0 mol -1 horizontal line is lower by half because the acid is present in only half the quantity. Q9: 6.5 cm3 Answers from page 23. Q10: 20 cm3 Q11: 10.4 cm3 s-1 Answers from page 26. Q12: a) A Q13: d) D Q14: b) B Q15: d) D © H ERIOT-WATT U NIVERSITY 199 200 ANSWERS: TOPIC 2 Controlling the rate - temperature and kinetic energy (page 27) Q16: c) Not changed Q17: a) Increased Q18: b) C and D Q19: a) Particles with sufficient energy to react. Q20: c) Roughly doubled. End of Topic 2 test (page 31) Q21: b) The activation energy is less for phosphorus than for magnesium. Q22: c) The reaction rate doubles. Q23: d) More particles have energy greater than the activation energy. Q24: a) the minimum kinetic energy required by colliding molecules for a reaction to occur. Q25: b) an intermediate stage at the top of the activation energy barrier. Q26: c) potential energy diagrams. Q27: 0.025 Either curve can be used: Rate = = 0.375 15 Change in concentration Change in time ( 12 mark) ( 12 mark) = 0.025 ( 12 ) mol l-1 s-1 ( 12 mark) Allow ±0.001 only. Two significant figures must be shown. (- 12 otherwise) Q28: At 40◦ C, rate = 0.032 s-1 ( 12 mark) 1 1 Rate ( 2 mark) 1 ( 12 mark) = 0.032 = 31.25 seconds ( 12 mark Time = - units must be given.) Q29: Increasing temperature increases kinetic energy (or speed of movement) of molecules ( 12 mark) The (potential) energy generated in collisions increases (collisions have more force) ( 12 mark) More collisions will result in the activation energy being attained (1 mark) Q30: b) 60◦ C Q31: b) a small rise in temperature results in a large increase in reaction rate. © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 3 3 Catalysis Test your prior knowledge (page 37) Q1: a) average kinetic energy of all the particles; Q2: b) Endothermic Q3: a) The minimum kinetic energy required by colliding molecules for a reaction to occur. Addition of hydrogen to ethene (page 38) Q4: c) adsorption. Q5: b) They get weaker. Q6: None Q7: c) The catalyst speeds up the reaction without taking part. Catalysis by cobalt(II) chloride solution (page 40) Q8: b) Co2+ (aq) ions Q9: c) Co3+ (aq) ions Q10: b) Co2+ (aq) ions Q11: This is a poor definition. It is correct to say that the Co2+ (aq) ions do speed up the reaction, but it is incorrect to say that they do not take part in the reaction. The pink Co2+ (aq) ions are changed into something green (probably Co 3+ (aq) ions) which are later changed back into pink Co 2+ (aq) ions. So the Co2+ (aq) ions can be recovered at the end. Answers from page 42. Q12: d) Cl © H ERIOT-WATT U NIVERSITY 201 202 ANSWERS: TOPIC 3 Industrial uses of catalysts (page 42) Q13: Catalyst Process Type Purpose Haber Iron Solid Production of ammonia Washing powders Protease Enzyme Breakdown of protein stains Catalytic converters Pt, Pd, Rb Solid Reduction of pollution Brewing Zymase Enzyme Production of ethanol Catalytic cracking Al2 O3 Solid Production of alkenes Food processing Rennin Enzyme Production of cheese Ostwald Pt gauze Solid Production of nitric acid Monsanto Rhodium complex Solution Ethanoic acid from methanol and CO Answers from page 45. Q14: c) It will decrease. Q15: c) Positive Answers from page 46. Q16: d) Endothermic Answers from page 47. Q17: a) Positive Q18: © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 3 203 Answers from page 49. Q19: d) ΔH =+283 kJ mol-1 Q20: b) ΔH = Hproducts - Hreactants Q21: c) Exothermic Q22: d) ΔH = -1414 kJ mol-1 Q23: 6835 kJ Q24: +150 kJ Q25: 4NH3 (g) + 5O2 (g) → 4NO(g) + 6H2 O(g) Q26: -906 kJ Q27: -226.5 kJ mol-1 Q28: 1359 kJ Answers from page 52. Q29: b) B Q30: The enthalpy change (ΔH) is given by the expression: ΔH(reaction) = ΔH(products) - ΔH(reactants) From the graph: Enthalpy of reactants = −1200 kJ Enthalpy of products = −630 kJ So, ΔH = (−630) − (−1200) kJ = +570 kJ Q31: d) -890 kJ Answers from page 54. Q32: d) +110 kJ Q33: c) +55 kJ Q34: If the activation energy is less, the barrier will be lower and it will be easier for colliding molecules to get over the barrier. There will be more molecules with enough energy to collide successfully and so the reaction rate will increase. In general, the lower the activation energy, the faster the reaction. © H ERIOT-WATT U NIVERSITY 204 ANSWERS: TOPIC 3 Answers from page 55. Q35: a) A Q36: d) D Q37: c) It will increase. How catalysts work (page 56) Q38: b) High activation energy, few successful collisions. Q39: c) Low activation energy, lots of successful collisions. Q40: c) It speeds up the reaction by lowering the activation energy. Q41: b) No effect. Mechanism of catalysis (page 58) Q42: a) X Q43: c) 3 Q44: d) 4 Q45: b) Decreases it Q46: b) W Q47: a) X End of Topic 3 test (page 62) Q48: a) molecules always have energy above the activation energy. Q49: a) the minimum kinetic energy required by colliding molecules for a reaction to occur. Q50: d) © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 3 Q51: e) Q52: c) Q53: a) A very unstable intermediate is formed when bonds within the reactant molecules begin to break and new bonds begin to form. Q54: Exothermic Q55: 10 kJ Q56: 35 kJ Q57: -25 kJ Q58: +25 kJ Q59: a) 1. Energy of reactants / Reactant molecules collide with correct geometry. 2. Energy of activated complex / The collision has enough energy to overcome the Ea . 3. Activation energy of the forward reaction / Activated Complex: bonds in the reactant molecules break and new bonds begin to form. 4. Enthalpy change of the forward reaction b) The product molecule(s) has less energy than the reactants. © H ERIOT-WATT U NIVERSITY 205 206 ANSWERS: TOPIC 4 4 The Periodic Table Test your prior knowledge (page 69) Q1: a) Atomic number Q2: • Bromine • Chlorine • Fluorine • Hydrogen • Iodine • Nitrogen • Oxygen Q3: Particle Charge Mass Location Proton +1 1 Nucleus Neutron 0 1 Nucleus Electron -1 0 Orbiting nucleus or in shells Q4: b) Period Q5: b) Group Newlands' octaves (page 70) Q6: c) Atomic mass Q7: a) Both are gases. Q8: c) Both are reactive metals. Q9: When listed in order of increasing atomic mass, similar properties appear with every eighth element. If the elements are numbered in order, element 1 has properties in common with element 8, element 2 has properties in common with element 9, etc. This is similar to musical notation. Notes in a scale are described as the letters A to G. The note after G is A and so on. A to G is said to be one octave. The same applies if the notes are described as 'doh-ray-me-fah-soh- etc'. Q10: 3 Q11: Chromium, manganese and iron are metals. Newlands' table are non-metals. The elements above them in © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 4 Mendeleev's Periodic Table (page 72) Q12: Mendeleev believed that elements, as yet undiscovered, would fit in these spaces. Q13: Iodine has similar chemical properties to bromine and so fits better in the column which contains bromine. Q14: Noble gases The modern Periodic Table (page 74) Q15: Atomic number Melting and boiling points across a Period (page 75) Q16: Carbon Q17: Argon Q18: Helium Melting and boiling points down a Group (page 76) Q19: Lithium Q20: Caesium Q21: Melting point and boiling point both decrease and so the forces must be getting weaker. Atomic size (page 77) Q22: c) The number of electron shells increases. Q23: d) The number of protons increases. © H ERIOT-WATT U NIVERSITY 207 208 ANSWERS: TOPIC 4 End of Topic 4 test (page 80) Q24: c) Mendeleev left gaps for elements which had not yet been discovered. Q25: c) There is a steady decrease in atomic size across a period from left to right and d) There is a decrease in first ionisation energy on going down Group 0. Q26: a) Atomic number Q27: • Bromine • Chlorine • Fluorine • Hydrogen • Iodine • Nitrogen • Oxygen Q28: b) The alkali metals. Q29: a) The halogens. Q30: c) The noble gases. Q31: a) number of occupied energy shells. Q32: c) The right hand side. © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 5 209 5 Bonding and structure Test your prior knowledge (page 85) Q1: a) a shared pair of electrons. Q2: d) delocalised electrons. Q3: b) transfer of electrons. Q4: a) have high melting and boiling points. Q5: d) conduct electricity when molten or in solution. Answers from page 86. Q6: You have probably thought of gold, nuggets of which are found in rocks and as gold particles in some river beds. Platinum and other platinum group metals (rhodium, iridium, ruthenium and osmium) are also found native. Sulfur is also found as yellow crystals in rocks and around volcanos. Copper is also found in elemental form, as well as in other ores. Gold Sulfur Native copper Answers from page 87. Q7: Niobium is used in steel superalloys which can resist high temperatures and are used in jet engine components and heat shields. Construction of a sodium atom (page 89) Q8: Argon © H ERIOT-WATT U NIVERSITY 210 ANSWERS: TOPIC 5 Q9: Gain an outer electron. Q10: Lose the outer electron. Q11: A positive sodium ion (Na + ) Q12: Neon Metallic bonds (page 90) Q13: The group 1 metals sodium and potassium are easily cut with a knife (which is made from hard metals - iron and chromium). Mercury is liquid (melting point -39 ◦ C) and gallium melts in the hand (at 30 ◦ C). Copper lattice (page 91) Q14: c) delocalised. Q15: d) Lattice Q16: Each aluminium atom will ionise to produce three electrons and a Al 3+ ion. Sodium will produce only one electron (to carry the electric current) from each atom. Answers from page 93. Q17: 4 Q18: 1 Q19: b) share, electron Q20: d) positively charged nuclei and negatively charged shared electrons. Q21: Diamond (page 98) Q22: 4 © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 5 Q23: a) Single covalent. Graphite structure (page 99) Q24: c) stronger. Q25: 6 Fullerenes (page 101) Q26: Diamond and graphite are network covalently bonded, so that it would require a very large input of energy to separate the atoms in a solution. 'Buckyballs' are covalent molecular bonded, with strong bonds binding the C 60 atoms into a molecule, but much weaker bonds between the molecules in the solid. These can be easily broken to form a solution. Ionic bond formation (page 103) Q27: 1 Q28: 7 Q29: 8 Q30: Both ions have a stable noble gas electron configuration. Answers from page 104. Q31: c) large Q32: c) positive caesium ions and negative chloride ions. Q33: c) 3 : 2 Sodium chloride ionic lattice (page 105) Q34: c) An ionic lattice. End of Topic 5 test (page 108) Q35: a) Calcium sulfide Q36: a) Sulfur Q37: b) delocalised electrons. Q38: b) Metallic © H ERIOT-WATT U NIVERSITY 211 212 ANSWERS: TOPIC 5 Q39: a) Covalent and c) Made up of discrete molecules. Q40: a) Boron Q41: d) Phosphorus and f) Sulfur Q42: b) Chlorine and e) Sodium Q43: c) C Q44: a) Ionic Q45: f) SiO2 (s) Q46: a) NH4 Cl (s) and e) Na2 CO3 (s) Q47: Q48: The bonds between the layers of carbon in graphite are comparatively weak, which means that the layers can easily slide over each other providing lubrication. Q49: The carbon atoms in graphite have electrons not involved on the covalent bonds between atoms within the layers. These delocalised electrons permit conduction of electricity. The boron atoms in boron nitride have no free electrons once they have formed three covalent bonds, so there is no delocalised pool of electrons to conduct electricity. Q50: The nitrogen atoms in the layers have two free electrons which can form strong covalent bonds between adjacent layers in boron nitride. Q51: b) Monatomic © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 6 213 6 Periodic Table trends Test your prior knowledge (page 115) Q1: Particle Charge Mass Location Proton +1 1 Nucleus Neutron 0 1 Nucleus Electron -1 0 Orbiting nucleus or in shells Q2: c) number of outer electrons. Q3: a) number of occupied energy shells. Q4: a) Atomic number Q5: a) a shared pair of electrons. Covalent radius - relative sizes (page 116) Q6: b) A decrease Q7: a) An increase Q8: The covalent radius is defined as half the distance between the nuclei of bonded atoms. Noble gases do not form bonds because they are so unreactive. The trends in covalent radius (page 117) Q9: c) The number of electron shells increases. Q10: d) The number of protons increases. © H ERIOT-WATT U NIVERSITY 214 ANSWERS: TOPIC 6 Ionisation energies (page 121) Q11: Q12: Yes Q13: The shape of the first part of the graph (from atomic number 3-10) is repeated for elements 11-18 Q14: Noble gases Q15: Alkali metals Q16: a) A steady increase. Q17: b) A steady decrease. Q18: Metal 1st Ionisation energy / kJ mol-1 Lithium 526 Sodium 502 Potassium 425 Rubidium 409 Caesium 382 © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 6 First ionisation energy - Going down a group (page 122) Q19: The nuclear charge increases on going down a group which should make the outer electron more difficult to remove. (1 mark.) However, the covalent radius increases and so the outer electron is further away from the nucleus. The electrostatic forces get weaker as the distance between the charges increases. This should make the outer electron easier to remove. (1 mark) On going down the group, there are more and more inner electrons which prevent the outer electron from experiencing the full effect of the nuclear charge. These inner electrons increasingly screen the outer electron from the nucleus. Consequently, the outer electron becomes easier to remove as the atom gets bigger, i.e. the first ionisation energy decreases on going down a group. (1 mark) First ionisation energy - Going across a Period (page 123) Q20: On going from left to right across Period 2, the nuclear charge increases. The electrons are held more tightly making it more difficult to remove one of the outer electrons. (1 mark) Each additional electron goes into the second shell. This makes little difference to the screening effect. Consequently, the increased nuclear charge pulls in the electrons and the covalent radius decreases. (1 mark) Because the outer electrons are closer to the nucleus, they are more strongly held and so the first ionisation increases across the Period. (1 mark) Q21: b) It increases. Q22: a) It increases. Q23: c) It stays the same. Q24: c) It stays the same. Q25: a) It decreases. Electronegativity (page 125) Q26: b) Bromine Q27: c) C-S Q28: a) Group 3 © H ERIOT-WATT U NIVERSITY 215 216 ANSWERS: TOPIC 6 Answers from page 126. Q29: 1 Q30: 7 Q31: Noble gases Q32: The noble gases do not generally form bonds with other elements. Answers from page 127. Q33: c) Electronegativity values increase on going from left to right across a Period and decrease on going down a Group. Summary: Periodic Table trends (page 128) Q34: End of Topic 6 test (page 130) Q35: c) Mendeleev left gaps for elements which had not yet been discovered. Q36: 3. There is a decrease in first ionisation energy on going down Group 0, and 6. There is a steady decrease in atomic size across a Period from left to right. Q37: b) Atomic size Q38: b) B Q39: d) more occupied electron shells? Q40: c) protons? Q41: c) Mg(g) → Mg+ (g) + 2eQ42: d) F(g) → F+ (g) + eQ43: 4. The first ionisation energy decreases from Li to Cs, and 5. Electronegativity decreases from Li to Cs. © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 6 Q44: d) increasing nuclear charge? Q45: d) 6 Q46: b) Atomic Number Q47: d) Atomic mass Q48: Electronegativity Q49: Fluorine Q50: d) Q51: On going from left to right across Period 2, the nuclear charge increases. The electrons are held more tightly making it more difficult to remove one of the outer electrons. (1 mark) Each additional electron goes into the second shell. This makes little difference to the screening effect. Consequently, the increased nuclear charge pulls in the electrons and the covalent radius decreases. (1 mark) Because the outer electrons are closer to the nucleus, they are more strongly held and so the first ionisation increases across the Period. (1 mark) Q52: The nuclear charge increases on going down a group which should make the outer electron more difficult to remove. (1 mark) However, the covalent radius increases and so the outer electron is further away from the nucleus. The electrostatic forces get weaker as the distance between the charges increases. This should make the outer electron easier to remove. (1 mark) On going down the Group, there are more and more inner electrons which prevent the outer electron from experiencing the full effect of the nuclear charge. These inner electrons increasingly screen the outer electron from the nucleus. Consequently, the outer electron becomes easier to remove as the atom gets bigger, i.e. the first ionisation energy decreases on going down a Group. (1 mark) Q53: b) They generally do not form bonds with other elements. © H ERIOT-WATT U NIVERSITY 217 218 ANSWERS: TOPIC 7 7 Bonding continuum and polar covalent bonding Test your prior knowledge (page 139) Q1: b) a shared pair of electrons. Q2: c) They generally do not form bonds with other elements. Q3: a) Fluorine Q4: c) Hydrogen Electronegativity (page 140) Q5: b) 50% Q6: c) C-S Q7: a) X is Na and Y is O Q8: c) Hydrogen chloride (HCl) Q9: a) Hydrogen fluoride (HF) Q10: d) Ammonia (NH 3 ) Direction of charge (page 143) Q11: © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 7 219 Answers from page 145. Q12: • Pure covalent - electronegativity difference 0; Br - Br • Polar covalent - electronegativity difference 0.6; • Ionic - electronegativity difference 2.0; K+ ...... Br- Q13: c) Ionic Q14: a) Pure covalent Q15: b) Fluorine Permanent dipole interactions (page 146) Q16: b) B Q17: c) C Q18: b) B Predicting molecular polarity (page 148) Q19: Detecting polar molecules (page 149) Q20: a) Water Q21: a) Water Q22: b) It is attracted by the rod. Q23: a) It is not affected by the rod. Answers from page 151. Q24: a) Polar © H ERIOT-WATT U NIVERSITY δ+ H - Brδ- 220 ANSWERS: TOPIC 7 End of Topic 7 test (page 153) Q25: d) Calcium sulfide Q26: a) Sulfur Q27: c) delocalised electrons. Q28: b) Metallic Q29: a) Covalent, and b) Made up of discrete molecules. Q30: a) Boron Q31: d) Phosphorus, and f) Sulfur Q32: b) Chlorine, and e) Sodium Q33: a) Cl-F Q34: C Q35: c) BeCl2 Q36: c) Ionic Q37: f) SiO2 (s) Q38: a) NH4 Cl (s), and e) Na2 CO3 (s) Q39: b) CH3 OH (l) © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 8 8 Intermolecular forces Test your prior knowledge (page 160) Q1: c) Electronegativity Q2: a) Fluorine Q3: c) increases. Induced dipoles (page 163) Q4: b) unevenly spread. Q5: b) a temporary dipole. Q6: b) The temporary dipole causes an induced dipole. Q7: b) intermolecular. Answers from page 164. Q8: Krypton Q9: Krypton Q10: Krypton Q11: c) The larger the atom the stronger the London dispersion forces. Answers from page 164. Q12: The energy required to break forces which are only effective at such low temperatures is very small, and in fact, London dispersion forces of attraction are weaker than all other types of bonding. Answers from page 165. Q13: As the alkane molecules in the family get bigger, from methane to butane, the boiling point increases (butane at -1 ◦ C is a higher temperature than methane at -164 ◦ C). (1 mark) Since boiling point depends on the strength of the London dispersion forces, butane must have stronger London dispersion forces than methane. (1 mark) Therefore the larger the molecule the stronger the London dispersion forces. (1 mark) Q14: Butane Q15: Butane © H ERIOT-WATT U NIVERSITY 221 222 ANSWERS: TOPIC 8 Q16: Butane Q17: c) The larger the molecule the stronger the London dispersion forces. Strength of hydrogen bonds (page 167) Q18: As size increases, London dispersion forces increase, so one might expect water (the smallest) to have the lowest boiling point. However, the boiling point of water goes against the trend and is much higher than might be expected. This is because water molecules exhibit hydrogen bonding between the molecules, due to the highly polar character of the water molecule. Although the other hydrides have some permanent dipole-permanent dipole interactions, the hydrogen bonded permanent dipole-permanent dipole interactions in water are stronger than other forms of permanent dipole-permanent dipole interactions and London dispersion forces, as evidenced by the high boiling point. Q19: London dispersion forces Q20: Water Q21: Water Q22: b) The hydrogen bonds are stronger than London dispersion forces and the other permanent dipole-permanent dipole interactions. Q23: a) Covalent bonds are much stronger than hydrogen bonds. Answers from page 169. Q24: All of the above. Hydrides of non-metals (page 170) Q25: b) No Q26: NH3 , H2 O and HF Q27: b) Van der Waals attractions Testing viscosity (page 172) Q28: Methanol Q29: Methanol Q30: Glycerol Q31: 0.5 m s-1 Q32: a) 0.8 m s-1 © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 8 Q33: 1.25 seconds Answers from page 176. Q34: Ammonia is a polar substance which can form hydrogen bonds with water; nitrogen is non-polar and unable to bond to water. End of Topic 8 test (page 180) Q35: d) a temporary dipole causing an induced dipole. Q36: b) Covalent bonds Q37: b) The smaller the molecule the weaker the force. Q38: a) HF Q39: b) The London dispersion forces become stronger. e) The relative atomic mass increases. Q40: London dispersion forces. Q41: Hydrogen bonding (a form of permanent-permanent dipole interaction) between the molecules. Q42: c) He (g) Q43: a) NH3 (l) Q44: a) NH3 (l) f) HBr (l) Q45: b) CCl4 (l) Q46: Hydrogen bonding Q47: © H ERIOT-WATT U NIVERSITY 223 224 ANSWERS: TOPIC 9 9 End of unit test End of Unit 1 test (page 184) Q1: b) decreases. Q2: b) endothermic. Q3: c) large. Q4: c) Carbon monoxide Q5: d) the number of occupied energy levels decreases. Q6: b) a small rise in temperature results in a large increase in reaction rate. Q7: b) 20 kJ mol-1 Q8: a) -30 kJ mol-1 Q9: b) H C1 Q10: d) London dispersion forces Q11: b) number of protons. Q12: b) bonds between molecules are weaker than bonds between ions. Q13: a) London dispersion forces are much weaker than covalent bonds. Q14: Fe(s) Cl2(g) Heat Iced waterr FeCl3(g) Q15: Polar covalent Q16: b) Ionic ⇒ polar covalent ⇒ pure covalent © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 9 225 Q17: The number of protons increases OR greater nuclear charge OR greater nuclear attraction. Q18: Electronegativity Q19: Same electronegativity values. Q20: Bigger atom so electron is further from the nucleus; inner electrons shield (screen) the outer electron from attraction of the nucleus. Q21: Cl(g) + e- → Cl- (g) Q22: b) B Q23: 100 cm3 Q24: 190 kJ Q25: -10 kJ Q26: b) decrease Q27: c) have no effect on Q28: 26 kJ mol-1 Q29: -33 kJ mol-1 Q30: Definition Chemical term For a chemical reaction to occur, particles must collide. Collision theory The minimum kinetic energy required by colliding particles before a reaction will occur. Activation energy Intermediate stage at the top of the energy barrier when reactants change to products. Activated complex The difference in potential energy between products and reactants. Enthalpy change A substance that alters the rate of a reaction without being used up. © H ERIOT-WATT U NIVERSITY Catalyst 226 ANSWERS: TOPIC 9 Q31: Definition Chemical term Regular arrangement of positively charged ions surrounded by delocalised electrons. Metallic bonding The electrostatic force of attraction of a shared pair of electrons for two positive nuclei. Covalent bonding The electrostatic attraction between negative and positive ions. Weak bonds between molecules. Ionic bonding Intermolecular forces A slight negative or positive charge caused by the uneven distribution of electrons. Bonds holding atoms together within molecules. Dipole Intramolecular forces Q32: Definition Chemical term Half the distance between the nuclei of two covalently bonded atoms of an element. Covalent radius The strength of the attraction of an element for the electrons of its bonding electrons. Electronegativity The energy required to remove an electron from a gaseous atom to form an ion with a single positive charge. Ionisation energy The temperature at which a substance changes from a solid to a liquid. Melting point The temperature at which a substance changes from a liquid to a gas. Boiling point Q33: 1. Covalent bonding 2. Hydrogen bonding 3. Permanent dipole - permanent dipole interactions 4. London dispersion forces Q34: • Going across a period, atomic size decreases. • Going down a group, atomic size increases. • Going down a group, electronegativity decreases. • Going across a period, electronegativity increases. • Going across a period, boiling point decreases. • Going down a group, boiling point increases. © H ERIOT-WATT U NIVERSITY ANSWERS: TOPIC 9 Q35: Covalent bonds not being broken OR Intermolecular bonds that are breaking. Any alternative wording that recognises that covalent bonds are not broken when covalent substances melt or boil. Q36: Formula refers to the ratio of Mg 2+ :Cl- ions (in lattice) OR alternative wording ie in the lattice there are twice as many chloride ions as magnesium ions OR Mg 2+ ions surrounded by > 2 Cl - ions OR Cl- surrounded by >1 Mg 2+ . Any reference to 'chlorine ions' is not acceptable. Q37: As a general rule, award yourself a half mark, up to a maximum of three marks in total, for each point you have successfully made (NB this is a general rule only, there are no half marks awarded at Higher): • Propane molecules are held together by weak intermolecular forces / ethanol molecules are held together by strong intermolecular forces. • The only intermolecular forces in propane are London Dispersion forces. • These are weak forces are due to momentary displacement of electrons between atoms creating temporary dipoles. • In response to these temporary dipoles an induced dipole can occur. • The intermolecular forces in ethanol are hydrogen bonds. • Hydrogen bonding arises because the O-H bond is highly polar (there is a large difference in the electronegativities of O and H). • The small positive dipole on H and small negative dipole on O strongly attract. • This causes ethanol to have a higher boiling point than propane as more energy is needed to overcome the stronger hydrogen bonds than the weak London Dispersion forces. © H ERIOT-WATT U NIVERSITY 227