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Temperature Temperature Scales Boiling point of water Fahrenheit Celcius Kelvin 212 oF 100 oC 373 K 180 oF Freezing point of water 32 oF 100 oC 0 oC 100 K 273 K Notice that 1 kelvin degree = 1 degree Celcius Temperature Scales Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 136 Temperature Scales • Temperature can be subjective and so fixed scales had to be introduced. • The boiling point and freezing point of water are two such points. • Celsius scale (oC) – The Celsius scale divides the range from freezing to boiling into 100 divisions. – Original scale had freezing as 100 and boiling as 0. – Today freezing is 0 oC and boiling is 100 oC. • Fahrenheit scale (oF) • Mercury and alcohol thermometers rely on thermal expansion Temperature is Average Kinetic Energy *Vector = gives direction and magnitude “HOT” Fast “COLD” Slow Kinetic Energy (KE) = ½ m v 2 Total kinetic energy is what we call heat. Heat is measured with an instrument called a calorimeter. Temperature is measured with an instrument called a thermometer. Heat versus Temperature lower temperature Fractions of particles higher temperature TOTAL = Heat Kinetic ENERGY Kinetic energy Molecular Velocities molecules sorted by speed Fractions of particles many different molecular speeds the Maxwell speed distribution speed http://antoine.frostburg.edu/chem/senese/101/gases/slides/sld016.htm Hot vs. Cold Tea Many molecules have an intermediate kinetic energy Low temperature (iced tea) High temperature (hot tea) Percent of molecules Few molecules have a very high kinetic energy Kinetic energy Equal Masses of Hot and Cold Water Thin metal wall Insulated box Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 291 Water Molecules in Hot and Cold Water Hot water 90 oC Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 291 Cold Water 10 oC Water Molecules in the same temperature water Water (50 oC) Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 291 Water (50 oC) Temperature vs. Heat Different Alike Measured with a Thermometer Have Kinetic Energy Topic Average Kinetic Energy oCelcius (or Kelvin) Temperature Different Measured with a Calorimeter Topic A Property of Matter Heat Total Kinetic Energy Joules (calories) Energy (a) Radiant energy (c) Chemical energy (b) Thermal energy (d) Nuclear energy Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (e) Electrical energy Potential energy Energy in kinetic energy Energy out kinetic energy The energy something possesses due to its motion, depending on mass and velocity. Energy A is the ability to do work or produce heat C B Kinetic Energy – energy of motion KE = ½ m v 2 mass velocity (speed) Potential Energy – stored energy Batteries (chemical potential energy) Spring in a watch (mechanical potential energy) Water trapped above a dam (gravitational potential energy) School Bus or Bullet? Which has more kinetic energy; a slow moving school bus or a fast moving bullet? Recall: KE = ½ m v 2 BUS KE = ½ m v 2 KE(bus) = ½ (10,000 lbs) (0.5 mph)2 BULLET KE = ½ m v 2 KE(bullet) = ½ (0.002 lbs) (240 mph)2 Either may have more KE, it depends on the mass of the bus and the velocity of the bullet. Which is a more important factor: mass or velocity? Why? (Velocity)2 Energy Kinetic Energy Potential Energy energy due to the composition or energy of motion position of an object. (Chemical Potential Energy – energy Work – weight lifted stored in a substance because of through a height its composition. The potential energy results from the arrangement of the atoms Heat – symbol is q; and the strength of the bonds that nergy in the process join them. Stored energy is f flowing from a warm released when bonds are bject to a cooler one broken.) Units of energy are the same, regardless of the form of energy Units of energy Most common units of energy metric calorie where 1 calorie (cal) is the amount of energy needed to raise the temperature of 1 g of water by 1°C. kilocalorie called a Calorie with a capital “C” sometimes called nutritional calories 1 Calorie = 1000 calories SI joule (J), defined as 1(kilogram•meter2)/second2, energy is also expressed in kilojoules (1 kJ = 103J). One cal = 4.184 J or 1J = 0.2390 cal. Energy Transformations if molecules collide with enough force to break them into atoms, a ELEMENT CHEMICAL REACTION can take place hydrogen molecule, H2 COMPOUND MIXTURE ELEMENT oxygen molecule, O2 a mixture of hydrogen and oxygen molecules water, H2O 2 H2 + O2 Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. 2 H2O + E The Zeppelin LZ 129 Hindenburg catching fire on May 6, 1937 at Lakehurst Naval Air Station in New Jersey. S.S. Hindenburg • German zeppelin luxury liner • Exploded on maiden voyage • Filled with hydrogen gas 35 people died when the Hindenburg exploded. May 1937 at Lakehurst, New Jersey Hydrogen is the most effective buoyant gas, but is it highly flammable. The disastrous fire in the Hindenburg, a hydrogen-filled dirigible, in 1937 led to the replacement of hydrogen by nonflammable helium. Exothermic vs. Endothermic Decomposition of Nitrogen Triiodide Decomposition of Nitrogen Triiodide N2 NI3 2 NI3(s) I2 N2(g) + 3 I2(g) Direction of Heat Flow Surroundings EXOthermic qsys < 0 ENDOthermic qsys > 0 System H2O(s) + heat H2O(l) H2O(l) H2O(s) + heat melting freezing System Kotz, Purcell, Chemistry & Chemical Reactivity 1991, page 207 Conservation of Matter Reactants yield Products Conservation of Energy in a Chemical Reaction In this example, the energy Endothermic of the reactants Reaction and products increases, while the energy of the surroundings decreases. Reactant + Energy Product In every case, however, the total energy does not change. Surroundings Energy Surroundings System System Myers, Oldham, Tocci, Chemistry, 2004, page 41 Before reaction After reaction Conservation of Energy in a Chemical Reaction In this example, the energy Exothermic of the reactants Reaction and products decreases, while the energy of the surroundings increases. Reactant Product + Energy In every case, however, the total energy does not change. Energy Surroundings Myers, Oldham, Tocci, Chemistry, 2004, page 41 System Before reaction Surroundings System After reaction Exothermic Reaction -DH Exothermic Reactants Products + Energy 10 energy = 8 energy + 2 energy Energy of reactants Energy Energy of products Reactants -DH Exothermic Products Reaction Progress Endothermic Reaction +DH Endothermic Energy + Reactants Products 2 energy + 8 energy = 10 energy Energy of reactants Energy Energy of products Products Reactants +DH Endothermic Reaction progress Heat Capacity and Specific Heat Thermal Expansion • Most objects e-x-p-a-n-d when heated • Large structures such as bridges must be built to leave room for thermal expansion • All features expand together COLD HOT Cracks in sidewalk. Specific Heat Specific Heat Heat absorbed or released specific heat mass in grams Specific Heats of Some Substances Specific Heat Substance (cal/ g oC) Water Alcohol Wood Aluminum Sand Iron Copper Silver Gold 1.00 0.58 0.42 0.22 0.19 0.11 0.093 0.057 0.031 (J/g oC) 4.18 2.4 1.8 0.90 0.79 0.46 0.39 0.24 0.13 Examples: How much heat is absorbed when a 4.68 g piece of metal experiences a temperature change of 182°C? (Cp = .301 J/g°C) q = m Cp ΔT = (4.68 g)(.301 J/g°C)(182°C) = 256.37 J =256 J (answer in 3 sig figs) Examples: The temperature of a sample of water increases from 20.0°C to 46.6°C as it absorbs 5650 J of heat. What is the mass of the sample? ΔT = 46.6°C – 20.0°C = 26.6 °C q = m Cp ΔT 5650 J = m (4.184 J/g°C)(26.6°C) 5650 J = m (111.2944 J/g) 111.2944 J/g 111.2944 J/g m = 50.766 g or 50.8 g Try: How much heat is released to the surroundings when 200 g of water at 96.0 °C cools to 25.0 °C? Answer = 59 400 J