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Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Acknowledgements No extract from any source held under copyright by any individual or organisation has been included in this booklet. The author would like to thank the following people for their assistance in the development of these materials: Alan McDowall The Carnegie Group is committed to providing an excellent customer service and to producing materials in formats to best meet the needs of our customers. Should you require this material to be produced in an alternative format, please contact Sandra Archibald. Tel: 01383 845149; email: [email protected] © Carnegie College - All rights reserved. No part of this publication may be reproduced or transmitted in any form by any means (electronic, photocopying, recording) without the prior written consent of Carnegie College. © Carnegie College DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Contents Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Equations 1 Covalent Bonding 1 Ionic Bonding 3 Nomenclature 5 –IDE and –ATE 5 -ATE and –ITE 5 Metallic Bonding 5 Covalent Bonding: A Second Look 6 Discrete Covalent Molecules 7 Covalent Network Molecules 7 Chemical Formula 8 Balance 12 The Bonding and Formula Progress Checklist 18 Answers to SAQs © Carnegie College 19 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Equations By the end of this section you should be able to: Outline the three main types of bonding Understand and use nomenclature accurately Use chemical formula accurately Balance chemical equations There are three main types of bonding: Covalent Ionic Metallic Covalent Bonding Atoms form bonds in order to increase stability; remember that an element is most stable when it has a full outer electron shell, that is when it has the same outer electron configuration as the noble gases. i) Covalent Bonding in the elements occurs in the non-metals. For example hydrogen has one outer electron; to gain a full outer shell like the noble gas helium it needs one more electron. Hydrogen is able to achieve this stability by sharing its one outer electron with another atom of hydrogen. We call this shared pair of electrons a single covalent bond. Covalent bonding also occurs in other elements forming diatomic molecules, such as Oxygen (O2) and Nitrogen (N2). In covalent bonding there is a sharing of electrons to give stability to the outer shell of each atom involved. Oxygen has the electron arrangement 2)6. Four of the outer electrons are paired up, leaving two others. Two oxygen atoms can share these two outer electrons to give a stable covalent bond. Two bonds in fact. Nitrogen has the electron configuration 2)5, so three electrons are left unpaired in the outer shell. A nitrogen atom can therefore bond with another nitrogen atom to from N2, which has three covalent bonds. The group 7 elements (the halogens) also form diatomic molecules; how many pairs of electrons are shared in the halogens? © Carnegie College 1 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions ii) Carnegie College Covalent Bonding in Molecular compounds occurs when more than one type of non-metal element join together to increase stability. You will recognise some common covalently bonded compounds. Water (H2O), carbon dioxide (CO2) and methane (CH4) the gas that heats many of our homes. In all of these examples electrons are shared giving a stable electron arrangement. For water we say that the molecular formula is H2O but the structural formula is drawn as H H O For Carbon dioxide CO2 is the molecular formula while the structural formula is shown O=C=O and for methane CH4 and H H C H H All discrete covalent molecules have low melting and boiling points and tend to be liquids and gases at room temperature. Between all molecules in the liquid or solid state weak forces called van der waals’ forces exist these forces become larger as the size of the molecule increases, it is these forces that enable us to liquefy and solidify gases. We can also say that since the electrons are held tightly in a bond and because covalent compounds are electrically neutral they do not conduct electricity; this is because there are no free electrons to carry the charge. When molecules are formed between atoms of different elements polar covalent bonding occurs. In polar covalent bonding the electrons are not shared equally between the different elements. That is to say some elements have a greater affinity for electrons than others. The atom with the greater electron attracting power acquires a very slight negative charge shown δ- while those with less attracting power acquire a slight positive charge δ+. Let’s consider water This bonding allows δ+ δ+ weak bonds to be formed within the structure of water and gives H H water higher melting and boiling points than would be expected Oδin a molecule of this size, these bonds are called hydrogen bonds and give water a lattice structure. This ordering of the water molecules means that when water freezes ice is less dense than water and floats on the top, allowing, as we know aquatic species to survive during the winter. Hδ+ Hδ+ Hδ+ Oδ- Hδ+ OδHδ+ Hydrogen Bond Hδ+ Oδ- © Carnegie College 2 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College iii) Covalent Networks are unusual for molecules joined by covalent bonding in that they have high melting and boiling points. A good example is diamond. A diamond is many millions of carbon atoms linked together by single covalent bonds in a tetrahedral arrangement. Diamonds are strong, rigid and solid at room temperature. Graphite is another form of carbon that exists as a covalent network; in this case the structure is made of layered rings linked together by van der waals’ forces. In graphite some of the electrons are free to move about the structure, we say these electrons are ‘delocalised’ and are capable of conducting electricity. Carbon in the form of graphite is unusual as it is the only non-metal that can conduct electricity. Ionic Bonding Ionic bonding occurs when one atom has a much larger ‘pull’ on electrons than another. Alkali metals and Halogens form ionic bonds. In ionic bonding one atom, the one with the greater electronegativity removes the electron from another atom with a smaller electronegativity. Both end up with noble gas electron arrangement in the outer shell. A chlorine atom with an electron configuration 2)8)7 will try to pick up an electron to give an electron configuration 2)8)8 and form the chloride ion Cl-. A Sodium atom with an electron configuration 2)8)1 will try to loss an electron to give an electron configuration 2)8) and form the sodium ion Na+. In general metals lose electrons to achieve a full outer shell and form positive ions, while non-metals gain electrons to achieve a full outer shell and form negative ions. Now try SAQ 1 on the following page © Carnegie College 3 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College 1 What will be the size and type (positive or negative) formed by each of the elements below? Lithium Calcium Aluminium Nitrogen Oxygen Fluorine Check your answer at the end of this booklet. © Carnegie College 4 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College The overall charge on an ionic compound must be zero you will learn more about this in a short while. All ionic compounds are solid at room temperature and do not conduct electricity when solid, however when ionic compounds are dissolved (and most are soluble in water) or when they are melted ionic compounds are able to conduct electricity due to the movement of ions. Nomenclature Many chemicals have traditional names, but all chemicals have a scientific name. Acetic acid is the traditional name of the acid in vinegar; ethanoic acid is the scientific name. These names can tell you a lot about a compound. They can tell you both the number and type of atoms that a molecule contains. Many compounds contain more than one type of atom. –IDE If a chemical has a name that ends in –IDE, this indicates that it is made up of two different types of atoms. So sodium chloride contains two types of atoms: Sodium and Chlorine. The metal sodium retains its name and the chlorine is renamed chloride. (A notable exception to this rule is Hydroxides(OH-), sodium hydroxide contains three elements – sodium, oxygen and hydrogen. The number of each atom can be denoted by the prefixes: Mono di tri tetra one two three four So carbon dioxide contains two oxygen; its formula is CO2. -ATE and –ITE Names that end in –ate and –ite contain three different types of atoms, one of which is oxygen. Compounds with names ending in ate have a greater proportion of oxygen than those with names ending in ite. Metallic Bonding Metals make up the left and ‘middle’ section of the periodic table. Clearly identify the metals on your periodic table. © Carnegie College 5 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College In metals the outer electrons are delocalised (delocalised means free to move). This produces a regular matrix of positively charged ions in a sea of electrons. This delocalisation is why metals are good at conducting electricity and heat. Unlike ionic compounds metals conduct due to movement of electrons rather than movement of ions. In addition since all the positive ions are of equal size, metals are malleable and ductile. This means that metals can be hammered into different shapes and drawn into wires. + + + + + + + + + + + + + + Properties of metals: Electrical conductivity Thermal conductivity Ductility Malleability they can conduct electricity they can conduct heat they can be drawn into wire they can be beaten and rolled into sheets Most metals except Mercury and the alkali metals have high melting and boiling points. Covalent Bonding: A Second Look Covalent bonding can be described at the ________ ____________ of electrons. There are two forms of covalent bonding: Covalent Discrete Molecules Covalent Network Molecule © Carnegie College 6 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Discrete Covalent Molecules This type of molecule can be divided into two types; pure covalent bonding and polar covalent bonding. Pure covalent bonding occurs between atoms of the same element, for example Oxygen (O2) or Sulphur (S8). In these cases the electrons are shared equally between the atoms. Where more than one type of atom is involved then electrons are unequally shared; polar covalent bonding – this can lead to areas of positive and negative charge within the molecule. These charged areas are much less than the charge on a proton or an electron and are shown by the Greek symbol delta + or -. Water is a good example. OδHδ+ Hδ+ Discrete covalently bonded compounds usually have low melting and boiling points. Covalent Network Molecules These are covalently bonded structures such as Carbon (diamond) or Silicon Dioxde (Quartz and Sand). These have four bonds and form very strong stable network structures such as diamond, graphite and sand. It takes a lot of energy to melt these compounds, due to the strong nature of the bonds. © Carnegie College 7 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Bonding Summary Compound Ionic: Solid does not conduct electricity. Conducts electricity when molten or dissolved High Melting Points Covalent: Does not conduct electricity Covalent Network High Melting Point Solids Covalent Discrete Low Melting Point Solids Low boiling point Gasses Question Name the only covalently bonded element that can conduct electricity. Chemical Formula Many atoms can be found as ions. An ion is an atom that has gained or lost an electron. Atoms gain or lose electrons to get full outer shells and increase their stability. The alkali metals produce a charge of one positive and would be written as X+. So Sodium would be written as Na+. While Magnesium, which looses two electrons when it forms ions, can be written as Mg2+. The halogens gain an electron and have a one negative charge. So Chlorine would be written as Cl-. While oxygen in group 6 would gain two electrons and be written as O2-. For a compound to be stable, the electrical changes must balance out. That is to say that the overall charge on the compound will be zero. So when a negative ion joins with a positive ion the amount of the positive charge must equal that of the negative charge. This is fine when the charges are 1+ and 1-. Then we just need one of each atom. However, if we have an atom with a charge of 2-, and another with a charge of 1+, we would need two of the 1+ atoms to balance out the 2- of the other. Let’s look at Oxygen. This has a charge of 2- and so is written as O2- so if this reacts with Sodium which is 1+, how many Sodium do we need? That’s right two. © Carnegie College 8 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College So the chemical formula would be: O2-(1) Na+(2). This could be written as ONa2. But due to convention the positive ion, ie the hydrogen or metal is named first and it would be written as Na2O. If you know the charge on each of the ions you can easily work out the chemical formula. The way to do this is to cross multiply. So if the negative ions have a charge of 3- and the positive ion a charge of 2+, you simply multiply the positive ion with the 3 and the negative ion with 2, ie X3- Y2+ the X is multiplied by the 2 giving X2 and the Y is multiplied by 3 to give Y3. This would give six positive and negative changes and the formula would be X2Y3. We can do this as a series of steps until we become more confident. For simple negative ions, that is those that are found on the periodic table the ending in the name is –ide, so a chlorine atom becomes a chloride ion and an oxygen atom becomes an oxide ion. Lets us try that with Potassium Oxide. Step 1: Write down the names of the ions involved. Potassium Oxide Step 2: Write down the symbols K+ O2- Step 3: Write down the charges K 1 O 2 Step 4: Cross multiply the charges K 1 2 O 2 1 Step 5: Write down the formula of the compound K2O1 © Carnegie College 9 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Step 6: Simplify if possible K2O Try the examples below 2 a) Sodium chloride b) Magnesium oxide c) Calcium chloride d) Aluminium oxide Check your results with those given at the end of the booklet. Sometimes an ion is made up of a group of non-metals that carry an overall charge; these are called compound ions and can be found on page 4 of your data book. Examples include the hydroxide ion OH-, sulphate ion SO42- and the ammonium ion NH4+. In this case it is the size of the charge that is swapped in order to get the formula. Follow the steps outlined in the simple example below to get the answer. For example Sodium carbonate Sodium Na+ 1 2 carbonate CO322 1 Na2 (CO3)1 The brackets are important in many compounds involving compound ions as it tells us that the number outside the bracket refers to the whole ion and not just part of it. If the number is 1 then it is appropriate at this stage to remove both the number and the brackets so our answer becomes: Na2CO3 © Carnegie College 10 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Try the examples below 3 a) Potassium Dichromate b) Ammonium Carbonate c) Sodium Nitrate d) Magnesium phosphate Check your results with those given at the end of the booklet. In addition to the above it is important to note that the transition elements are capable of forming ions of more than one size. For example copper forms Cu+ ions and Cu2+ ions and iron forms Fe2+ and Fe3+ ions. In examples like these the size of the charge is always indicated using roman numerals, so a compound containing the Fe2+ ion would for example be shown as Iron (II) chloride while a compound containing Fe3+ would be Iron (III) chloride. Try the examples below. 4 a) Lead(II) nitrate b) Iron(III)sulphate c) Copper(I) chloride d) Silver(I) nitrate (In fact unless told otherwise it is safe to assume that silver will always form 1+ ions). Check your results with those given at the end of the booklet. © Carnegie College 11 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Balance When chemicals come together and react, the starting material and the product often appear completely different. Example 1: Think of common table salt. This is made of the reactive alkali metal sodium and the poisonous gas chlorine, neither of which you would want to eat; yet when these two elements react, a food seasoning is produced. The combining of sodium and chlorine can be written as a word equation where the reactants are shown on the left and the products on the right of the arrow. Reactants Products Sodium + chlorine Sodium chloride This is called a word equation We can replace the words with chemical formula, which can be worked out using the rules of valency and by knowing which elements form diatomic molecules. The equation above becomes Na + Cl2 NaCl When we count up the numbers of each element on each side of the arrow they should be equal. Element Sodium Chlorine Number of LHS 1 2 Number on RHS 1 1 As you can see the number of each type of element is not balanced on both sides of the equation. We have to add more molecules of some compounds to make the equation balance. In chemistry it is very important that any equations we write should balance. So how can we balance this equation? We have 2 chlorines on the LHS but only one on the RHS; we need to add a 2 to the LHS. Na + Cl2 2NaCl You will note that this 2 also affects the number of sodium on the RHS. Whenever a number is added in front of a compound it affects all the elements in that compound, so now we have 2 sodium on the RHS and only one on the LHS, to make the equation balance a 2 will need to be placed in front of the sodium on the LHS. 2Na + Cl2 © Carnegie College 2NaCl 12 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College The equation is now balanced. Element Sodium Chlorine Number of LHS 2 2 Number on RHS 2 2 Example 2: A Human Example Think of the food you eat. You take in sugars and you breathe out carbon dioxide and water. This can be written as a word equation: Glucose + Oxygen Carbon Dioxide + Water We could write this in its chemical form: C6H12O6 + O2 CO2 + H2O A question of balance When we count up the numbers of each element on each side of the arrow they should be equal. Element Carbon Hydrogen Oxygen Number of LHS 6 12 8 Number on RHS 1 2 3 As you can see the number of each type of element is not balanced on both sides of the equation. We have to add more molecules of some compounds to make the equation balance. So how can we balance this equation? C6H12O6 + O2 CO2 + H2O We need 12 Hydrogen and six Carbons on the RHS, so we need six H2O and six CO2 so giving: C6H12O6 + O2 6CO2 + 6H2O We now have 18 oxygen on the RHS and only eight on the LHS. So to make it balance we need to add six O2 to the LHS. C6H12O6 + 6O2 © Carnegie College 6CO2 + 6H2O 13 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College This equation now balances out: Element Carbon Hydrogen Oxygen Number of LHS 6 12 18 Number on RHS 6 12 18 Try the examples in SAQ 5 on the following page. © Carnegie College 14 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College 5 1) When methane (CH4) burns in oxygen (O2), carbon dioxide (CO2) and water (H2O) are produced. Write a word equation, a formula equation and a balanced chemical equation for this reaction. 2) Hydrogen and oxygen combine in an explosive reaction to form water, write word, formula and balanced chemical equations for this reaction. 3) Balance each of the following formula equations a Mg + HCl MgCl2 + H2 b NaOH + H2SO4 Na2SO4 + H2O c AgNO3 + Cu Ag + Cu(NO3)2 d C2H4 + O2 CO2 + H2O SAQ 5 continues over the page © Carnegie College 15 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College SAQ 5 continued e C6H12O6 C2H5OH + CO2 f SnO2 + HCl SnCl4 + H2O Check your results with those given at the end of the booklet. © Carnegie College 16 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Additional information can be added to balanced chemical equations by the use of state symbols. s- solid l- liquid g- gas aq- aqueous( dissolved in water) So our first example would become 2Na(s) + Cl2(g) © Carnegie College 2NaCl(s) 17 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College The Bonding and Formula Progress Checklist Tick the boxes only if you can: Topic Outline the three main types of bonding Understand and use nomenclature accurately Use chemical formula accurately Balance chemical equations © Carnegie College 18 Understand(?) DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions Carnegie College Answers to SAQs Answer to SAQ 1 1+ 2+ 3+ 321- Answers to SAQ 2 Sodium chloride Magnesium Oxide Calcium chloride Aluminium oxide NaCl MgO CaCl2 Al2O3 Answer to SAQ 3 Potassium dichromate Ammonium carbonate Sodium Nitrate Magnesium phosphate K2Cr2O7 (NH4)2CO3 NaNO3 Mg3(PO4)2 Answer to SAQ 4 Lead (II) nitrate Iron(IIl) sulphate Copper(I)chloride Silver(I)nitrate Pb(NO3)2 Fe2(SO4)3 CuCl AgNO3 Answer to SAQ 5 Methane + Oxygen carbon dioxide + water CH4 + O2 CO2 + H2O CH4 + 2O2 CO2 + 2H2O Hydrogen + Oxygen © Carnegie College Water 19 DH2K 34 Fundamental Chemistry: Theory and Practice Topic 3 – Bonding, Chemical Symbolism and Balanced Chemical Reactions H2 + O2 2H2 + O2 H2O 2H2O Mg + 2HCl MgCl2 + H2 2NaOH + H2SO4 Na2SO4 + 2H2O 2AgNO3 + Cu 2Ag + Cu(NO3)2 C2H4 + 3O2 2CO2 + 2H2O C6H12O6 2C2H5OH + 2CO2 SnO2 + 4HCl SnCl4 + 2H2O © Carnegie College Carnegie College 20 DH2K 34