CHAPTER 8 Writing Chemical Equations © 2013 Marshall Cavendish International (Singapore) Private Limited

8.1Chemical Equations 8.2Ionic Equations Writing Chemical Equations Chapter 8 2

Learning Outcomes interpret chemical equations with state symbols; write balanced chemical equations with state symbols. At the end of this section, you should be able to: Chemical Equations 8.1 3

What is a Chemical Equation? A chemical equation shows what happens in a chemical reaction. It tells us which reactants and products are involved in the reaction; the relative amounts of reactants and products; the physical states of the reactants and products of the reaction. sodium + chlorine → sodium chloride (word equation) 2 Na + C l 2 → 2 NaC l (chemical equation) Example: Chemical Equations 8.1 4

Balancing a Chemical Equation A balanced chemical equation must contain equal numbers of atoms of each element on both sides of the equation. The reactant(s) are written on the left-hand side of the equation. The product(s) are written on the right-hand side of the equation. The state symbols (s) – solid (g) – gas ( l ) – liquid (aq) – aqueous solution 2H 2 (g) + O 2 (g) → 2H 2 O( l ) Chemical Equations 8.1 The chemical equation is balanced by adding a number in front of the chemical formula. This is the same as multiplying the formula by that number. 5

Writing a Balanced Chemical Equation Step 1: Write down the chemical formulae of the reactants and products to get the chemical equation. Example: Reacting hydrogen and oxygen to get water H 2 + O 2 H 2 O Chemical Equations 8.1 6

Step 2: Check the number of atoms of each element in the formulae on both sides of the equation. If the equation is not balanced, proceed to step 3. Writing a Balanced Chemical Equation H 2 + O 2 H 2 O Chemical Equations 8.1 Left-hand sideRight-hand side 2 H atoms 2 O atoms1 O atom 7

Step 3a: To balance the number of oxygen atoms, put a ‘2’ in front of H 2 O. This means that two molecules of water are formed. The equation is still not balanced because there are four hydrogen atoms on the right-hand side and only two on the left-hand side. Writing a Balanced Chemical Equation H 2 + O 2 2H 2 O Chemical Equations 8.1 8

Step 3b:To balance the equation, we need to put a ‘2’ in front of H 2. Writing a Balanced Chemical Equation 2H 2 + O 2 2H 2 O Chemical Equations 8.1 Left-hand sideRight-hand side 4 H atoms 2 O atoms 9

Step 4: Add the state symbols to indicate the physical state of each reactant and product. Writing a Balanced Chemical Equation 2H 2 (g) + O 2 (g) 2H 2 O( l ) Chemical Equations 8.1 StateSymbol Solid (s) Liquid (l)(l) Gas (g) Aqueous (aq) Aqueous means dissolved in water 10

8.1Chemical Equations 8.2Ionic Equations Writing Chemical Equations Chapter 8 11

Learning Outcome write ionic equations with state symbols. At the end of this section, you should be able to: Ionic Equations

What is an Ionic Equation? An ionic equation is a simplified chemical equation that shows the reactions involving ions in aqueous solution. In chemical reactions, there are ions that do not take part in the chemical reaction. Such ions are called spectator ions. Spectator Ions Ionic Equations

Writing an Ionic Equation HC l (aq) + NaOH(aq) → NaC l (aq) + H 2 O( l ) Let’s look at the reaction between hydrochloric acid and sodium hydroxide. hydrochloric acid + sodium hydroxide sodium chloride + water H + (aq) + C l – (aq)Na + (aq) + OH – (aq)Na + (aq) + C l – (aq) Na + (aq) and C l – (aq) are spectator ions. Removing them from the equation, we are left with: H + (aq) + OH – (aq) → H 2 O( l ) Ionic Equations

How to Write an Ionic Equation Step 1: Write a balanced chemical equation including state symbols. HC l (aq) + NaOH(aq) → NaC l (aq) + H 2 O( l ) Ionic Equations 8.2 Example: Reaction of hydrochloric acid with sodium hydroxide 15

+ H 2 O( l ) Step 2: Rewrite the chemical equation in terms of ions. HC l H + (aq) + C l – (aq) NaOH Na + (aq) + OH – (aq)+ NaC l Na + (aq) + C l – (aq) Ionic Equations How to Write an Ionic Equation

H + (aq) + C l – (aq) + Na + (aq) + OH – (aq) → Na + (aq) + C l – (aq) + H 2 O( l ) Step 3: Cancel out the spectator ions. Step 4: Write the ionic equation. H + (aq) + OH – (aq)  H 2 O( l ) Ionic Equations

Insoluble solids (e.g metals, insoluble salts) Covalent liquids (such as water) Gases Do not break them into ions when writing an ionic equation. These substances should be written in full, for example, H 2 O( l ) and AgC l (s). Substances that do not ionise in solution: Ionic Equations

Example: Reaction of sodium chloride with silver nitrate solution Step 1: Write a balanced chemical equation including state symbols. NaC l (aq) + AgNO 3 (aq) → AgC l (s) + NaNO 3 (aq) Ionic Equations

Step 2: Rewrite the chemical equation in terms of ions. Na + (aq) + C l – (aq) + → AgC l (s) + Na + (aq) + NO 3 – (aq) Ag + (aq) + NO 3 – (aq) Ionic Equations

Step 3: Cancel out the spectator ions. Step 4: Write the ionic equation. Na + (aq) + C l – (aq) + Ag + (aq) + NO 3 – (aq) → AgC l (s) + Na + (aq) + NO 3 – (aq) C l – (aq) + Ag + (aq) → AgC l (s) Can you give another example of ionic precipitation? Ionic Equations

Aqueous silver nitrate consists of Ag + and NO 3 – ions. Silver chloride precipitates as a white solid. Aqueous sodium chloride consists of Na + and C l – ions. Aqueous sodium nitrate consists of Na + (aq) + NO 3 – (aq) which remained unchanged throughout the reaction. URL Ionic Equations

Writing Chemical Equations Chapter 8 Concept Map 23

Writing Chemical Equations Chapter 8 24 Acknowledgements (slide 1) chemical reactions © Aushulz | Wikimedia Commons | CC BY 3.0 ( (slide 22) © Marshall Cavendish International (Singapore) The URLs are valid as at 15 October 2012.