WJEC Chemistry for AS Level Student Book: 2nd Edition

Worked example Sodium carbonate reacts with dilute hydrochloric acid to give carbon dioxide and a solution of sodium chloride. Write a balanced chemical equation including state symbols for this reaction. sodium carbonate + hydrochloric acid A sodium chloride + carbon dioxide + water Writing the formulae gives: Na 2 CO 3 + HCl A NaCl + CO 2 + H 2 O Number of atoms on L.H.S. = 2Na + 1C + 3O + 1H + 1Cl Number of atoms on R.H.S. = 1Na + 1C + 3O + 2H +1Cl Start by balancing Na atoms, so multiply NaCl on the R.H.S. by 2: Na 2 CO 3 + HCl A 2NaCl + CO 2 + H 2 O Number of atoms on L.H.S. = 2Na + 1C + 3O + 1H + 1Cl Number of atoms on R.H.S. = 2Na + 1C + 3O + 2H +2Cl Next balance H atoms by multiplying HCl on the L.H.S. by 2: Na 2 CO 3 + 2HCl A 2NaCl + CO 2 + H 2 O Number of atoms on L.H.S. = 2Na + 1C + 3O + 2H + 2Cl Number of atoms on R.H.S. = 2Na + 1C + 3O + 2H +2Cl Equation is now balanced. Add state symbols. Na 2 CO 3 (s) + 2HCl(aq) A 2NaCl(aq) + CO 2 (g) + H 2 O(l) Ionic equations Many reactions involve ions in solutions. However, in these reactions not all of the ions take part in any chemical change. An ionic equation may help to show what is happening. Ionic equations are frequently used for displacement and precipitation reactions. Worked example 1 When zinc powder is added to copper(II) sulfate solution, copper is displaced and a red-brown deposit is formed on the zinc. The chemical equation for the reaction is: Zn(s) + CuSO 4 (aq) A ZnSO 4 (aq) + Cu(s) Writing out all of the ions gives: Zn(s) + Cu 2+ (aq) + SO 4 2− (aq) A Zn 2+ (aq) + SO 4 2− (aq) + Cu(s) There is repetition here. The SO 4 2− (aq) ions have not taken part in any chemical change at all. They have been present unchanged throughout. They are called spectator ions and are left out of the ionic equation, which is written: Zn(s) + Cu 2+ (aq) A Zn 2+ (aq) + Cu(s) An ionic equation provides a shorter equation which focuses attention on the changes taking place. Stretch & challenge Balance the following equation: Cu + HNO 3 A Cu(NO 3 ) 2 + NO + H 2 O. 7 Knowledge check When a solution of sodium sulfate is added to a solution of barium chloride, a white precipitate of barium sulfate forms. Write an ionic equation, including state symbols, for this reaction. 14 WJEC Chemistry for AS Level