WJEC Chemistry for A2: Study and Rev Guide

3.2 Redox reactions Pointer You do not need to include state symbols like (s) or (aq) unless the question speci cally asks you to. Pointer The titrations undertaken in the rst year of the course will help when performing redox titrations. These skills can be examined in unit 3 but also in both the unit 5 assessments. QUICKFIRE QUICKFIRE QUICKFIRE  Write a balanced equation for the oxidation of Cl − by acidi ed MnO 4 − . In a redox reaction both oxidation and reduction occur together, so to work out the overall equation by combining two half-equations, one written as a reduction and one as an oxidation, both equations must contain the same number of electrons. Example: Cr 2 O 7 2– reacting with Fe 2+ . The two relevant half-equations for this reaction are: Cr 2 O 7 2– (aq) + 14 H + (aq) + 6e – 2 Cr 3+ (aq) + 7 H 2 O (l) Fe 3+ (aq) + e – Fe 2+ (aq) Both these are reductions as written above, but as we are starting with Fe 2+ (aq) rather than Fe 3+ (aq) we need to reverse the second reaction to make it into an oxidation: Fe 2+ (aq) Fe 3+ (aq) + e – To get the same number of electrons in each we must multiply this by 6 to give: 6 Fe 2+ (aq) 6 Fe 3+ (aq) + 6 e – Then we add both equations together, and then cancel out the electrons and anything else that is present in both reactants and products: Cr 2 O 7 2– + 14 H + + 6 e – + 6 Fe 2+ 2 Cr 3+ + 7 H 2 O + 6 Fe 3+ + 6 e – Cr 2 O 7 2– + 14 H + + 6 Fe 2+ 2 Cr 3+ + 7 H 2 O + 6 Fe 3+ In this example we must cancel out electrons only, but if there are any other species such as H + that are the same on both sides of the equation then these are cancelled out as well. Redox titrations You should be familiar with the technique of titration from your previous work in the AS course. Most redox titrations are carried out in the same way as acid-base titrations, except that an indicator is not always needed as the colours of the reactants allow the end point to be seen. 13 3.2 Redox reactions