WJEC Chemistry for A2: Study and Rev Guide

3.1 Redox and standard electrode potential Redox reactions Grade boost If you are asked to identify oxidation or reduction then clearly state the oxidation state of the atom at the start of the reaction and at the end of the reaction and then state whether it has been oxidised (more positive), reduced (more negative) or neither (no change). QUICKFIRE  Work out the oxidation states of the atoms underlined in the following list: S 8 , Fe 3+ , NaCl , H 2 O , F 2 O , CaH 2 , AlCl 4 – , NaOCl , NaIO 3 , MnO 4 – . Pointer Oxidising agents become reduced as they oxidise something else. Reducing agents become oxidised as they reduce something else. Pointer Do not mix up charges and oxidation states – they are similar but not the same: +3 and −2 are oxidation states but 3+ and 2– are charges. In redox reactions, something is oxidised and something else is reduced. These processes can be defined in terms of electrons or in terms of oxidation states. Oxidation states Oxidation states measure how much an atom has been oxidised compared with the element. Oxidation states allow us to measure oxidation and reduction in covalent compounds as well as ionic compounds. The oxidation state of an atom is: 0 in elements, e.g. in Xe, O 2 , C 60 . Equal to the charge in ions, e.g. +2 in Fe 2+ . Negative for the most electronegative atom in a compound. +1 for group 1 metals in compounds and +2 for group 2 metals in compounds. +1 for hydrogen, except in metal hydrides where it is −1. −1 for fluorine in its compounds. −2 for oxygen in its compounds except in peroxides where it is −1. −1 for halogens in metal halides. The sum of all the oxidation states of atoms in a compound equals the charge on the species – for a neutral species the sum equals zero. Oxidation is where oxidation states become more positive; reduction is where oxidation states become less positive or more negative. Examples Cr 2 O 7 2– + 14 H + + 6 Fe 2+ 2 Cr 3+ + 6 Fe 3+ + 7 H 2 O Orange Green The oxidation state of chromium in the reactants is +6, and in the products it is +3. The chromium has been reduced – we say it is an oxidising agent. When we add an alkali (e.g. sodium hydroxide) to a solution containing dichromate(VI) ions, the reaction below occurs: Cr 2 O 7 2– + 2 OH – 2 CrO 4 2– + H 2 O Orange Yellow The oxidation state of chromium in the reactants is +6, and in the products it is +6. It has not been oxidised or reduced – this is not a redox reaction. Oxidation = loss of electrons OR oxidation state becoming more positive. Reduction = gain of electrons OR oxidation state becoming more negative. Key Terms 8 A2 Chemistry: Study and Revision Guide