1. Starter w.c. 31.10.11 In the following redox reactions use oxidation numbers to explain what has been oxidised and what has been reduced: a.N 2 + 3H 2  2NH 3 b.3Mg + 2Fe(NO 3 ) 3  3Mg(NO 3 ) 2 + 2Fe c.MnO 2 + 4HCl  MnCl 2 + Cl 2 + 2H 2 O

2. Answers a.N 2 + 3H 2  2NH 3 0 0 -3 +1 Nitrogen reduced: oxidation number decreases from 0 in N 2 to -3 in NH 3; hydrogen oxidised: oxidation number increases from 0 in H 2 to +1 in NH 3 3Mg + 2Fe(NO 3 ) 3  3Mg(NO 3 ) 2 + 2Fe 0 +3 +5 -2 +2 +5 -2 0 Iron reduced: oxidation number decreases from +3 in Fe(NO 3 ) 3 to 0 in Fe ; magnesium oxidised: oxidation number increases from 0 in Mg to +2 in Mg(NO 3 ) 2

3. Answers c. MnO 2 + 4HCl  MnCl 2 + Cl 2 + 2H 2 O +4 -2 +1 -1 +2 -1 0 +1 -2 Manganese reduced: oxidation number decreases from +4 in MnO 2 to +2 in MnCl 2; chlorine oxidised: oxidation number increases from -1 in HCl to 0 in Cl 2