RI Y4 Chem 2022 FE Revision Redox ANS.pdf

1 Practice Questions 1 B 2 C 3 B 4 oxidation: Mg  Mg2+ + 2e− reduction: 2H+ + 2e−  H2 5 Cu2+ gains electrons to form Cu. Hence, copper(II) ions, Cu2+, are reduced to copper, Cu. Zn loses electrons to form Zn2+ – this is oxidation Hence, zinc, Zn, is oxidized to zinc ions, Zn2+. 6 The oxidation state of nitrogen, N, increased from −3 in NH3 to 0 in N2. Thus NH3 is oxidised. The oxidation state of copper, Cu, decreased from +2 in CuO to 0 in Cu. Thus CuO is reduced. 7 The oxidation state of nitrogen, N, increased from +4 in NO 2 to +5 in HNO3. Thus NO2 is oxidised. The oxidation state of oxygen, O, decreased from 0 in O2 to −2 in HNO3. Thus O2 is reduced. 8 The oxidation state of chromium, Cr, decreased from +6 in Cr 2O72− to +3 in Cr3+. Thus Cr2O72− is reduced. The oxidation state of sulfur, S, increased from +4 in SO32− to +6 in SO42−. Thus SO32− is oxidised. 9 (a) amount of H 2O2 reacted = 0.0470 x 0.0250 = 0.001175 mol (b) amount of KMnO 4 reacted = 0.0200 x 0.0235 = 0.000470 mol (c) x:y = 0.000470:0.001175 = 2:5 (d) The purple KMnO 4 solution becomes colourless. (e) The oxidation state of oxygen, O, increased from −1 in H 2O2 to 0 in O2. Thus H2O2 is oxidised, which means H2O2 is the reducing agent. 10 A is oxidation. CH4 gained an oxygen to form CH4O. B is oxidation. CH 4O lost 2 hydrogen to form CH2O. C is oxidation. CH2O gained an oxygen to form CH2O2. D is oxidation. CH 2O2 lost 2 hydrogen to form CO2. Raffles Institution Raffles Programme Year Four Chemistry Name: _________________________________ ( ) Class: _________ Date: ______________ 2022 FE Revision – Redox ANSWERS