IJC_H2_CHEM_P3_ANS

Answers (Prelim 2 H2 Paper 3) 1 PRELIM 2 © INNOVA 9647/03/2011 [Turn over 1 (a) (i) Manganese(IV) oxide, MnO 2 is the most common starting material for the production of compounds of manganese of other oxidation states. With reference to the Data Booklet , state which of the following ions would convert manganese(IV) oxide to manganese(II) sulfate in an acidic solution. P b 4+(aq) / Pb2+(aq) E = +1.69V C o 3+(aq) / Co2+(aq) E = +1.82V F e 3+(aq) / Fe2+(aq) E = +0.77V Co3+(aq) / Co2+(aq) E  = + 1.82V Pb4+(aq) / Pb2+(aq) E  = + 1.69V MnO2(s) / Mn2+(aq) E  = + 1.23V Fe3+(aq) / Fe2+(aq) E  = + 0.77V Only can choose Fe2+(aq) (ii) Calculate the E cell value to prove that the reaction between MnO 2 and the ion you have chosen in a(i) is feasible and write a balanced equation for the reaction. Eq(1) MnO 2 + 4H+ + 2e Mn2+ + 2H2O +1.23 Eq(2) Fe 3+ + e Fe2+ +0.77 Overall: MnO2 (s) + 4H+ (aq) + 2Fe2+ (aq)  Mn2+ (aq) + 2H2O (aq) + 2Fe3+ (aq) E θ cell = (+1.23) – (+0.77) = +0.46V Since Eθ cell > 0, reaction is feasible. (iii) What would be the colour of the final solution observed? Orange (Pale pink + Yellow) Also accept yellow [5] (b) (i) Write the full electronic configuration of Zn2+ and Mn2+. Zn2+ : 1s2 2s2 2p6 3s2 3p6 3d10 Mn2+: 1s2 2s2 2p6 3s2 3p6 3d5 (ii) Explain why manganese forms compounds that are often coloured whereas compounds of zinc are usually white. Compounds of Mn is coloured due to the presence of incompletely filled 3d– orbitals in the Mn 2+ metal ions. In the complex ion, the electrostatic field produced by the donated lone pairs on the ligands, splits the five orbitals into a group of three and a group of two . An electron in a lower d orbital energy can absorb radiation and be promoted into the higher orbital energy level by absorbing photons of light in the visible light region (d-d transitions). The colour seen is the complement of those absorbed in the visible region of the spectrum. Zn2+ is colourless because its d orbitals are completely filled. It is not possible to promote an electron to the higher energy level. [5]

Answers (Prelim 2 H2 Paper 3) 2 PRELIM 2 © INNOVA 9647/03/2011 [Turn over (c) MnO2 undergoes the following reaction as shown in the schematic diagram below: Step I: Oxidation of MnO 2 in molten KOH with oxygen from the air produces a green compound, A. Step II: Reaction of A with dilute acid produced 0.174 g of MnO 2 and a purple solution of compound B was observed. A solution of compound B is just decolourised by 40.0 cm3 of 0.500 mol dm-3 iron(II) sulphate. (i) B contains potassium, mangane