2019 DHS Prelim H2 Chem P2 ANS

© DHS 2019 [Turn over Name: Answers Index Number: Class: DUNMAN HIGH SCHOOL Prelims Examination 2019 Year 6 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions Additional Materials: Data Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Write in dark blue or black pen. 3 You may use an HB pencil for any diagrams or graphs. 4 Do not use staples, paper clips, glue or correction fluid. 5 Write your answers in the spaces provided on this question paper. The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings in calculations. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use Question No. Section A Marks 1 21 2 14 3 12 4 13 5 15 Total 75

2 © DHS 2019 9729/02 Answer all questions in the spaces provided. 1 This question explores the chemistry of transition metal complexes. (a) Cobalt cations readily form complexes with ligands. Some of the cobalt complexes with their absorption frequencies are shown in Table 1.1. The absorption frequency is the frequency of light absorbed for each complex, and is proportional to the energy of light absorbed. Table 1.1 Complex Absorption Frequency / cm–1 [Co(CN)6]4– 15300 [Co(CN)6]3– 33500 [Co(H2O)6]2+ 9300 [Co(H2O)6]3+ 18200 [Co(NH3)6]3+ 22870 (i) State the electronic configuration of the cobalt cation in [Co(CN)6]3–. Electronic configuration: 1s2 2s2 2p6 3s2 3p6 3d6 [1] (ii) Using the information in Table 1.1, state the relationship between the oxidation state of the metal cation and the absorption frequency of its complex. A higher oxidation state of the metal cation will result in a higher absorption frequency of the complex. [1] (iii) Stronger field ligands generate a d orbital splitting pattern with a larger energy gap when bonded to transition metal cations. Using the information in Table 1.1, rank CN –, H2O and NH 3 in order of increasing ligand field strength. Ligand field strength: H2O < NH3 < CN– [1] (iv) Hence, predict a value for the absorption frequency of [Co(NH3)6]2+. Any value between 9300 and 15300 (excluding boundaries) [1] (b) In the presence of an octahedral ligand field, the 3 d orbitals of cobalt are split into two energy levels.

3 © DHS 2019 9729/02 [Turn Over (i) Using the axes below, draw the shape of a 3d orbital of cobalt of  a higher energy level,  a lower energy level, in the presence of an octahedral ligand field. Higher energy level Lower energy level (accept dx2–y2) (accept dxy, dyz) [2] (ii) Hence, explain why the 3d orbitals of cobalt are split into two energy levels in the presence of an octahedral ligand field.