2019 DHS Prelim H2 Chem P2 QP

This question paper consists of 21 printed pages and 1 blank page. © DHS 2019 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Prelims Examination 2019 Year 6 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 18 September 2019 2 hours 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–. ……………………………………………………………………………………..[1] (ii) Using the information in Table 1.1, s tate the relationship between the oxidation state of the metal cation and the absorption frequency of its 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. ……………………………………………………………………………………..[1] (iv) Hence, predict a value for the absorption frequency of [Co(NH3)6]2+. ……………………………………………………………………………………..[1]

3 © DHS 2019 9729/02 [Turn Over (b) In the presence of an octahedral ligand field, the 3 d orbitals of cobalt are split into two energy levels. (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 [2] (ii) Hence, explain why the 3d orbitals of cobalt are split into two energy levels in the presence of an octahedral ligand field. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………..[2]

4 © DHS 2019 9729/02 (iii) In the presence of a tetrahedral ligand field, the ligands approach in between the axes. Complete Fig 1.1 to show how the degenerate 3d orbitals of cobalt will split in a tetrahedral ligand field. Label all the orbitals in your diagram. Energy Fig 1.1 [2] (c) Another cobalt complex, [CoCl6]3 undergoes ligand exchange with CN – ligands to form [Co(CN)6]3– according to the following equilibrium. [CoCl6]3(aq) + 6CN–(aq) [Co(CN)6]3–(aq) + 6Cl–(aq) The effect of pH on the concentration of [Co(CN)6]3– formed is shown in the graph in Fig 1.2 . Fig 1.2 (i) Write an equation that describes the dissociation of the weak acid, HCN, in water. ……………………………………………………………………………………..[1] pH conc. of [Co(CN)6]3–

5 © DHS 2019 9729/02 [Turn Over (ii) Hence, using Le Chatelier’s Principle, explain the shape of the graph in Fig 1.2. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………..[2] (iii) Explain why the Gibbs free e nergy change and enthalpy change for the forward reaction are approximately equal. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………..[1] (iv) Hence deduce the sign of the enthalpy change of the forward reaction given that it is spontaneous. ……………………………………………………………………………………….. ……………………………………………………………………………………..[1]

6 © DHS 2019 9729/02 (d) Similar to organic compounds, transition metal complexes may exhibit stereoisomerism. (i) For square planar complexes, cis-trans isomerism can be exhibited for MA2B2 type complexes (where M is the metal cation, and A and B are different ligands). cis and trans isomers are differentiated in the following way: cis: Both A ligands next to one another trans: Both A ligands are separated by a B ligand [Pt(Cl)2(NH3)2] is a square planar complex. Complete the diagrams below to show the structures of its cis and trans isomers. Cis isomer Pt Trans isomer Pt [1] (ii) For any transition metal complex, enantiomerism is exhibited if it has:  Mirror images that are non-superimposable and  No internal plane of symmetry Deduce if the trans isomer of [Pt(Cl)2(NH3)2] can exhibit enantiomerism. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………..[1]

7 © DHS 2019 9729/02 [Turn Over (e) The cyclopentadienyl anion is a common ligand in transition metal complexes. This anion can be generated via the following reaction between tert–butoxide anion and cyclopentadiene. H H O+ H OH+ Cyclopentadiene tert-butoxide cyclopentadienyl anion An organic compound is considered aromatic if all of the following three criteria are met:  it is a cyclic planar compound,  it has a delocalised π electron system, and  it has (4n + 2) π electrons, where n is an integer. One such example is benzene. (i) State the type of reaction that generates the cyclopentadienyl anion from cyclopentadiene. ……………………………………………………………………………………..[1] (ii) Given that the cyclopentadienyl anion is aromatic, explain fully how it meets the three criteria stated above. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………..[3] [Total: 21]

8 © DHS 2019 9729/02 2 Halogenoalkanes are widely used commercially as they are important precursors for many organic synthesis. (a) One method of synthesising halogenoalkanes is the free radical substitution of alkanes. For example, butane can be chlorinated in the presence of UV light. However, this is not a preferred method of synthesising halogenoalkanes as there are multiple by –products. For example, one by –product of the monochlorination of butane is shown below. 3,4–dimethylhexane (i) Draw the mechanism of the reaction between butane and chlorine that would result in the formation of 3,4–dimethylhexane. Show the movement of electrons by using suitable curly arrows and indicate any unpaired electron using a dot (). [3] (ii)