2022 MI H2 Chem Prelim Paper 3 - Question

Class Adm No Candidate Name: This question paper consists of 23 printed pages and 1 blank page. 2022 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 19 September 2022 2 hours Candidates answer on the Question Paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so. Write your name, class and admission number in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the page at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question A B Total 1 2 3 4 5 / 6 Marks 16 12 18 14 20 80

2 Section A Answer all the questions in this section. 1 (a) The Bunsen cell is an electrochemical cell built to provide electrical energy. It consists of a zinc anode in dilute sulfuric acid, separated by a salt bridge from a carbon cathode in dilute nitric acid. Nitrogen dioxide, NO 2, is the only gas formed from the reaction. At the anode, zinc decreases in size during the reaction. (i) Draw a fully labelled diagram of the Bunsen cell. [3] (ii) Use data from the Data Booklet to construct an overall equation for the reaction. [2] (iii) Calculate the cell potential, E, for the reaction. [1] (iv) The experimental E of the Bunsen cell is +1.9 V. Use ideas about reversible reactions to suggest why the experimental E may differ from the calculated value in (iii). [2] ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….……………………………………………………………………………….

3 [Turn over (b) Nitrogen dioxide, NO2, and dinitrogen tetroxide, N2O4, exists at equilibrium as a mixture: 2NO2(g) ⇌ N2O4(g) (i) Draw the ‘dot-and-cross’ diagram for N2O4. [1] (ii) The standard enthalpy changes of formation for these gases are shown. gas NO2 N2O4 ΔHfo / kJ mol–1 +33.2 +9.2 Calculate the standard enthalpy change for the reaction shown above. [1] (iii) Explain why the sign of the value calculated in (ii) is as such. [1] (iv) The standard entropy change for the reaction is –175.8 J mol–1 K-1. Explain why the value is negative. [2] (v) Explain how the Gibbs free energy change, ΔG, of the reaction will vary with temperature. [2] (vi) A sample of NO2(g) was sealed in a container. Its initial pressure was 1.00 bar. After reaching equilibrium, 0.351 bar of N2O4(g) was formed. Calculate the equilibrium constant, Kp, for this equilibrium. [1] [Total: 16] ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….……………………………………………………………………………….

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5 [Turn over 2 (a) The copper(II) ion forms a coloured complex ion with six water molecules. (i) Use your knowledge of VSEPR theory to name the shape of and state the bond angle in the complex. Explain your reasoning. [2] (ii) Explain why the 3d orbitals of the copper(II) ion are split into 2 separate energy levels. [2] The water molecules on the z-axis (axial ligands) can move further away from the copper(II) ion (Fig. 2.1), resulting in a further splitting of the orbitals into separate energy levels (Fig. 2.2). This is known as Jahn–Teller distortion. As the new ene rgy levels (Fig. 2.2) are close together, the electrons remain in their respective orbitals. The complex is stabilised as the Cu2+ electrons now have a lower overall energy. Jahn–Teller distortions will only be able to decrease the overall energy if the metal ion has an odd number of 3d electrons. Fig. 2.1 Fig. 2.2 S OH OH OH OH OH OH S OH OH OH OH OH OH H2O H2O H2O H2O OH2 OH2 Cu 2+ Cu 2+ H2O H2O OH2 OH2 H2O H2O Jahn–Teller distortion xy yz xz x 2 -y 2 z 2 x 2 -y 2 z 2 net decrease in energy for one electron no change in energy level

6 (iii) Explain why the z2 orbital is lower in energy after the further splitting. [1] (iv) Predict if each of the ions will exhibit Jahn–Teller distortion: 1) Co2+ 2) Ni2+. Explain your reasoning. [2] (v) Unlike complex ions of Cu2+, complex ions of Cu+ are not coloured. Write the electronic configuration of Cu + and explain why its complex ions are not coloured. [2] ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….………………………………………………………………………………. ……….………….…………………………………