23MIPrelim Answers (H2 Chem Paper 3)

Class Adm No Candidate Name: This question paper consists of 25 printed pages and 1 blank page. 2023 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 18 September 2023 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 Marks 24 19 17 20 80

2 Section A Answer all the questions in this section. 1 Aluminium is the most abundant metal in the Earth's crust. It has a great affinity towards oxygen, forming a protective layer of oxide on the surface when exposed to air. (a) Aluminium is protected from corrosion by a protective oxide layer. The thickness of the aluminium oxide layer on the surface of aluminium can be increased by anodising the aluminium. H2SO4 is used as an electrolyte in the anodising of aluminium. (i) Draw a well-labelled diagram to illustrate the electrolysis set-up for the anodisation of aluminium. [2] (ii) Write chemical equations to show the reactions at the anode and cathode during anodising. Include the overall equation in your answer. [3] (iii) The aluminium piece to be anodised has a surface area of 29.2 cm2. Determine the current required to pass through the set -up for 1.8 hours to form a 0.2 mm-thick protective layer of Al2O3 on the aluminium piece. [Density of Al2O3 is 3.95 g cm−3] [4] (iv) Give one example of an anodised aluminium object, and explain the advantages of anodising it. [1] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… • Correct orientation of the battery • Correct labeling of both the electrodes with the material used • Correct cell. i.e. An electrolytic cell and not a electrochemical cell (ii) Anode: 2H2O → O2 + 4H+ + 4e- 4Al + 3O2 → 2Al2O3 Cathode: 2H+ + 2e- → H2 Overall: 2Al + 3H2O → Al2O3 + 3H2

3 [Turn over ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (b) Aluminium sulfate, A l2(SO4)3, decomposes upon heating, forming aluminium oxide, sulfur dioxide gas and oxygen gas. In an experiment, a 150 g sample of impure aluminium sulfate was heated until the sample reached a constant mass of 83.6 g. (i) Write a balanced equation, with state symbols, for the decomposition of solid aluminium sulfate. [1] (ii) Determine the percentage purity of the sample, assuming the impurities remained inert during the decomposition. [4] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (iii) Volume of Al2O3 = 29.2 × 0.02 = 0.584 cm3 Mass of Al2O3 = 3.95 × 0.584 = 2.3068 g Amount of Al2O3 = 2.3068 2(27.0)+3(16.0) = 0.02262 𝑚𝑜𝑙 Amount of O2 needed = 0.02262 × 3 2 = 0.033923 𝑚𝑜𝑙 Amount of e- = 4 × 0.03392 = 0.13569 mol Q = 0.13569 × 96500 = 13094 C Current needed = 13094 1.8×60×60 = 2.02 𝐴 (iv) Examples of anodized aluminium objects: drink cans, windows frames and grilles Anodised aluminium objects are more resistant to corrosion. Moreover, they can be decorative as well since the aluminium oxide layer is able to absorb dyes. (i) Al2(SO4)3(s) → Al2O3(s) + 3SO2(g) + 3 2O2(g) (ii) Mass of gas lost = 150 – 83.6 = 66.4 g Let the amount of O2 be a, hence amount of SO2 = 2a Mass of gas lost = a(32.0) + 2a(32.1+ 32.0) = 66.4 32a + 128.2a = 66.4 a = 0.41448 mol amount of Al2(SO4)3 = 2 3 × 0.41448 = 0.2763 𝑚𝑜𝑙 mass of Al2(SO4)3 = 0.2763 × [2(27.0) + 3(32.1) + 12(16.0)] = 94.58 g % purity = 94.58 150 = 63.1%

4 ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (c) Aluminium chloride is an active ingredient used in skin medication to control excessive sweating. In the vapour phase, a dynamic equilibrium is established between aluminium chloride and its dimer as follows: 2AlCl3(g) ⇌ Al2Cl6(g) (i) Aluminium chloride is often described as electron deficient. Explain what is meant by electron deficient. [1] (ii) With the aid of a diagram, explain how the dimer is formed. [2] (iii) Explain what is meant by dynamic equilibrium. [1] (iv) Write an expression for the equilibrium constant, Kp, for the dimerisation. [1] (v) In an experiment, 10 atm of A lCl3(g) was allowed to reach equilibrium in a sealed vessel. At equilibrium it was found that the total pressure was 6.8 atm. Calculate the value of Kp, stating its units. [3] (vi) Suggest a reason why the dimerisation would not take place readily if th e AlCl3 is placed in an aqueous system. [1] [Total: 24] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………