2022 MI H2 Chem Prelim Paper 2 - Question

Class Adm No Candidate Name: This question paper consists of 17 printed pages and 1 blank page. 2022 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 14 Sep 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 on all the work you hand in. 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 1 2 3 4 5 Total Marks 20 13 24 9 9 75

2 Answer all questions in the spaces provided. 1 Many household tools are made from stainless steel as it is highly resistant to corrosion and rust. Stainless steel contains iron, chromium, manganese, nickel, molybdenum, silicon and carbon which interacts with oxygen from water and air to form oxides. Of these elements, chromium has the highest resistance to corrosion. Despite the high resistance, rusting still occurs in a humid environment. A ‘Do -it-Yourself’ (DIY) kit has been launched in the market to remove rust from household tools and the set-up is shown in Figure 1.1. Figure 1.1 (a) (i) State if the iron sheet is the anode or the cathode. ………………………………………………………………………………………………….[1] (ii) Use of Data Booklet is relevant to this question. State and explain the observations found at the iron sheet electrode. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. ………………………………………………………………………………………………….[2] Rusty tool Na2CO3 (aq) Iron sheet

3 [Turn over (iii) Suggest why carbonate ions from Na2CO3 electrolyte cannot be oxidised at the anode. …………………………………………………………………………………………………….. ………………………………………………………………………………………………….[1] (iv) A consumer saw the DIY kit and decided to purchase it and try out on his rusted dumbbell. He prepared the set up as shown in Figure 1.1 with a 20 A power source and switched it on for 6 hours. Determine the change in mass of the iron sheet. [2] (b) Fe2O3 is the main constituent of the rust found on household items. The standard electrode potentials involving the iron containing species can be found in Table 1.1. Table 1.1 Half-equations Eθ/V 3Fe2O3 + H2O +2e ⇌ 2OH– + 2Fe3O4 –0.62 Fe2O3 + 3H2O + 6e ⇌ 2Fe + 6OH– –0.88 Fe3O4 + 4H2O + 8e ⇌ 3Fe + 8OH– –0.91 A layer was coated around the rusty tool after using the DIY kit. With reference to the standard electrode potentials given in Table 1.1, explain fully whether the layer coated is iron. ……………………………………………………………………………………………………..…… ………………………………………………………………………………………………..………… …………………………………………………………………………………………..……………… …………………………………………………………………………………………..……………… ……………………………………………………………………………………..…………………… ………………………………………………………………………………..……………………...[2]

4 (c) The metal alloy that made up the main components of stainless steel are transition metals. (i) Define transition metal. …………………………………………………………………………………………………….. ………………………………………………………………………………………………….[1] (ii) State a physical property of transition metals which makes them a suitable material used for household tools compared to main group metals. ………………………………………………………………………………………………….[1] (iii) Iron(III) ions catalyse the reaction between I- ions and S2O82- ions through homogeneous catalysis. Explain the term homogeneous catalyst. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..………………...[1] (iv) By considering relevant Eo values from the Data Booklet, and using balanced equations, determine whether chromium(III) ions are suitable catalyst for the reaction between I– ions and S2O82- ions. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..…………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..…………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..…………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..………………...[2]

5 [Turn over (v) Describe the structure and bonding in the element chromium. Draw a diagram to illustrate your answer. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………………………..………………..[2] (d) Chromium(III) oxide has the same acid-base behaviour as aluminium oxide. (i) State the type of oxide chromium(III) oxide exists as in terms of its acid-base behaviour. ………………………………………………………………..………………………………..[1] (ii) By quoting relevant data from the Data Booklet, explain why c hromium(III) oxide and aluminium oxide have the same acid-base behaviour. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..………………………………...[2] (iii) Use of Data Booklet is relevant to this question. Describe the observations when dilute NaOH is slowly added to the solution containing chromium(III) ions until in excess. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ………………………………………………………………..………………………………..[2] [Total: 20]

6 2 (a) An aqueous solution of chlorine dioxide undergoes disproportionation in sodium hydroxide to form chlorate(V) ion, ClO3– , and hypochlorite ion, C lO–. The reaction is catalysed by OBr– ion. 4ClO2(aq) + 4OH–(aq) → 3ClO3–(aq) + ClO–(aq) + 2H2O(l) To study the kinetics of this reaction at 25 °C, initial rates were measured using various concentrations of ClO2, OH– and OBr–. experiment [ClO2]/ mol dm–3 [OH–]/ mol dm–3 [OBr-] / mol dm-3 initial rate/ mol dm–3 s–1 1 0.02 0.05 0.01 2.30 x 10–3 2 0.10 0.02 0.03 6.90 x 10–2 3 0.02 0.10 0.01 4.60 x 10–3 4 0.04 0.02 0.03 1.10 x 10-2 (i) Explain, in terms of oxidation numbers, why this reaction is a disproportionation reaction. ………………………………………………………………..…………………………………… ………………………………………………………………..…………………………………… ……