2021 MI Prelim P2 Qns

Class Adm No Candidate Name: This question paper consists of 21 printed pages and 1 blank page. 2021 End-of-Year Exams Pre-University 2 H1 CHEMISTRY 8873/02 Paper 2 Structured Questions 20 Sep 2021 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. Section A Answer all the questions. Section B Answer one question. 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. Section A Section B Question 1 2 3 4 5 6 Total Marks 18 17 14 8 20 20 80

2 Section A Answer all the questions in this section in the spaces provided. 1 Chromic acid, H 2CrO4, is commonly used as a glassware cleaning reagent in laboratories. It is prepared by adding 60.0 g of potassium dichromate( VI), K 2Cr2O7, in 150 cm 3 of warm distilled water at 35.0 °C, and then slowly adding excess concentrated sulfuric acid to produce a 1 dm 3 chromic acid solution. During this process, the solution reached a maximum temperature of 38.5 °C. [Assume for the solution, density = 1.71 g cm–3 ; specific heat capacity = 4.18 J g–1 K–1.] Chromic acid is a strong acid which dissociates according to the following equation: H2CrO4 → H+ + HCrO4– ------------- Ka Chromic acid is also a strong oxidising agent, in which the HCrO 4– ion produced can be used to oxidise aldehydes (R-CHO) to carboxylic acids (R-COOH), where R represents an alkyl group. In the process, green Cr3+ ions are produced. The rate equation for this reaction is rate = k [R-CHO] [HCrO4–] (a) (i) Write a balanced ionic equation for the conversion of dichromate(VI) ions, Cr2O72–, into H2CrO4 during the preparation process. ………………………………………………………………………………………………… [1] (ii) Calculate the amount of K2Cr2O7 used in the preparation of chromic acid. [1] (iii) Hence, calculate the concentration of H 2CrO4 in the 1 dm 3 solution formed from the preparation process outlined above. [1]

3 [Turn over (iv) Calculate the mass of the 1 dm3 chromic acid solution. [1] (v) Hence, calculate the enthalpy change for the reaction in (a)(i). [2] (b) (i) Determine the oxidation numbers of C in R-CHO and in R-COOH. [Assume that the R group does not contribute to the oxidation number of C.] R-CHO: …………………………………… R-COOH: ………………………………... [1] (ii) Determine the oxidation numbers of Cr in HCrO4– and in Cr3+. HCrO4– : …………………………………… Cr3+ : ………………………………... [1] (iii) Hence, or otherwise, determine the molar ratio in which HCrO4– reacts with R-CHO. HCrO4– : R-CHO : [1]

4 (iv) Sketch, on the axes below, how the rate of the oxidation reaction varies with the concentration of R-CHO at: I: 298 K II: 398 K For this question, assume that the concentration of HCrO4– remains constant. [2] (v) With the aid of a Maxwell -Boltzmann distribution diagram, explain how the rate of the reaction between R-CHO and HCrO 4– changed when the temperature was increased from 298 K to 398 K. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [3] rate [R-CHO]

5 [Turn over (c) (i) By considering the electronic configuration of the Cr 3+ ion, determine the number of unpaired electrons for the Cr3+ ion in the ground state. ………………………………………………………………………………………………... [1] (ii) Suggest if the value of the dissociation constant of chromic acid, Ka, is larger or smaller than 1. Explain your answer. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………... [1] (iii) Given that H 2CrO4 is a simple molecule containing two Cr=O double bonds and two Cr –O single bonds, draw the displayed formula of H 2CrO4 and state the predominant intermolecular interaction between two molecules of H2CrO4. …………………………………………………………………………………………………[2] [Total: 18]

6 2 At a fixed temperature, nitrogen dioxide, NO2(g), was placed into a closed vessel of fixed volume and allowed to reach dynamic equilibrium. The equilibrium concentration of NO 2(g) was found to be 0.800 mol dm–3 and the equilibrium constant, Kc, has a value of 0.400. 2NO2(g) ⇌ N2O4(g) (a) (i) Define the term dynamic equilibrium. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (ii) Write an expression for the equilibrium constant, Kc, and state its units. [2] (iii) Calculate the equilibrium concentration of N2O4(g). [1] (iv) Using your answer in (a)(iii), calculate the initial concentration of NO2(g) that was placed into the closed vessel. [2]

7 [Turn over (v) Using your answer in (a)(iii), calculate the average molecular mass, to 4 significant figures, of all the gases in the mixture at equilibrium. [2] (b) (i) Draw a ‘dot-and-cross’ diagram for the NO2 molecule. [1] (ii) Based on your answer in (b)(i) and using information from the Data Booklet, suggest a value for the enthalpy change for the dimerisation of nitrogen dioxide. Include the sign and units in your answer. 2NO2(g) ⇌ N2O4(g) [1] (iii) Hence, draw a well-labelled energy profile diagram for the reaction in (b)(ii). [2]

8 (iv) Hence, or otherwise, explain how increasing the temperature will affect the position of equilibrium and suggest a value for the equilibrium constant. 2NO2(g) ⇌ N2O4(g) …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (c) (i) Explain why the first ionisation energy of nitrogen is higher than that of oxygen. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (ii) Explain if the ionic radius of oxygen is smaller or larger than that of nitrogen. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] [Total: 17]

9 [Turn over 3 (a) When magnesium chloride, MgC l2, and phosphorus pentachloride, PC l5, are separately placed in a beaker of hot water, they produce solutions of different pH values. Write balanced chemical equations, with state symbols, for the separate reactions of magnesium chloride and phosphorus pentachloride with hot water, and suggest the pH of the resulting solutions. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………....….. [4] (b) A period 3 oxide, A, was subjected to the following reactions.