2021 MI Prelim P2 Ans

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 xxth 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, with the general formula 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–] For Examiners’ Use (a) (i) Write a balanced ionic equation for the conversion of dichromate(VI) ions, Cr2O72–, into H2CrO4 during the preparation process. ………………………………………………………………………………………………… [1] Cr2O72– + H2O + 2H+ → 2H2CrO4 ; (ii) Calculate the amount of K2Cr2O7 used in the preparation of chromic acid. [1] Amount of K2Cr2O7 = 60.0 ÷ [ 2(39.1) + 2(52.0) + 7(16.0) ] = 60.0 ÷ 294.2 = 0.204 mol (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 Amount of K2Cr2O7 : Cr2O72– : H2CrO4 = 1 : 1 : 2 (ecf from (a)(i)) Concentration of H2CrO4 = 2(0.204) ÷ 1 = 0.408 mol dm–3 ; (iv) Calculate the mass of the 1 dm3 chromic acid solution. [1] Mass of solution = 1.71 × 1000 = 1710 g ; (v) Hence, calculate the enthalpy change for the reaction in (a)(i). [2] Q = mcΔT = (1710)(4.18)(38.5 - 35.0) = 25 017 J ΔH = –Q / n = –25 017 / 0.204 = –122 632 J mol–1 = –122 kJ mol–1 (3 s.f.) (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] For Examiners’ Use R-CHO: +1 ; R-COOH: +3 ; (ii) Determine the oxidation numbers of Cr in HCrO4– and in Cr3+. HCrO4– : …………………………………… Cr3+ : ………………………………... [1] HCrO4– : +6 Cr3+ : +3 ; (both) (iii) Hence, or otherwise, determine the molar ratio in which HCrO4– reacts with R-CHO. HCrO4– : R-CHO : [1] 2 : 3 (iv) Sketch, on the ax es 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.

4 [2] ECF for rate-conc graph drawn for 0 or 2nd order but correct shows increase in T leading to increase in gradient. (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. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… 398 K ; 298 K ;

5 [Turn over ………………………………………………………………………………………………… [3]  When temperature is increased, average kinetic energy of particles increases.  Proportion of particles with Energy ≥ Ea increases,  Frequency of effective collision increases, leading to increase in rate of reaction. (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] For Examiners’ Use (1s2 2s2 2p6 3s2 3p6 3d3 )  3 unpaired electrons. (ii) Suggest if the value of the dissociation constant of chromic acid, Ka, is larger or smaller than 1. Explain your answer. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………... [1] Ka is larger than 1 as chromic acid is a strong acid and hence dissociates completely / concentration of products is much larger than concentration of reactants in K a expression / larger numerator and small denominator or OWTTE ; (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. - molecules with energy ≥ Ea at T = T1 - molecules with energy ≥ Ea at T = T2 Fraction of molecules Kinetic energy T1 T2 Ea Ea : Activation energy

6 …………………………………………………………………………………………………[2] [Total: 18] ; Hydrogen bonding ; 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) For Examiners’ Use (a) (i) Define the term dynamic equilibrium. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] A dynamic equilibrium is a reversible reaction in which the forward and backward reaction rates are the same and the overall / net concentration of reactants and products remain unchanged. (ii) Write an expression for the equilibrium constant, Kc, and state its units. [2] Kc = [𝑁2𝑂4] [𝑁𝑂2]2 units: mol–1 dm3 (iii) Calculate the equilibrium concentration of N2O4(g).

7 [Turn over [1] Kc = [𝑁2𝑂4] [𝑁𝑂2]2 = 0.400 = [𝑁2𝑂4] (0.800)2 [N2O4]eqm = 0.400 × (0.800)2 = 0.256 mol dm-3 ; (iv) Using your answer in (a)(iii), calculate the initial concentration of NO2(g) that was placed into the closed vessel. [2] 2NO2(g) → N2O4(g) Initial conc / mol dm-3 x 0 Change in conc / mol dm-3 –2 × (0.256) +0.256 Eqm conc / mol dm-3 x – 0.512 = 0.800 0.256 ; (table not required) x = 0.800 + 0.512 = 1.312 = 1.31 mol dm-3 (3 s.f.) ; (v) Using