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.