2020 ACJC Prelim Paper 3 Answers

9 Index No. Name Form Class Tutorial Class 2CH________ Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY H2 9729/03 Paper 3 Free Response 1 September 2020 2 hours Cover page READ THESE INSTRUCTIONS FIRST Write your index number and name, form class and tutorial class 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 in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. The use of an approved 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. Please fill in the question number for the question attempted. Section Question No. For Examiner’s Use Marks A 1 2 3 B Presentation of answers TOTAL: 80 m This document consists of 33 printed pages and 1 blank page. 9729/03/Prelim/20 © ACJC 2020 ANGLO-CHINESE JUNIOR COLLEGE Department of Chemistry [Turn over

2 © ACJC2020 9729/03/Prelim/2020 [Turn over Section A Answer all the questions in this section. 1 Sodium borohydride, NaBH4, reduces carbonyl compounds to secondary alcohols as shown below. CH3COCH3 + NaBH4 + H2O  CH3CH(OH)CH3 + BH3 + NaOH (a) (i) The kinetics of this reaction was studied using propanone in the presence of aqueous NaBH4. Table 1.1 shows the experimental data obtained. Table 1.1 experiment initial concentration / mol dm3 initial rate of formation of CH3CH(OH)CH3 / mol dm3 s1 [NaBH4] [CH3COCH3] 1 0.0100 0.200 6.60 × 105 2 0.0080 0.200 5.28 × 105 3 0.0100 0.100 3.30 × 105 By showing approp riate working, deduce the order of reaction with respect to each compound. Hence write a rate equation for the reaction. [3] Comparing experiments 1 and 2, When [NaBH4] is increased 1.25 times, rate is increased 1.25 times. Rate is directly proportional to [NaBH4] Order of reaction with respect to NaBH4 = 1 Comparing experiments 1 and 3, When [propanone] is doubled, rate is doubled. Rate is directly proportional to [propanone] Order of reaction with respect to propanone = 1 Rate = k [NaBH4] [propanone] (ii) Calculate the val