2020 ACJC Prelim Paper 3 Qns

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 Candidates answer on the Question Paper. 2 hours Additional Materials: Data Booklet 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 the 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 29 printed pages and 1 blank page. 9729/03/Prelim/20 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2020 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 appropriate working, deduce the order of reaction with respect to each compound. Hence write a rate equation for the reaction. [3] (ii) Calculate the value of the rate constant, stating its units. [2] ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… ………………………………………………………………………………………….. ……………………………………………………………………………………...…… …………………………………………………………………………………………...

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4 © ACJC2020 9729/03/Prelim/2020 [Turn over (b) The structural formula of the BH4 is shown below. The reaction between propanone and sodium borohydride occurs in the following manner.  In the first step, the BH4– ion transfers a hydride to the carbonyl carbon of propanone, forming the CH3CH(O–)CH3 ion.  In the next step, the CH3CH(O–)CH3 ion accepts a proton from water to form CH3CH(OH)CH3. (i) By referring to your rate equation in (a)(i) and the steps described above, name and describe the mechanism for the reaction between propanone and sodium borohydride. In your answer, you should s how all charges and lone pairs and sho w the movement of electrons by curly arrows. [3] (ii) Experiment 1 was repeated under identical conditions using propanal. Explain how the initial rate of reaction will change. [2] ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… ……………………………………………………………………………………...……

5 © ACJC2020 9729/03/Prelim/2020 [Turn over (c) (i) Propanone reacts with 2,4-dinitrophenylhydrazine to form an orange precipitate. Draw the structure of the orange precipitate. [1] (ii) Describe a simple chemical test that could distinguish between propanone and propanal. [2] (iii) Suggest a suitable three step reaction pathway to convert propan -2-ol to 2-methylpropanoic acid. [5] …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... ……………………………………………………………………………………...…… …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………...

6 © ACJC2020 9729/03/Prelim/2020 [Turn over (d) In the presence of a strong base, carbonyl compounds undergo the aldol condensation to form a 3-hydroxy