ASR 2020 J2Prelim H2Chem P3 QP

ASRJC JC2 PRELIM 2020 9729/03/H2 [Turn Over ANDERSON SERANGOON JUNIOR COLLEGE 2020 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 20 /____ CHEMISTRY 9729/03 Paper 3 Free Response 21 September 2020 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and register 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. 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 the booklet. The question number must be clearly shown. 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. *Please circle the question you have attempted. For Examiner’s Use 1 /16 2 /21 3 /23 4* /20 5* /20 Total /80 This document consists of 32 printed pages. Paper 1 Paper 2 Paper 3 Paper 4 Percentage Grade /30 /75 /80 /55 /100

2 ASRJC JC2 PRELIM 2020 9729/03/H2 Section A Answer all questions in this section. 1 Many naturally occurring organic compounds contain either acidic groups or basic groups or both. The strength of the acid or the base depends on the structure of the molecule. (a) The pKa values of ethanoic acid and two amino acids are given in Table 1.1 below. Table 1.1 name structure pKa1 pKa2 pKa3 ethanoic acid CH3COOH 4.7 – – serine (ser) H2N CH CO2H CH2 OH 2.2 9.2 – aspartic acid (asp) H2N CH CO2H CH2 CO2H 1.9 3.7 9.6 (i) Suggest two reasons why the pKa1 value of serine is so much less than the p Ka1 of ethanoic acid. [2] (ii) Draw a skeletal structure for the dipeptide, asp–ser. [2] Electrophoresis is a technique used to separate charged particles placed in an electric field. The system consists of two oppositely –charged electrodes connected by a conducting medium, typically a gel. The separation of the charged particles is based on their electrical charge and Mr. A sample consisting of serine and aspartic acid was analysed by electrophoresis using a gel buffered at pH 5.7. The small quantity of the mixture was placed at the centre of the gel, at equal distance from the two electrodes. (iii) Using the information given in Table 1.1, suggest the structures of the major species present in the buffer solution. Label the structures clearly. [2] (iv) Hence, describe the relative positions of the two species in the gel after the separation by the electrophoresis process. Explain your answer. You may find it helpful to draw a diagram to illustrate your descriptions. [2] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………………………………

3 ASRJC JC2 PRELIM 2020 9729/03/H2 [Turn Over ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………………………………

4 ASRJC JC2 PRELIM 2020 9729/03/H2 (b) A student prepa red a buffer solution of pH 5.7 by adding solid sodium hydroxide to 200 cm3 of 0.200 mol dm–3 ethanoic acid. (i) With reference to the information in Table 1.1, calculate the mass of sodium hydroxide the student would have used. [2] A buffer solution generally lost its effectiveness when one component of the buffer pair is less than about 10% of the other. (ii) Calculate the percentage of ethanoic acid component present in the buffer solution in (b)(i) and hence comment on the effectiveness of the buffer. [2] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………………………………

5 ASRJC JC2 PRELIM 2020 9729/03/H2 [Turn Over (c) Nicotinic acid and its amide, nicotinamide, are present in Vitamin B3 and are used to increase good cholesterol levels. N O OH N O NH2 1 2 nicotinic acid nicotinamide The structure of nicotinamide consists of a single delocalised system of electrons which includes:  an electron in the unhybridised p–orbital of the nitrogen atom in the six–membered ring  electrons from the carbon atoms of the six–membered ring  two electrons in the  bond of the >C=O group  two electrons in lone pair on the nitrogen atom of the amide group (i) State the number of delocalised electrons in one molecule of nicotinamide. [1] (ii) Using the information provided, suggest the H–N–H bond angle in the NH 2 group in nicotinamide. [1] (iii) Predict and explain the relative basicity of the two N atoms, labeled N 1 and N2, in the nicotinamide molecule. [2] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ……………………………………………………………………………………………