2023 NJC H2 Chemistry Prelim Paper 2 QP-1

NJC SH2 Preliminary Examination 9729/02/23 1 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/02 28 August 2023 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use paper clips, highlighters, glue or correction fluid. Answers all questions. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /9 2 /10 3 /12 4 /13 5 /13 6 /18 Paper 2 Total /75 Marks Weightings Paper 1 /30 15% Paper 2 /75 30% Overall Percentage Paper 3 /80 35% Paper 4 /55 20% Grade This document consists of 22 printed pages.

NJC SH2 Preliminary Examination 9729/02/23 2 Answer all the questions in the spaces provided. 1 (a) Thioacetic acid, CH 3COSH, and ethanoic acid, CH 3COOH, both act as weak acids in solution. thioacetic acid ethanoic acid (i) An equilibrium is set up when CH3COSH is added to water. Write the equation for this equilibrium. …………………………………………………………………………………………….. [1] (ii) 0.150 mol of CH3COSH dissolves in 250 cm 3 of distilled water to produce a solution of pH 1.79. Calculate the pKa of CH3COSH. [2]

NJC SH2 Preliminary Examination 9729/02/23 3 [Turn over (iii) CH3COOH has a pKa value of 4.76. With reference to the Data Booklet, explain the difference in the pKa values of CH3COOH and CH3COSH. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2] (b) (i) State the type of reaction involved when CH 2ClCOOH and CH 2BrCOOH are synthesized from CH3COOH. …………………………………………………………………………………………….. [1] (ii) Prop-2-en-1-amine, CH 2=CHCH2NH2, commonly used as a pharmaceutical intermediate, can be synthesized from CH2BrCOOH. Complete the following synthetic route for the conversion of CH2BrCOOH into CH2=CHCH2NH2. Suggest the structure of the intermediate compounds, and reagents and conditions for step 1 and 2. C CBr H H OH O heat with Al2O3 step 1 step 2 CH2=CHCH2NH2 [3] [Total: 9]

NJC SH2 Preliminary Examination 9729/02/23 4 2 (a) A student studies the reaction of CH 3CHClCOOH with aqueous NH 3 to determine the reaction mechanism. The student finds that when CH3CHClCOOH and NH3 are added in a 1 : 1 stoichiometric ratio, the conjugate acid and base of the reactants are quickly formed. reaction I CH3CHClCOOH + NH3 ⎯→ CH3CHClCOO− + NH4+ Identify the conjugate acid-base pairs in reaction I. conjugate acid-base pair 1: ……………………………….……………………… conjugate acid-base pair 2: …………………………………………….………… [1] (b) In an excess of NH3, CH3CHClCOO− undergoes a nucleophilic substitution reaction. reaction II CH3CHClCOO− + NH3 ⎯→ CH3CH(NH2)COO− + H+ + Cl− (i) Describe the effect of an increase in temperature on the rate of reaction of CH3CHClCOO− and NH3, illustrating your answer using the Boltzmann distribution curve. [3]

NJC SH2 Preliminary Examination 9729/02/23 5 [Turn over A s tudent mixes CH 3CHClCOO− with a large excess of NH3 and monitored the [CH3CHClCOO−] with time. The graph in Fig. 2.1 shows the results obtained. Fig. 2.1 (ii) Use the graph in Fig . 2.1, deduce the order of reaction with respect to CH3CHClCOO− in reaction II. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2]

NJC SH2 Preliminary Examination 9729/02/23 6 (iii) Explain why a large excess of NH 3 needs to be used to obtain the results in Fig. 2.1. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [1] (iv) The student measured the effect of changing concentration of NH3 on the rate of reaction II. Table 2.1 shows the results obtained. Table 2.1 experiment [CH3CHClCOO−] / mol dm−3 [NH3] / mol dm−3 Initial rate of reaction / mol dm−3 s−1 1 0.00120 0.00300 1.47  10−5 2 0.00120 0.00450 2.21  10−5 Use the information in Table 2.1 and your answer in b(ii) to determine whether the nucleophilic substitution reaction proceeds via SN1 or SN2 mechanism. Explain your answer. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2] (v) When an excess of CH 3CHClCOO− is used in reaction II, further substitution reactions occur. One product has the formula C6H9NO42−, suggest its structure. [1] [Total: 10]

NJC SH2 Preliminary Examination 9729/02/23 7 [Turn over 3 This question is related to the properties of Group 17 elements and their compounds. (a) Table 3.1 shows the reactions between concentrated sulfuric acid and sodium halides. Table 3.1 halide reaction NaCl NaCl + H2SO4 ⎯→ HCl + NaHSO4 NaBr 2NaBr + 2H2SO4 ⎯→ Br2 + SO2 + Na2SO4 + 2H2O NaI 8NaI + 5H2SO4 ⎯→ 4I2 + H2S + 4Na2SO4 + 4H2O (i) Suggest the role of concentrated sulfuric acid in its reaction with NaCl. …………………………………………………………………………………………….. [1] (ii) By considering the changes in oxidation states of S, deduce whether NaBr or NaI is a stronger reducing agent. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2]

NJC SH2 Preliminary Examination 9729/02/23 8 (iii) ∆H1, is defined as the energy required to remove one mole of electrons from one mole of gaseous halide ions to form one mole of gaseous halogen atom. X−(g) ⎯→ X(g) + e− ∆H1 With reference to the ionic radii in the Data Booklet, suggest a reason for the trend of ∆H1 shown in Table 3.2. Table 3.2 halide ion ∆H1 / kJ mol−1 Cl−(g) +349 Br−(g) +324 I−(g) +295 …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2]

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