2023 NJC H2 Chem Prelim P2 (Ans)

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 23 printed pages and 1 blank page.

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. CH3COSH + H2O CH3COS− + H3O+ or CH3COSH CH3COS− + H+ *Partial dissociation (reversible arrow) [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. Initial [CH3COSH] = 0.150 250/1000 = 0.600 mol dm−3 Equilibrium [H+] = 10−1.79 = 1.6218 × 10−2 mol dm−3 CH3COSH + H2O CH3COS− + H3O+ Initial / mol dm−3 0.600 0 0 Change / mol dm−3 −1.6218 × 10−2 +1.6218 × 10−2 +1.6218 × 10−2 Eqm / mol dm−3 0.58378 1.6218 × 10−2 1.6218 × 10−2 Ka = [CH3COS- ][H3O+] [CH3COSH] = (1.6218  10−2)2 0.58378 = 4.5055 × 10−4 pKa = −lg(4.5055 × 10−4) = 3.346 = 3.35 [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. The O−H bond (460 kJ mol −1) in CH3COOH is stronger than S−H bond (347 kJ mol−1) in CH3COSH, thus it is harder for O−H bond to break to release H + and CH3COOH has a smaller extent of dissociation, it has a smaller Ka than CH3COSH and hence a larger pKa than CH3COSH. [2] (b) (i) State the type of reaction involved when CH 2ClCOOH and CH 2BrCOOH are synthesized from CH3COOH.