2022 DHS Y6 H2 Prelim Paper 2_Suggested Solutions

© DHS 2022 9729/02 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 13 September 2022 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. 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. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 12 2 12 3 14 4 16 5 21 Total 75 This document consists of 18 printed pages.

2 © DHS 2022 9729/02 Answer all the questions in the spaces provided. 1 (a) Describe the thermal decomposition of the hydrogen halides HCl, HBr and H I and explain any variation in their thermal stabilities. [3] Hydrogen halides thermally decompose to give its constituent elements / show equation: 2HX → H2 + X2. HCl does not decompose even on strong heating. HBr decomposes on strong heating while HI decomposes readily in the presence of a hot rod. Thermal stability decreases in the order HC l > HBr > H I as bond energy / bond strength of the H –X bond decreases from HC l to HBr to H I. As such, decreasing amount of energy is required to decompose the hydrogen halide from HCl to HI. (b) Tert-butyl alcohol reacts with hydrogen chloride according to the equation shown. (CH3)3COH + HCl → (CH3)3CCl + H2O This reaction occurs in three steps. step 1 protonation of –OH group in (CH3)3COH to produce (CH3)3COH2+ cation step 2 loss of H2O molecule from (CH3)3COH2+ to produce a carbocation step 3 chloride ion reacts with carbocation to produce (CH3)3CCl (i) Describe the mechanisms which occur in steps 2 and 3. Use curly arrows to show the movement of electrons and label the slow step. [3] CH3 C O + CH3 CH3 H H CH3 C + CH3 CH3 + H2O CH3 C + CH3 CH3 + Cl slow CH3 C CH3 CH3 Cl (ii) An enantiomerically pure alcohol, where the carbon atom bonded to the –OH group is chiral, was used for the reaction in (b). Use your answer in (b)(i) to deduce the stereochemical outcome of this reaction. Explain your reasoning. [2] The reaction will produce a racemic mixture / the product will be formed as a 50 : 50 mixture of enantiomers.

3 © DHS 2022 9729/02 [Turn over The carbocation intermediate is (trigonal) planar around the positively charged C atom. The chloride ion can attack the positively charged C atom from the top and botto