RI 2021 Year 6 March TP QP

Name: ( ) Class: 21S0____ RAFFLES INSTITUTION 2021 YEAR 6 TERM 2 TIMED PRACTICE Higher 2 CHEMISTRY 9729 23 March 2021 2 hours READ THESE INSTRUCTIONS FIRST Write your name, index number and civics tutorial group in the spaces provided on this cover sheet. This paper consists of two sections B and C. Section B (35 marks) consists of 2 structured questions. Answer all questions in the spaces provided on the Question Paper. Section C (40 marks) consists of 2 free-response questions. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the blank spaces, blank pages and the pages at the end of this booklet. The question number must be clearly shown. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. Write in dark blue or black pen. You may be subject to penalty for writing answers in pencil. Question Marks B1 / 13 B2 / 22 C1 / 21 C2 / 19 Total / 75 Percentage / 100 Penalty Overall / 100 Grade This document consists of 22 printed pages and 1 blank page. This document is copyrighted, please do not reproduce it without permission

2 Section B (35 marks) This section consists of 2 questions. Answer all the questions in this section in the spaces provided. 1 Ammonia, NH3, is an important reagent in organic and inorganic chemical reactions. The solubility of ammonia in water is different from that in organic solvents. When aqueous ammonia is shaken with an organic solvent, an equilibrium is established. NH3(org) NH3(aq), KD = 3 3 [NH (aq)] [NH (org)] The equilibrium constant, KD, is known as the distribution coefficient. It is the ratio of the concentration of ammonia in water to the concentration of ammonia in the organic solvent. When trichloromethane is used as the organic solvent, the value of KD at 25 °C for the above equilibrium is known to be approximately 25. The accurate value of KD can be determined by the distribution of ammonia into the two layers followed by an acid-base titration. A student poured 50 cm 3 of 1.04 mol dm –3 aqueous ammonia into a separating funnel, followed by 50 cm3 of trichloromethane. The stopper was replaced and the separating funnel was shaken. The ammonia originally in the aqueous layer was redistributed into the aqueous and organic layers. Since the volumes of the aqueous and organic layers are equal, [NH3(aq)] + [NH3(org)] = 1.04 mol dm–3. The student obtained 25.0 cm 3 of the aqueous layer and titrated it against 0.100 mol dm –3 hydrochloric acid. She decided that the concentration of ammonia in the aqueous layer was too high and needed to be diluted before titration. separating funnel stopper tap aqueous layer organic layer This document is copyrighted, please do not reproduce it without permission

3 (a) Use the approximate value of KD to calculate the volume of 0.100 mol dm –3 hydrochloric acid required to titrate 25.0 cm3 of the aqueous layer, and explain why the concentration of ammonia in the aqueous layer would have been unsuitable for use in the student’s titration. [3] (b) Plan a procedure for the determination of the concentration of ammonia in the aqueous layer. You may assume that you are provided with: • the separating funnel and its contents, prepared by the student , which has reached equilibrium, • 0.100 mol dm–3 hydrochloric acid, • the apparatus and chemicals normally found in a school Chemistry laboratory. Your plan should include: • calculation of an appropriate volume of the aqueous layer to be used for dilution, • the procedure you would follow to o obtain the volume of the aqueous layer from the separating funnel without the organic layer, o dilute the aqueous layer to an appropriate volume, o carry out the titration. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… This document is copyrighted, please do not reproduce it without permission

4 …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[6] This document is copyrighted, please do not reproduce it without permission

5 (c) A mixture of aqueous ammonia and ammonium chloride can be us ed to prepare a buffer solution (pKb of NH3 = 4.75). (i) Write two equations to show how the buffer solution can control pH. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ………………………………………………………………..…………………………[2] (ii) Calculate the mass of solid ammonium chloride to be added to 1 dm3 of 0.80 mol dm–3 aqueous ammonia to form a buffer solution of pH 9.0. [2] [Total:13] This document is copyrighted, please do not reproduce it without permission

6 2 Compound X can be synthesised from phenol and 1,2 -dihydroxybenzene (catechol) in several steps. (a) The following table compares the pKa values of phenol with catechol. compound pKa1 pKa2 phenol 10.0 – catechol 9.45 12.8 . (i) Calculate the pH of a 0.750 mol dm–3 solution of aqueous phenol. [2] (ii) Draw the structure of the monoanion of catechol and explain why catechol has a lower pKa1 than phenol. ………………………………………………………………………………………….… ………………………………………………………………………………………….… ………………………………………………………………………………….………[2] This document is copyrighted, please do not reproduce it without permission

7 (b) Phenol can be converted to intermediate D via a series of steps. In the above reactions, the –OCH3 group remains unaltered. (i) Draw the structure of compound C in the above reaction scheme and state the reagents and conditions required for reactions 2 and 3. reaction 2 .............................................................................................................. reaction 3 .............................................................................................................. [3] This document is copyrighted, please do not reproduce it without permission

8 Reaction 1 occurs in several steps. The first