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 Th