HCI Prelim P2

This document consists of 20 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 14S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9647/02 2 September 2015 2 hours INSTRUCTIONS TO CANDIDATES 1) Write your name, CT group, centre number and index number clearly in the spaces at the top of this page. 2) Answer all questions in the spaces provided in this Question Paper. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. Do not use staples, paper clips, highlighters, glue or correction fluid. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use 1 / 12 2 / 10 3 / 17 4 / 9 5 / 9  6 / 15  Deductions Total / 72       Calculator Model:

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 2 1 Planning (P) For Examiner's use When excess ammonia is added to an aqueous solution containing transition metal ion M2+, a complex ion [M(NH3)n]2+, where n has a maximum value of 6, is formed. When this solution is shaken with trichloromethane (density = 1.49 g cm–3), an equilibrium mixture consisting of two immiscible layers is obtained. The aqueous layer contains the complex ion and uncomplexed ‘free’ ammonia while the organic layer contains only ‘free’ ammonia. The ratio of the concentrations of ‘free’ ammonia in the two immiscible layers at equilibrium is a constant, known as the partition ratio, K. NH3(aq) ⇌      NH3(trichloromethane) K=[NH3(trichloromethane)][NH3(aq)]=0.4 To determine the value of n in [M(NH3)n]2+, the equilibrium mixture is prepared. A sample is withdrawn from the organic layer and the ‘free’ ammonia is titrated with standard hydrochloric acid. As the aqueous acid is unable to dissolve in the organic layer, an equal volume of water is added to the organic layer before titration so that the ‘free’ ammonia can be neutralised by the acid. The titration results can be used to determine the concentration of ‘free’ ammonia in the organic and aqueous layers. The amount of ammonia used to form the complex ion and hence the value of n can then be determined. (a) Explain why the value of the partition ratio, K, of ammonia between trichloromethane and water is less than 1. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………[1] (b) With the aid of an equation, explain why the concentration of ‘free’ ammonia in the aqueous layer cannot be determined by titrating the aqueous layer with standard hydrochloric acid. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………