JJC_H2_CHEM_P3_Qn

[Turn over JURONG JUNIOR COLLEGE 2015 JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS 15S CHEMISTRY 9647/03 Paper 3 Free Response 15 September 2015 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number 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 staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. At the end of the examination, fasten all your work securely together. This document consists of 10 printed pages.

2 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 Answer any four questions. 1 (a) Sodium amide (NaNH 2), commonly known as ‘sodamide’, can be prepared by reacting metallic sodium with ammonia gas. reaction 1 2Na(s) + 2NH3(g)  2NaNH2(s) + H2(g) (i) Use suitable bond energies data from the Data Booklet to calculate the enthalpy change of formation of NH3(g). [2] (ii) The enthalpy change of formation of ‘sodamide’ is 119 kJ mol1. Use the above information and your answer in (a)(i) to calculate the enthalpy change for reaction 1. [2] (iii) Use oxidation states to explain why reaction 1 is a redox reaction. [2] When dissolved in liquid amm onia, ‘sodamide’ produces NH2  ion which acts as a strong base in organic reactions. (iv) Draw a ‘dot-and-cross’ diagram for a NH 2  ion. In your diagram, use a third symbol ‘’ for any additional electrons responsible for the overall negative charge. [1] (v) Write an equation to show how NH 2  ion acts as a Brønsted-Lowry base when it reacts with methanol, CH3OH. [1] (b) ‘Tris’, (CH2OH)3CNH2, and tert-butylamine, (CH 3)3CNH2, are weak monoprotic organic bases. (i) Predict and explain whether ‘Tris’ has a smaller or larger Kb value as compared to tert-butylamine. [2] ‘Tris’ (Mr = 121) is widely used as a component of buffers. A buffer solution A of pH 7.40 was prepared by mixing solid ‘Tris’ with 100 cm 3 of 0.500 mol dm3 hydrochloric acid and making it up to 1.00 dm 3 with deionised water at 25 °C. The Kb of ‘Tris’ is 1.20  106 mol dm3 at 25 °C. (ii) Write an equation for the reaction between ‘Tris’ and hydrochloric acid. [1] (iii) Calculate the [base] [salt] ratio in buffer A and hence the mass of ‘Tris’ to be used. [3]

3 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 [Turn over (c) Isocy