RI 2018 Prelim P3 QP

© Raffles Institution 2018 9729/03/S/18 [Turn Over RAFFLES INSTITUTION 2018 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9729/03 Paper 3 Free Response 20 September 2018 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write your name, class and 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. Section A Answer all questions. Section B Answer one question. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything in it. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together, with the cover page on top. The cover page is found on page 14. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 13 printed pages and 1 blank page.

2 © Raffles Institution 2018 9729/03/S/18 [Turn Over Section A Answer all the questions in this section. 1 (a) In the aldol reaction, two carbonyl compounds react with each other in the presence of a base catalyst to form a hydroxy carbonyl compound. The aldol reaction between two ethanal molecules, in the presence of OH − ions, is shown below. (i) A mixture of ethanal and propanone was subjected to the aldol reaction. Apart from 3-hydroxybutanal, suggest three other possible products formed. Ignore stereoisomers in your answer. [2] To determine the kinetics of reaction 1, two experiments, I and II, were carried out and the graphs of the concentration of ethanal against time were plotted. The graphs on page 13 show the results of experiments I and II with [OH–] = 1.0 mol dm−3 and 2.0 mol dm−3 respectively. For (a)(ii) and (a)(iii), draw clearly any construction lines on the graphs on page 13. (ii) OH− ions act as catalyst in the aldol reaction. Using the graph for Experiment I, determine the order of reaction with respect to ethanal. Explain your reasoning. [2] (iii) By finding the initial rates, determine the order of reaction with respect to OH–. [2] (iv) Hence, write an overall rate equation for this reaction. [1] (v) With reference to Experiment I, calculate a value for the rate constant and state its units. [2] (b) A is an intermediate formed in reaction 1. In a separate reaction, A was used a s a nucleophile to react with an optically active sample of 2-bromobutane. The product formed was found to rotate plane polarised light. (i) Describe the mechanism of this reaction. In your answer, show relevant lone pairs and dipoles, and use curly arrows to indicate the movement of electrons. [3]

© Raffles Institution 2018 9729/03/S/18 [Turn Over 3 (ii) When conducted at a higher temperature, the same reaction occurred at a higher rate. With the aid of the Boltzmann distribution, explain why this is so. [3] (iii) In another reaction with A, 2 -chlorobutane was used instead of 2 -bromobutane. With reference to the Data Booklet , predict and explain the effect on the rate of reaction. [1] (iv) Instead of A, hydroxide ions were used to react with 2 -bromobutane via the same mechanism in (b)(i). Suggest why the reaction between hydroxide ions and 2 -bromobutane is faster than that between A and 2-bromobutane. [1] (c) The following shows a reaction scheme to synthesise an ester D from compound B. (i) Suggest the structure of compound C. [1] (ii) Suggest reagents and conditions for steps 1 and 2. [2] [Total: 20]

© Raffles Institution 2018 9729/03/S/18 [Turn Over 4 2 This question is about the halogens and their compounds. Halogens react readily with lead to form lead halides. The melting points of some of the lead halides are given in Table 2.1. Table 2.1 compound melting point / ºC PbF2 824 PbCl2 501 PbBr2 373 PbCl4 –15 (a) (i) Explain, in terms of structure and bonding, why the melting point s of the lead( II) halides decrease from PbF2 to PbBr2. [2] (ii) Explain why PbCl4 is a covalent compound and account for its low melting point. [2] (iii) In an experiment, an excess of solid PbC l2 was added to a solution of 0.050 mol dm –3 NaCl and allowed to reach equilibrium at 298 K. The mixture was then filtered. 25.0 cm3 of the filtrate required the addition of 3.58  10–5 mol of Na I for the first trace of PbI2 precipitate to appear. Given that the solubility product of Pb I2 is 9.80  10–9 mol3 dm–9, calculate the solubility product of PbCl2. [3] When concentrated hydrochloric acid is added to a mixture containing the white precipitate of PbCl2, the precipitate dissolves to form a colourless solution containing an ionic compound E. (iv) E contains 0.6% H; 59.0% Pb; 40.4% C l by mass, and has a relative formula mass of 351.0. The ratio of the cation to anion in E is 2 : 1. Suggest the identity of the complex ion present in E. [2] (v) Hence, explain the above observation with relevant ionic equations. [2] (b) Halogens react with hydrogen to form hydrogen halides which may decompose upon heating. (i) Write an equation for the thermal decomposition of hydrogen iodide. [1]

© Raffles Institution 2018 9729/03/S/18 [Turn Over 5 (ii) In an experiment, three glass jars were filled with hydrogen chloride, hydrogen bromide and hydrogen i odide gases respectively. A red hot wire was inserted into each jar for a fixed period of time. The observations were recorded in Table 2.2. Table 2.2 gas in glass jar observations hydrogen chloride no observable change hydrogen bromide reddish brown vapour formed slowly hydrogen iodide purple fumes evolved vigorously Explain these observations. [2] (c) Interhalogen species are commonly formed between halogens, where the central atom is the less electronegative of the two halogens. Draw the dot -and-cross diagram of the IF4− ion. In your diagram, use the symbols ‘ •’ and ‘’ to distinguish the electrons from the iodine and fluorine atoms, and the symbol ‘ ’ for any additional electron responsible for the overall negative charge. State the shape and bond angle in IF4− ion. [2] (d) Use of the Data Booklet is relevant to this question. Halogens are strong oxidising agents which have important industrial applications. Chlorine, for example, is used as a disinfectant. Show, with appropriate calculations, that manganese(IV) oxide is a product of the redox reaction between chlorine and manganese metal. Hence, write the overall equation for the reaction. [3] (e) Write a balanced equation for the reaction of PCl5 with water. [1] [Total: 20]

© Raffles Institution 2018 9729/03/S/18 [Turn Over 6 3 Chromium is a hard bluish –white metal that is extremely resistant to chemical attack at room temperature. Chromium metal is an important component in stainless steel. (a) Write the electronic configuration of Cr. [1] (b) The following sequence of reactions involving chromium illustrates many of the characteristic properties of transition metals. (i) State the types of reaction in steps I and II. Give the formula of the chromium-containing species, F and G. [3] (ii) The variety of colours shown by chromium compounds in this reaction scheme is a typical property of transition metals. State one other characteristic property of transition metals that is illustrated by this reaction scheme. Briefly explain why transition