RI 2019 Prelim P2 QP

1 © Raffles Institution 2019 9729/02/S/19 [Turn Over RAFFLES INSTITUTION 2019 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 17 September 2019 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces provided at the top of this page. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. Do not write anything in it. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 8 2 / 7 3 / 16 4 / 9 5 / 15 6 / 20 Total / 75 This document consists of 23 printed pages and 1 blank page.

2 © Raffles Institution 2019 9729/02/S/19 [Turn Over 1 Silicon dioxide is made up of the two most abundant elements on earth, and is the second most abundant mineral. (a) Silicon dioxide is known to occur in as many as 12 different crystalline forms. Unlike SiO2, silicon disulfide, SiS2, lacks such variety in structures. SiS2 reacts quickly with liquid ammonia to form compound X, which has the following composition by mass. Si 48.3%, N 48.1%, H 3.6% Determine the empirical formula of X. [1] (b) Explain the difference in the first ionisation energies of silicon and phosphorus. ……..…………………………………………………………………………………………….. ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………........................ [2] Aluminium and phosphorus are adjacent to silicon in the Periodic Table yet their oxides behave differently as compared to silicon dioxide. (c) Write equations to illustrate the acid -base behaviour of the oxides of aluminium and silicon. ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………….. [3]

3 © Raffles Institution 2019 9729/02/S/19 [Turn Over (d) Explain, in terms of structure and bonding, why the melting point of P4O10 is lower than that of SiO2. ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ……………………………………………………………………………………………........... ………………………………………………………………………………………………... [2] [Total: 8]

4 © Raffles Institution 2019 9729/02/S/19 [Turn Over 2 A reaction is represented by the following equation. xC(g) ⇌ yA(g) + B(g); H > 0 The reaction was studied by monitoring the amounts of C, A and B over time in a reaction vessel with a fixed volume of 2 dm3. The initial amounts of C, A and B used were 1.80 mol, 0.40 mol and 0.40 mol respectively. Fig. 2.1 shows the results that were obtained. Fig. 2.1 (a) Determine the values of x and y in the above equation, showing clearly any calculations or reasoning. [2]

5 © Raffles Institution 2019 9729/02/S/19 [Turn Over (b) Calculate the equilibrium constant, Kc, immediately before time t1. Include units in your answer. [2] (c) Based on the Kc value calculated in (b), state whether the G of the reaction at the given temperature is less than, equal to or greater than zero. ………………………………………………………………………………………………... [1] (d) A change was made to the system at time t2. State the change that was made. ………….…………………………………………………………………………………….. [1] (e) At time t3, an inert gas was added to the system at constant volume and temperature. Sketch the effect of this change on the amounts of C, A and B in the space after the dotted vertical line on Fig. 2.1. [1] [Total: 7]

6 © Raffles Institution 2019 9729/02/S/19 [Turn Over 3 This question is about 1,3-butadiene. 1,3-butadiene undergoes an exothermic electrophilic addition reaction with an equimolar amount of HBr to produce a mixture of 1,2 -addition and 1,4 -addition products as shown in Table 3.1. In this case, 1,2- and 1,4-addition refer to the positions o f the carbon atoms in the diene at which H and Br atoms were added. Table 3.1 type of addition structure of product % yield at 45 oC structure of carbocation intermediate 1,2-addition X 0 Y 15 Refer to your answer in (b). 1,4-addition Z 85 (a) State the total number of mono -brominated products, including stereoisomers, formed when 1,3-butadiene reacts with an equimolar amount of HBr at 45 oC. ……………………………………………………………………………………................... [1] (b) Describe the mechanism for the reaction between 1,3 -butadiene and HBr to form compound Y. Show relevant lone pairs and dipoles, and use curly arrows to indicate the movement of electron pairs. [2] 1,3-butadiene

7 © Raffles Institution 2019 9729/02/S/19 [Turn Over (c) In Table 3.1, d raw the structure of the carbocation intermediate generated during the formation of compound X. [1] (d) (i) By c onsidering the structures of t he respective carbocation intermediates in Table 3.1, account for the percentage yield of each of the compounds stated below. (1) compound X ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………… [1] (2) compound Z ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………… [2]

8 © Raffles Institution 2019 9729/02/S/19 [Turn Over (ii) Y is energetically more stable than X. On the same axes, sketch two reaction pathway diagrams to illustrate the formation of compounds X and Y from 1,3-butadiene, labelling the reactants, intermediate and product for each diagram. [3] (e) Draw the structural formula of the major product formed when 1,3-butadiene reacts with an equimolar amount of aqueous bromine at 45 oC. [1] energy / kJ mol–1 reaction pathway

9 © Raffles Institution 2019 9729/02/S/19 [Turn Over (f) (i) 1,3-butadiene can be used to synthesise compound W via the reaction scheme below. Complete the reaction scheme above by drawing the structure of the intermediate compound in the box provided. Suggest appropria