DHS Prelim H2 CHEM P3 QP

This question paper consists of 15 printed pages and 1 blank page. © DHS 2016 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination 2016 Year 6 H2 CHEMISTRY 9647/03 Paper 3 Free Response 20 September 2016 2 hours Additional Materials: Data Booklet Writing Papers Graph Paper INSTRUCTIONS TO CANDIDATES Write your name, index number and class on this question paper and on the Cover Sheet provided. 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. Start each question on a fresh sheet of paper. A Data Booklet is provided. 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 work securely together with the Cover Sheet on top.  Hand in the question paper separately. The total marks for this paper is 80 marks.

2 © DHS 2016 9647/03 [Turn over Answer any four questions. 1 Blue #1 (Na2C37H34N2S3O9) is a synthetic food colouring often found in ice cream. (a) One of the main compounds used to produce Blue #1 is benzenesulfonic acid, which is produced by heating benzene under reflux with concentrated sulfuric acid for several hours. (i) The first step of the mechanism involves the protonation of one molecule of sulfuric acid by another and the loss of a molecule of water. Name the type of reaction and describe the mechanism. In your answer, show relevant lone pairs and charges, and use curly arrows to indicate the movement of electrons. [4] (ii) Benzenesulfonic acid is a useful starting material as it can be easily converted to phenol by heating with NaOH(aq) at 300°C followed by acidification. It also has similar chemical reactivity as nitrobenzene.  H2SO4 H2O 3000C HCl (aq)

3 © DHS 2016 9647/03 [Turn over Compound A can be synthesised from either benzenesulfonic acid or ethylbenzene in not more than 4 steps. A The first step of the synthesis using either benzenesulfonic acid or ethyl benzene is as shown below in Scheme 1 and 2 respectively: Scheme 1: Scheme 2: Identify the correct scheme that will give compound A and copy the synthetic scheme in your answer. Using information in (a)(ii), suggest the next three steps to synthesise compound A. Show all intermediates, reagents and conditions clearly in your synthetic route. [4] CH3CH2Cl / AlCl3 CH2CH3 CH2CH3 conc. H2SO4 heat CH2CH3

4 © DHS 2016 9647/03 [Turn over (b) Blue #1 can be oxidised by household bleach to form colourless products, as represented by the equation below. Na2C37H34N2S3O9 + bleach → products blue colourless colourless To study the kinetics of the reaction, a student used an UV spectrophotometer to study the absorbance of Blue #1 over time during the bleaching process. In experiment 1, the student mixes 3.0 cm3 of 0.1 mol dm–3 Blue #1, 1.0 cm3 of water and 0.5 cm 3 of 3 .5 mol dm –3 bleach together. The results of experiment 1 are as shown below. Time (s) 15 30 45 60 75 90 105 120 135 Absorbance 0.495 0.424 0.371 0.328 0.285 0.251 0.218 0.191 0.170 (i) Explain why bleach is used in large excess as compared to that of Blue #1. [1] (ii) Given that absorbance measured varies linearly with the concentration of Blue #1, use a suitable graphical method to determine the order of reaction with respect to the blue food colouring, Blue #1. [3] (iii) The student subsequently conducted a few more experiments using other volumes of Blue #1 and bleach. The table below shows the results obtained. Experiment Volume of Blue #1 / cm3 Volume of distilled water / cm3 Volume of bleach / cm3 Rate constant 1 3.0 1.0 0.5 2 4.0 0.0 0.5 0.00882 3 3.0 0.5 1.0 0.01698 Using results from (b)(ii), determine the rate constant for experiment 1. Hence, deduce the overall order of reaction, showing your reasoning clearly. [3] (c) (i) Household bleach is also often known as “chlorine water”. The active chemical responsible for the bleaching action in household bleach is the anion, OC l–, which is easily formed by the disproportionation of chlorine in water. Write a balanced equation bet ween chlorine and water, and hence, suggest the identity of the other compound formed. [1] (ii) Chlorine reacts with Period 3 elements, Mg and Si, to give chlorides. The two chlorides differ in their reaction with water. Describe their differences, and explain them in terms of the different structures and types of chemical bonding. Write an equation for the reaction, if any. [4] [Total: 20]

5 © DHS 2016 9647/03 [Turn over 2 Vanillin and vanillyl alcohol (derived from vanillin) are both used as flavouring, usually in sweet foods such as chocolate. Vanillyl alcohol is commonly synthesised via the reduction of vanillin using sodium borohydride, as shown below. OH O HO OH O OH 4 + NaBH4 (aq) + 4H2O (l) + H3BO3 (aq) + NaOH (aq) vanillin vanillyl alcohol 4 (a) (i) Draw a ‘dot–and–cross’ diagram to show the bonding in sodium borohydride. [1] (ii) Suggest why sodium borohydride is used in this reduction instead of lithium aluminium hydride. [1] (iii) Given that the melting points of vanillin and vanillyl alcohol are 355 K and 388 K respectively, predict and explain the sign of the entropy change of reaction at 360 K. [1] (iv) A higher temperature leads to a faster rate of reaction. However, thermodynamic factors must also be taken into consideration when considering appropriate conditions for a reaction. Explain how higher temperatures will affect the thermodynamic outcome. [2]

6 © DHS 2016 9647/03 [Turn over (b) The procedure for the reduction of vanillin to vanillyl alcohol is as follows: 1. Place 2 g of vanillin in a 25 cm3 round bottom flask followed by 4 cm3 ethanol. Add a magnetic stirrer, clamp the flask above a stir plate and commence stirring at room temperature to dissolve vanillin. 2. After the vanillin dissolves, add an ice bath under the flask to cool the solution. 3. In a separate reaction vial, dissolve 0.5 g of NaBH 4 in 3.8 cm3 of 1 mol dm–3 NaOH solution. 4. Using a glass pipette , slowly add the NaBH 4 solution dropwise to the vanillin solution over a period of 10 minutes. 5. Only after the addition is complete, remove the ice bath. This is because NaBH 4 decomposes at high temperatures. 6. Stir the resulting mixture for 10 minutes at room temperature. 7. Add 6 mol dm–3 HCl dropwise until the evolution of hydrogen gas stops. 8. Stir to allow the product to precipitate from solution. (i) State a reason why sodium borohydride must be added dropwise to the reaction mixture (in Step 4). [1] (ii) In Step 7, hydrochloric acid was added to react with excess sodium borohydride present in the reaction mixture. Write an equation for this reaction, given that boric acid (H 3BO3) is one of the products formed. [1] (iii) State one way you could modify the procedure to increase the isolated yield. [1] (iv) A