DHS H2 CHEM P2 Prelim

_______________________________________________________________________________________________ This question paper consists of 18 printed pages and 0 blank page. © DHS 2011 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/02 Paper 2 Structured 21 September 2011 2 hours Additional Materials: Data Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Answer all questions. 3 Write your answers in the spaces provided on the question paper. 4 A Data Booklet is provided. 5 The number of marks is given in brackets [ ] at the end of each question or part question. 6 You may use a calculator. FOR EXAMINER’S USE Question No. 1 2 3 4 5 6 Total % Marks 12 14 15 11 11 9 [72]

2 © DHS 2011 9647/02 [Turn over Answer all questions in the space provided. 1 Planning When aqueous sodium chloride, NaC l, is added to aqueous lead( II) nitrate, Pb(NO 3)2, a white precipitate of lead chloride, PbCl2, is produced. A suggested stoichiometric equation is Pb(NO3)2(aq) + 2NaCl(aq)  PbCl2(s) + 2NaNO3(aq) In separate experiments, different volumes of 0.20 mol dm –3 aqueous sodium chloride are added to a fixed volume of 0.10 mol dm –3 aqueous lead( II) nitrate. In each case, the precipitate, PbCl2 will be formed. This precipitate is then filtered, washed with distilled water and thoroughly dried. The mass of the precipitate is recorded. You are to plan an experiment to investigate this reaction in order to confirm or reject the stoichiometry of the equation. (a) (i) By considering the suggested stoichiometric equation, predict how the number of moles of the precipitate, PbC l2, will change as the number of moles of NaC l added increases. Prediction: (ii) Sketch the graph which would result if NaC l is the limiting reagent . Start your graph with no NaCl added. [2] PbCl2 /mol NaCl /mol 0

3 © DHS 2011 9647/02 [Turn over (b) You are provided with 250 cm 3 of 0.20 mol dm –3 aqueous sodium chloride and some solid lead(II) nitrate. (i) Outline how you would prepare 250 cm 3 of 0.10 mol dm –3 aqueous lead( II) nitrate. [Ar: N, 14.0; O, 16.0; Pb, 207.0] (ii) You will be carrying out five sets of experiments to obtain different masses of precipitate. Give a step by step description of how you would carry out one such experiment. You need to state only the following:  the volumes of each solution to be used,  how the volumes will be measured, and  how you would dry the precipitate. [6]

4 © DHS 2011 9647/02 [Turn over (c) In the table below  enter appropriate headings to show recorded and calculated data. The headings should include appropriate units,  enter the volumes from your plan in (b),  enter suitable volumes for four further experiments. experiment 1 2 3 4 5 [3] (d) How would you ensure that at the end of each experiment the precipitate is thoroughly dried? [1] [Total: 12]

5 © DHS 2011 9647/02 [Turn over 2 Iodine forms an intense blue complex with starch and many chemical reactions make use of this property. (a) In the iodine clock reaction, the following set of reactions take place and a colour change involving the iodine–starch complex happens after a fixed time delay, allowing the kinetics of the reaction to be determined. Step I: 2 I– (aq) + S2O8 2–(aq)  I2(aq) + 2SO4 2–(aq) Step II: I2 + 2S2O3 2–  2I– + S4O6 2– During an experiment to determine the rate equation for these reactions, different concentrations of KI, Na2S2O8 and Na2S2O3 are mixed according to the following table. The rates for the colour change in each solution are shown below. flask [ I –]/ mol dm–3 [S2O8 2–] / mol dm–3 [S2O3 2–] / mol dm–3 rate /s–1 1 0.10 0.10 0.06 0.0222 2 0.10 0.20 0.06 0.0434 3 0.10 0.30 0.06 0.0665 4 0.20 0.15 0.05 0.0685 5 0.20 0.15 0.12 0.0675 6 0.20 0.15 0.18 0.0672 (i) Deduce two sets of variables whose values can be plotted to obtain the order of reaction with respect to either S2O8 2– or S2O3 2–. plot : against :

6 © DHS 2011 9647/02 [Turn over (ii) On the grids provided, plot a graph using the variables in (i) and determine the order of reaction with respect to S2O8 2–. Label the axes and the graph. Order of reaction with respect to S2O8 2– : (iii) Plot another line on the axes above to determine the order of reaction with respect to S2O3 2–. Order of reaction with respect to S2O3 2– :

7 © DHS 2011 9647/02 [Turn over (iv) Using a non–graphical method, deduce the order of reaction with respect to I– . (v) Hence or otherwise, write the rate equation and decide if step I could be the rate determining step for the iodine clock reaction. [10] (b) In the determination of the mass percentage of ascorbic acid (C 6H8O6) present in vitamin C tablets, the following reaction involving iodine takes place. C6H8O6 + I2 → C6H6O6 + 2I– + 2H+ Due to the low solubility of iodine in water, it is generated in situ using potassium iodate(V), potassium iodide and sulfuric acid. In this reaction, iodine is the only product. (i) Write a balanced equation for the reaction between IO3 – and I– to obtain I2.

8 © DHS 2011 9647/02 [Turn over A 3.0 g sample of the tablet is crushed and dissolved in 100.0 cm 3 of dilute sulfuric acid. To this solution, 1.08 g of potassium iodate( V) and 4.32 g of potassium iodide is added. A 25.0 cm 3 aliquot is withdrawn and 21.60 cm 3 of 0.1 mol dm –3 thiosulfate is required to titrate with the excess iodine, using starch as an indicator. (ii) Calculate the total amount of iodine produced from reacting potassium iodate( V) with potassium iodide. (iii) Determine the total amount of excess iodine in the 100 cm3 solution. (iv) Hence or otherwise, obtain the mass percentage of ascorbic acid in the sample. [4] [Total: 14]

9 © DHS 2011 9647/02 [Turn over 3 Alkynes are hydrocarbons containing a C C triple bond. Reactions involving alkynes resemble that of corresponding alkenes. For example, alkynes react with bromine in tetrachloromethane to give dibromoalkenes. In the mild oxidation of alkynes, the π bonds are broken and a dicarbonyl compound is formed, while oxidation with powerful oxidising agents such as potassium manganate(VII) cleaves the CC triple bond.     (a) The following reaction scheme shows some reactions involving phenylacetylene (C6H5CCH), an alkyne that is useful in organic synthesis. (i) State the types of reaction for stages 1 and 2. stage 1 : stage 2 :

10 © DHS 2011 9647/02 [Turn over (ii) Draw the structures for the organic products A to D from the reaction scheme. A B C D (iii) Name and outline the mechanism for stage 3, using only equations. Clearly indicate the arrows and the intermediate. [9]