DHS H2 Chem 2012 Prelim P2 QP

Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/02 Paper 2 Structured 14 September 2012 Additional Materials: Data Booklet 2 hours 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 1 2 3 4 5 6 Total %No. Marks 12 14 8 18 10 10 [72] _______________________________________________________________________________________________ This question paper consists of 17 printed pages and 1 blank page. © DHS 2012 [Turn over

2 © DHS 2012 9647/02 [Turn over Answer all questions in the space provided. 1 Planning This question is about the kinetics of urease –catalysed hydrolysis of urea in phosphate and citrate buffer solutions. Urease is an enzyme that breaks down urea, CO(NH 2)2, into carbon dioxide and ammonia. The production of ammonia raises the pH of the buffered solution mixture and can be detected using a suitable acid–base indicator. CO(NH2)2 + H2O ® CO2 + NH3 Urease is active over a wide range of pH so a few indicators can be used provided that the original solution mixture is buffered at the right pH to start with. The colour change of bromomothymol blue and phenolphthalein in acid– base medium is shown in the table: Indicator pKind pH range Colour change bromothymol blue 7.0 5.9–7.6 Yellow(in acid) – blue (in alkali) phenolphthalein 9.3 8.3–10.0 Colourless(in acid)–pink (in alkali) Phosphate and citrate present in the buffer solutions do not react with urea or urease. In order to study the effect of the enzyme concentration on the rate of hydrolysis of urea, you are provided with · Choice of two indicators, bromothymol blue and phenolphthalein · Choice of two buffered solutions, phosphate buffer at pH 7 and citrate buffer at pH 5 · 2% of urea solution · 3% urease solution · distilled water In a preliminary experiment, a solution mixture containing 20 cm 3 of urea solution, 8 cm 3 of buffer solution, 6 cm 3 of urease solution, 6 cm3 of water took 40 seconds for the indicator to show a colour change. It is found that the time taken for the indicator in the reaction mixture to change colour doubled when urease used is diluted with an equal amount of water. (a) (i) Suggest the appropriate choice of indicator and the buff er solution used in the preliminary experiment. Choice of buffer solution: Choice of indicator:

3 © DHS 2012 9647/02 [Turn over (ii) Using the results of the preliminary experiment predict the relationship b etween the rate of hydrolysis of urea and concentration of urease. [2] (b) (i) A student designed a series of experiments to investigate the prediction in the preliminary experiment by initial rate method. Complete the table below to show · all volumes to be used such that a simple comparison of the time taken for end–point of the indicator can be made with the preliminary experiment. · how the measurements would be processed(that is, complete the heading for the last column) to enable the student to confirm or reject the preliminary prediction. Expt Volume of urea/ cm3 Volume of urease/ cm3 Volume of buffer/ cm3 Volume of water/ cm3 Time taken, t/ s P 20 6 8 6 40 1 2 3 P: Preliminary experiment

4 © DHS 2012 9647/02 [Turn over (ii) Outline the procedural steps that the student would take to carry out the designed experiments in (b)(i),stating any assumption(s) made. Your plan should include: o the apparatus used to measure the various volumes; o the sequence that the various solutions are mixed; o how the time for the colour change of indicator is measured; and o other experimental details to ensure the consistency of the experiment. You may also assume that standard laboratory apparatus are available. (iii) Explain how the results of the experiments will confirm or reject the preliminary prediction. [8]

5 © DHS 2012 9647/02 [Turn over (c) The rate of this reaction is also studied whe n the temperature is varied. Give a sketch of a graph to show how the rate will vary with temperature. Explain the shape of your sketch. [2] [Total: 12] 2 Ethanoic acid, also co mmonly known as acetic acid , is an important chemical due to its varied uses in the industry. (a) The preferred industrial method for its manufacture is by carbonylation of methanol, an exothermic reaction, accounting for 65% of the total world ethanoic acid manufacturing capacity. CH3OH(aq) + CO(g) CH3CO2H(aq) The conditions employed are: · 150 – 200 °C · 30 – 60 atm · rhodium–based catalyst Using Le Chatelier’s Principle, explain why the above moderate pressure i s chosen as the process condition. [2] T/ °C rate 20

6 © DHS 2012 9647/02 [Turn over (b) The graph below shows the changes in pH when a 25.0 cm 3 sample of vinegar, with ethanoic acid as its main component, is titrated against 0.45 mol dm –3 aqueous sodium hydroxide. (i) Using the information provided, show by calculations that the acid dissociation constant, Ka of ethanoic acid has an approximate numerical value of 1.85 x 10–5. (ii) State a suitable indicator for this titration. Explain your choice. Indicator: Reason for choice: [5] Region A

7 © DHS 2012 9647/02 [Turn over (c) The buffer produced at region A in (b) consists of a weak acid and its conjugate base. (i) Draw a diagram to illustrate the shape of the anion present. State the shape about each carbon atom and its respective bond angles. (ii) With clear reference to your drawing in c(i), state the type of hy bridisation present for each carbon atom in the anion. (iii) The boiling points of both components of the buffer are given below. weak acid 118 °C conjugate base 881 °C With reference to the structures and interactions present, account for the large difference in the boiling points shown. [7] [Total: 14]

8 © DHS 2012 9647/02 [Turn over 3 (a) Sodium chlorate (I), NaClO, found in household bleach produces chlorine upon addition of acidified dilute hydrochloric acid according to the equation below. ClO–(aq) + 2H+(aq) + Cl–(aq) ® Cl2(aq) + H2O(l) A 10.0 cm 3 sample of a household bleach is diluted to 100 cm 3. 25.0 cm 3 of this solution is then pipetted out into a conical flask containing dilute hydrochloric acid. Excess potassium iodide solution is added and the resultant solution turns brown. This resultant solution is then titrated against 0.100 mol dm –3 sodium thiosulfate solution and required 15.20 cm3 for complete reaction. (i) Identify the specie that is responsible for the brown colouration and state the reaction that has taken place. (ii) Calculate the concentration of the sodium chlorate(I) in the household bleach in g dm–3. concentration= ..........................g dm –3 [4]

9 © DHS 2012 9647/02 [Turn over (b) A small portion of three unlabelled samples of KC l, KBr and K I are r eacted with concentrated phosphoric acid separately and each of the hydrogen halide gases produced are immediately passed through identical set–ups as shown. (i) Suggest a hydrogen halide that can be identified using this set –up, clearly stating the equation of the reaction