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