DHS Prelim P4_Suggested_Solutions

This question paper consists of 18 printed pages and 0 blank page. © DHS 2018 [Turn over Suggested solutions DUNMAN HIGH SCHOOL Preliminary Examinations 2018 Year 6 H2 CHEMISTRY 9729/04 Paper 4 Practical 28 August 2018 2 hour 30 minutes INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Give details of the practical shift and laboratory where appropriate, in the boxes provided. 3 Write in dark blue or black pen. 4 You may use an HB pencil for any diagrams or graphs. 5 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. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 17 and 18. The number of marks is given in brackets, [ ], at the end of each question or part question. Shift Laboratory For Examiner’s Use Question No. Marks 1 14 2 13 3 28 Total 55

© DHS 2018 9729/04 Answer all questions in the spaces provided. 1 Investigation of reaction kinetics between triacetin and lipase Lipase is an enzyme that breaks down, hydrolyses, the lipids present in animal fats and vegetable oils. It does so by hydrolysing ester bonds. A general reaction scheme for this process is shown below; the size of the R group and its level of saturation will depend on the lipid used. CH2OCOR CHOCOR CH2OCOR + 3H2O CH2OH CHOH CH2OH + 3RCOOH glycerol fatty acid Olive oil is often used as a substrate in testing for lipase, with the oleic acid released being titrated with sodium hydroxide solution. Oleic acid, C17H33COOH, is a mono–unsaturated fatty acid. In this experiment, glycerol triethanoate (triacetin) is used as the substrate. Lipase breaks this substrate down into glycerol and ethanoic acid as shown below. CH2OCOCH3 CH2OCOCH3 CH2OCOCH3 + 3H2O CH2OH CHOH CH2OH + 3CH3COOH glycerol triethanoate (triacetin) The ethanoic acid produced lowers the pH of the solution. This change can be detected by using a suitable acid–base indicator, bromocresol purple which changes from purple at high pH value to yellow at lower pH value. Sodium carbonate is added to the triacetin to ensure that the pH is high enough at the start of the experiment. Only after the sodium carbonate has all reacted with the liberated ethanoic acid will the ethanoic acid concentration rise and the pH value decreases. The reaction time is measured from the initial point of mixing the solutions until the end–point of the indicator is reached. The following reagents are provided for this experiment. FA 1 is triacetin FA 2 is 0.5% sodium carbonate solution FA 3 is 8% lipase solution FA 4 is Bromocresol purple solution

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