CIE Nov 2016 v1 Paper 5 Q2

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Sucrose, C12H22O11, is a naturally occurring sugar found in sugarcane and many fruits. It can be hydrolysed in acidic solution to give glucose and fructose. All three molecules are chiral and will rotate the plane of polarised light. The degree of rotation is known as the optical rotation, . PMT CH2OH O OH CH2OH O HO + H2O CH2OH HO OH OH sucrose H+ catalyst CH2OH O OH CH2OH O HO HO OH OH OH glucose OH fructose CH2OH In the presence of excess water, the reaction can be considered to be first order with respect to sucrose concentration. The progress of the reaction can be monitored using a polarimeter, which measures the optical rotation, , of the solution. The more concentrated the solution, the greater the optical rotation of the solution. The concentration of sucrose at time t can be represented as (final), where final is the optical rotation of the solution after 6 hours. The mathematical relationship is given by the following equation. kt log10(final) = A2.30 A is a constant. k is the rate constant.UCLES 2016 9701/51/O/N/16 [Turn over<br /> PMT (a) The experimentally determined values of optical rotation during the hydrolysis of sucrose at 298 K are recorded below. Process the results to allow you to plot a graph of log10(final) against time, t. Calculate (final) and record it to 1 decimal place. Calculate log10(final) and record it to 2 decimal places. time / s optical rotation,(final) log10(final) 0 300 600 900 1200 1500 1800 2100 2400 2700 final 39.9 29.1 21.3 15.5 10.6 6.2 2.4 0.3 2.5 4.5 12.0 (b) (i) Plot a graph on the grid on page 9 to show how log10(final) varies with time, t. Use a cross () to plot each data point. Draw the line of best fit. [2] [2] (ii) State and explain whether the results and your graph confirm the relationship kt log10(final) = A2.30[1]UCLES 2016 9701/51/O/N/16<br /> PMT 1.8 1.7 1.6 1.5 1.4 1.3 log10()final 1.2 1.1 1.0 0.9 0.8 0.7 0.6 500 1000 1500 time / s 2000 2500 3000UCLES 2016 9701/51/O/N/16 [Turn over<br /> 10 (c) (i) Determine the gradient of the graph. State the co-ordinates of both points you used for your calculation. Record the value of the gradient to three significant figures. co-ordinates 1co-ordinates 2PMT gradient =s1 [2] (ii) Use the gradient value to calculate a value for k in the expression shown. kt log10(final) = A2.30 [2]UCLES 2016 9701/51/O/N/16<br /> (d) The graph below shows the results obtained from a second hydrolysis of sucrose reaction performed at a different temperature. 11 PMT 45.0 40.0 35.0 30.0 25.0final 20.0 15.0 10.0 5.0 0.0 400 800 1200 1600 2000 2400 2800 time / s (i) The point at time = 2000 s is considered to be anomalous. Suggest what caused the anomaly.[1]UCLES 2016 9701/51/O/N/16 [Turn over<br /> 12 (ii) Use the graph to determine the half-life, t, of this reaction. State the co-ordinates of both points you used in your calculation. co-ordinates 1co-ordinates 2PMT half-life =s [2] (iii) For a first-order reaction, the following relationship exists. half-life, t = 0.693 k' Use this relationship and your answer to (ii) to determine k', the rate constant for this second hydrolysis reaction. If you have been unable to determine the half-life of the reaction in (ii), you may use the value t = 500 s, though this is not the correct answer. k' =s1 [1] (iv) State whether the temperature of the second reaction was higher or lower than that of the first. Explain your answer with reference to the answers you obtained in (c)(ii) and (d)(iii). If you have been unable to calculate a value for k in (c)(ii), you may use the value k = 8.00104, though this is not the correct answer.[1] (v) Would the value of the half-life change if the reaction were repeated with twice the initial concentration of sucrose? Give a reason for your answer.[1] [Total: 15] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.UCLES 2016 9701/51/O/N/16<br />

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