DHS H2 Chem 2012 Prelim P3 Soln

2 © DHS 2012 9647/03 [Turn over Answer any four questions. 1 The study of organic compounds includes the collection of kinetic data and thermodynamic data of the compounds. (a) The kinetics of the hydrolysis of the ester, CH 3CH2CO2CH3, may be investigated by the following method. CH 3CH2CO2CH3 + H2O ® CH3CH2CO2H + CH3OH In a 1 dm 3 mixture, 0.350 mol of the ester was hydrolysed by heating with water and using hydrochloric acid as catalyst. The following results were obtained. Time/s Concentration of CH3CH2CO2H/mol dm–3 0 0 340 0.105 680 0.185 1080 0.243 1440 0.278 (i) Suggest how the progress of this reaction may be followed in order to obtain the results as stated. Taking samples of reaction at the stated intervals (i.e. at 340 s, 680 s, 1080 s and 1440 s) and quenching it with large amount of cold water. Titrate sample with dilute NaOH and suitable indicator. (ii) By drawing a suitable graph using the data given above, show that the reaction is first order with respect to the ester. It has been found that the hydrolysis reaction is first order with respect to the hydrochloric acid. y = -1E-07x2 + 0.0003x + 0.001 0 0.05 0.1 0.15 0.2 0.25 0.3 0 200 400 600 800 1000 1200 1400 1600 Time/s [RCOOH]/mo ldm The half life of the ester is approximately 640 seconds based on the data. Hence, the order of reaction with respect to the ester is 1 since a constant half life is established.

3 © DHS 2012 9647/03 [Turn over (iii) Deduce the units of the rate constant. Rate = k[ester][HCl] Units for k is mol–1 dm3 s–1 (iv) State and explain the effect of a catalyst on the rate constant. The catalyst increases the rate constant by decreasing the activation energy of the reaction and offers an alternative pathway for reaction. (v) Using suitable bond energy values from the Data Booklet, calculate the ∆H for the hydrolysis of the ester. ∆H = [ 8 ´ 410 + 2 ´ 350 + 2 ´ 360 + 740 + 2 ´ 460] – [8 ´ 410 + 2 ´ 350 + 2 ´ 360 + 740 + 2 ´ 460] = 0 kJ mol–1 (vi) Given that the standard enthalpy change of reaction for the hydrolysis is +7.6 kJ mol –1, suggest a reason for the difference between this given value and the value that you have calculated in (a)(v). The ester is not in gaseous phase which would make the calculation by bond energy inaccurate. [11] (b) Hydrolysis of ester can be achieved in the biologica l system by enzymes known as esterase. Enzymes are proteins that catalysed a specific chemical transformation in the biological system. Such enzymes are generally quaternary proteins. (i) Sketch and explain the graph showing how the rate of hydrolysis c hanges with increasing concentration of the ester. [ester] / mol dm-3 Rate At low concentration of ester, the rate of hydrolysis is roughly proportional to the concentration of ester and theref