Edexcel Jun 2011 Paper 6 Q2

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PMT 2 The tertiary halogenoalkane 2-chloro-2-methylpropane reacts with hydroxide ions in solution as follows: (CH3)3CCl + OH(cid:236) (cid:111) (CH3)3COH + Cl(cid:236) The progress of the reaction can be followed by titrating the reaction mixture with a solution of hydrochloric acid of known concentration. In an experiment designed to determine the order of the reaction, the following procedure was used. 1. 250 cm3 of an ethanolic solution of 2-chloro-2-methylpropane, of concentration 0.100 mol dm(cid:237)3, was placed in a flask in a water bath at 25C. 250 cm3 of aqueous sodium hydroxide solution, also of concentration 0.100 mol dm(cid:237)3, was placed in a similar flask in the same water bath. The temperature of the solutions was allowed to reach 25C. 2. A series of conical flasks were prepared, each containing about 40 cm3 of propanone. 3. The reaction was started by mixing the halogenoalkane solution and the sodium hydroxide solution in a large flask in the water bath. A clock was started as the solutions were mixed. 4. At intervals, a 25 cm3 pipette was used to withdraw samples of the reaction mixture. Each sample was added to a flask containing propanone and the time was noted. The propanone slows but does not completely stop the reaction. 5. Each sample was titrated immediately with a solution of hydrochloric acid of concentration 0.0500 mol dm(cid:237)3, using methyl orange as the indicator. (a) (i) What colour change would you see at the end point of the titration? (1)(ii) Explain why it is necessary to titrate the samples immediately after they have been withdrawn from the reaction mixture. State the effect, if any, on the titre if this were not done. (2)*P38481A0412*<br /> PMT (b) Suggest why it is necessary to use a solvent of aqueous ethanol rather than water alone for this reaction. (1)(c) In an experiment which was carried out as described above, the following data were obtained. Time/min Volume of HCl /cm3 1 25.5 5 22.5 12 18.5 20 15.5 32 11.5 49 8.0 (i) Using the axes below, plot a suitable graph of these data. 65 5.5 (2) Volume of HCl /cm3 Time/min *P38481A0512* Turn over<br /> PMT (ii) Show two successive half-life measurements on your graph and write their values below. (2) First half-lifeSecond half-life(iii) Explain how your answers to (ii) show that this reaction is first order. (1)(iv) Give the units of the rate constant for this reaction. (1)(v) Because the initial concentrations of the reactants are the same, it is not possible to tell whether the rate equation is of the form rate = k[(CH3)3CCl] or of the form rate = k[OH(cid:236)] Suggest a further experiment which could be carried out to show that it is in fact first order with respect to the halogenoalkane. (2)(d) In a further experiment to investigate the hydrolysis, a solution of 2-chloro-2-methylpropane in aqueous ethanol was prepared at room temperature. The pH of this solution was measured at intervals using narrow-range pH paper. The reaction occurring is (CH3)3CCl + H2O (cid:111) (CH3)3COH + H+ + Cl(cid:236) (i) Suggest what the initial pH of the mixture would be. Justify your answer. (1)*P38481A0612*<br /> PMT (ii) The pH rapidly falls to 2 or lower. Explain why this confirms that the rate of the hydrolysis of 2-chloro-2-methylpropane is independent of the hydroxide ion concentration in the reaction (CH3)3CCl + OH(cid:236) (cid:111) (CH3)3COH + Cl(cid:236) (2)(iii) Assuming that the reaction rate follows the rate equation rate = k[(CH3)3CCl] write the equation for the rate-determining step. (2) (Total for Question 2 = 17 marks) *P38481A0712* Turn over<br />

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