Edexcel Jun 2016 (IAL) Paper 6 Q

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2 Ethyl ethanoate, CH3COOC2H5, is hydrolysed in the presence of a catalyst of dilute hydrochloric acid, HCl(aq), according to the equation below. CH3COOC2H5(l) + H2O(l) (cid:85) C2H5OH(l) + CH3COOH(l) (cid:168)H = +3.5 kJ mol1 In an experiment carried out by a teacher to determine the equilibrium constant, Kc , the following steps are carried out.15.0 cm3 (0.153 mol) of ethyl ethanoate and 10.0 cm3 dilute hydrochloric acid, concentration 1.00 mol dm3, are mixed together. In this pre-equilibrium mixture, there is 0.556 mol of water.The mixture is left for several days, at room temperature and pressure, to reach equilibrium.Using a pipette, several 5.00 cm3 samples are taken from the 25.0 cm3 of equilibrium mixture.Each 5.00 cm3 sample is titrated with dilute sodium hydroxide solution from a burette. The concentration of the sodium hydroxide solution is 0.500 mol dm3.The mean titre of sodium hydroxide is 42.40 cm3. This neutralizes both the hydrochloric acid catalyst and the ethanoic acid in 5.0 cm3 of the equilibrium mixture. (a) (i) Show that the 25.0 cm3 of equilibrium mixture contains 0.0960 mol of CH3COOH. (3) *P46666A0624* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (ii) Deduce the number of moles of each of the other components in the equilibrium mixture. (3) Equilibrium moles of C2H5OH =mol Equilibrium moles of CH3COOC2H5 =mol Equilibrium moles of H2O =mol (iii) Give the expression for the equilibrium constant, Kc, for the reaction CH3COOC2H5(l) + H2O(l) (cid:85) C2H5OH(l) + CH3COOH(l) (1) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46666A0724* Turn over<br /> (iv) The equilibrium constant, Kc, is defined in terms of the concentrations in the equilibrium mixture. However, in this case, Kc can be calculated using moles rather than concentrations. Calculate the value of Kc from the data in parts (a)(i) and (a)(ii). Give your answer to three significant figures. (2) D O N O T W R I T E I N T H I S A R E A Kc =(v) Explain why it is possible, in this case, to calculate Kc using moles rather than concentrations. (1)(b) The experiment was repeated by a student whose value for Kc differed from the value calculated in (a)(iv). The student made several suggestions to explain this. State and explain how, if at all, each suggestion would affect the Kc value obtained by the student, compared with that from (a)(iv). (i) Suggestion 1The concentration of the sodium hydroxide solution used by the student was less than 0.500 mol dm3. (2) How the students value of Kc compared with that from (a)(iv)Explanation*P46666A0824* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (ii) Suggestion 2The mixture had been left in a warm part of the laboratory and the equilibrium had been reached at a temperature above room temperature. (2) How the students value of Kc compared with that from (a)(iv)Explanation(c) A second student repeated the original experiment using exactly the same method. However, when using the pipette and the burette, the readings were taken from the top of the meniscus. (i) How, if at all, will this affect the volume of the equilibrium mixture delivered from the pipette? Justify your answer. (1)(ii) How, if at all, will this affect the volume of sodium hydroxide solution delivered from the burette? Justify your answer. (1)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46666A0924* Turn over<br /> (iii) One of the students titres was 42.60 cm3 of sodium hydroxide solution. The burette has a maximum uncertainty of 0.05 cm3 for each reading. Calculate the percentage uncertainty in this titre. (1) Percentage uncertainty = % (Total for Question 2 = 17 marks) 10 *P46666A01024* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />

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