14 (B410U10-1) Examiner only 14. Emily was investigating the reversible reaction between methanol and ethanoic acid. CH3OH(l) + CH3COOH(l) a CH3COOCH3(l) + H2O(l) Reaction 1 She followed these instructions. • Heat 16.0 g of methanol with 36.0 g of ethanoic acid under reflux for one hour so that the reaction reaches equilibrium. • Titrate the contents of the flask with aqueous sodium hydroxide. Complete this titration as quickly as possible. • Note the volume of alkali used. The equation for the reaction with sodium hydroxide is as follows. CH3COOH(l) + NaOH(aq) CH3COONa(aq) + H2O(l) (a) Write the expression for the equilibrium constant, Kc, for the reaction between methanol and ethanoic acid, Reaction 1. [1] (b) Emily calculated that she had used 0.220 mol of sodium hydroxide in her titration. How many moles of ethanoic acid were present in the flask at equilibrium? [1] Number of moles of acid = .............................................. © WJEC CBAC Ltd. (B410U10-1) Turn over. 15 Examiner only (c) Emily also calculated that there were 0.12 mol of methanol in the flask at equilibrium. Her teacher told her that the equilibrium constant, Kc, for this reaction is 5.47. Emily looks at the equation for the equilibrium and sees that the number of moles of ester, CH3COOCH3, and water at equilibrium would be the same. Use these data, and the value found in (b), to calculate the number of moles of ester present at equilibrium. In this reaction you should assume that the concentration, in mol dm−3, is equal to the number of moles of each substance present. [3] Number of moles of ester = .............................................. (d) Another student carried out a similar experiment to that above to determine the value of the equilibrium constant, Kc. However he refluxed the mixture for only 30 minutes. When he calculated the value of Kc he found it to have a significantly lower value than that found in data books. Give a possible explanation for the difference in the value of Kc. [1] (e) Explain why the heating should be carried out under reflux. [1] © WJEC CBAC Ltd. (B410U10-1) 16 Examiner only (f) (i) Explain why the instructions said that the titration should be completed as quickly as possible. [2] (ii) What would be the effect on the value of Kc that Emily calculated if she took a long time carrying out the titration? Explain your answer. [1] (g) The value of Kc for this reaction decreases if the temperature at which it is carried out increases. Deduce whether the forward reaction is exothermic or endothermic. Explain your reasoning. [2] END OF PAPER © WJEC CBAC Ltd. 12 (B410U10-1) 17 Examiner only © WJEC CBAC Ltd. For continuation only. BLANK PAGE (B410U10-1) 18 © WJEC CBAC Ltd. BLANK PAGE (B410U10-1) 19 © WJEC CBAC Ltd.