VJC_H2_CHEM_P2_ANS_FINAL Prelim

 VJC 2011 9647/02/PRELIM/11 [Turn over 1 Victoria Junior College 2011 H2 Chemistry Prelim Exam 9647/2 Suggested Answers 1 Planning An acid can be represented by the general formula, H xA, where x represents the basicity of the acid. A bottle of a dilute aqueous solution of an acid, either monobasic or dibasic, was found. However, the label on the bottle had been damaged. Only the concentration of the acid, 1.00 mol dm 3 was readable. A student was given a 1.00 mol dm 3 sodium hydroxide solution. She was asked to determine the basicity of the acid in the bottl e by mixing different volumes of the acid and sodium hydroxide. (a) (i) Write an equation to represent the molar enthalpy change of neutralization between sodium hydroxide and HxA. 1 x HxA + NaOH  1 x NaxA + H2O [1] (ii) A student determined the basicity of the acid by performing the following experiments: Experiment 1: 30 cm3 of HxA(aq) was added to 60 cm3 of NaOH(aq) Experiment 2: 60 cm3 of HxA(aq) was added to 30 cm3 of NaOH(aq) The changes in temperature of the mixture were measured for Experiment 1 and Experiment 2 as T1 and T2 respectively. Suggest and explain the basicity of HxA if T1 = 2 x T2 Heat is released when water is formed in the neutralization process. The amount of heat released in Experiment 1 is twice that in Experiment 2. Since total volume of mixture remained unchanged, therefore, the number of moles of water formed in Experiment 1 is twice that in Experiment 2. H xA is a dibasic acid because in Experiment 1, all the acid and alkali are reacted while in Experiment 2, the alkali is the limiting reagent. Alternatively If x = 1 HA + NaOH  NaA + H 2O [2]

 VJC 2011 9647/02/PRELIM/11 [Turn over 2 Experiment H xA NaOH ηH2O 1 30 (limiting) 60 (excess) y 2 60 (excess) 30 (limiting) y Both experiments will produce same amount of water. Hence ∆T1 = ∆T2 (since total volume of mixture is fixed) Experiment H xA NaOH ηH2O 1 30 (limiting) 60 (limiting) 2y 2 60 (excess) 30 (limiting) y If x = 2 H2A + 2 NaOH  Na2A +2 H2O Experiment 1 will produce twice the amount of H2O. Hence ∆T1 = 2∆T2 (since total volume of mixture is fixed) ∴ x = 2 dibasic acid (b) For this part, you may assume that the acid in the bottle is a monobasic acid, HA. A student decided to determine whether the acid, HA, is a strong or weak acid by performing a series of experiments involving mixing of hydrochloric acid with sodium hydroxide and HA with sodium hydroxide. Explain how, from the suggested experiment, the student might be able to determine whether HA is a strong or weak acid. If HA is a weak acid, some of the heat evolve during neutralization is absorbed to dissociate the weak acid, HA, fully. Hence ∆T (HA/NaOH) < ∆T (HCl/NaOH). If HA is a strong acid, full dissociation occurs like HC l, ∆T (HA/NaOH) = ∆T (HCl/NaOH) [2] FA 1 is a solution of sodium hyd