PJC H2 CHEM P2

PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CANDIDATE NAME CT INDEX GROUP NUMBER CHEMISTRY 9647/02 Paper 2 Structured 23 September 2014 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, CT group and index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 21 printed pages. FOR EXAMINER’S USE Paper 2 1 / 12 5 / 12 2 / 15 3 / 10 Penalty sf units 4 / 23 Total / 72 1 3

© PJC 2014 9647/02/JC2 Prelim/2014 2 Answer all the questions in the spaces provided. 1 Planning The label of a bottle of an aqueous acid, represented by HxA, had been damaged. Only the concentration of the acid, 0.50 mol dm3 was known. A student was tasked to determine (1 ) if the acid is monobasic or dibasic, and (2) if the acid is a strong acid or a weak acid. (a) The student was given a 0.50 mol dm3 sodium hydroxide solution. She proposed to first determine the basicity of the acid in the bottle by mixing different v olumes of the acid and sodium hydroxide and measure the maximum temperature change, T, for each of the constant volume mixtures. The following experiments were performed: Experiment 1: 50 cm3 of HxA(aq) was added to 100 cm3 of NaOH(aq) Experiment 2: 100 cm3 of HxA(aq) was added to 50 cm3 of NaOH(aq) The changes in temperature of the mixture were measured for Experiment 1 and Experiment 2 as T1 and T2 respectively. (i) Write an equation to represent the standard enthalpy change of neutralisation between sodium hydroxide and HxA. (ii) Explain how the basicity of the acid can be determined by comparing the T1 and T2 values. [5]

© PJC 2014 9647/02/JC2 Prelim/2014 3 (b) It was determined that the acid in the bottle is a monobasic acid, HA. The student proceeded to determine if HA is a strong or weak acid using the following:  FA 1, a solution of 0.50 mol dm–3 sodium hydroxide.  FA 2, a solution of 0.50 mol dm–3 HA.  Thermometers  Styrofoam cups  Apparatus normally found in a college laboratory She performed a series of experiments by mixing different volumes of FA 1 with FA 2 and measuring the temperature changes of each of these mixtures. (i) Write a plan to determine the temperature changes, ∆T, for the following series of reactions between FA 1 and FA 2: Mixture Volume of FA 1 / cm3 Volume of FA 2 / cm3 1 15 45 2 25 35 3 35 25 4 45 15 5 50 10 Your plan should include the apparatus for measurement of volume. Indicate clearly the measurements that will be made during the experiment and show how these measurements can be used to obtain the temperature change, T.

© PJC 2014 9647/02/JC2 Prelim/2014 4

© PJC 2014 9647/02/JC2 Prelim/2014 5 (ii) The student performed the experiment for the series of reactions between FA 1 and FA 2 and the temperature changes were plotted as shown below. By means of two straight lines , determine the ∆Tmax for the ser ies of experiments.

© PJC 2014 9647/02/JC2 Prelim/2014 6 (iii) Using your graph in (b)(ii), calculate the enthalpy change of neutralisation for the reaction between HA and NaOH . [Assume that the heat capacity of the solution = 4.2 J K–1 cm–3.] (iv) Given that the enthalpy change of neutralis ation for reaction between HC l and NaOH is –57.3 kJ mol –1, deduce whether the unknown acid HA is a strong acid or a weak acid. Explain your answer. [7] [Total: 12]

© PJC 2014 9647/02/JC2 Prelim/2014 7 2 Magnesium metal can be manufactured by the electrolysis of molten magnesium salt, Z using the setup shown below. During the electrolysis process, 0.912 g of unknown gas Y was produced which occupied 500 cm3 volume at 200 °C and 1 atm. (a) (i) Determine the Mr of gas Y and hence identify it. (ii) Explain why you would expect the behavio ur of g as Y to be less ideal at low temperature. (iii) Write ion-electron equations for the reaction occurring at the cathode and anode. Cathode: Anode:

© PJC 2014 9647/02/JC2 Prelim/2014 8 (iv) Given that the electrolysis took place for 40 minutes, calculate the current used for the process. (v) Using relevant Eo data from the Data Booklet , explain why magnesium metal cannot be obtained by the electrolysis of aqueous magnesium salt. [8]

© PJC 2014 9647/02/JC2 Prelim/2014 9 (b) Ethylenediamine (en) can act as a bidentate ligand. The structure of en is shown below. NH2 NH2 ethylenediamine [Ni(NH3)6]2+(aq) and [Ni( en)3]2+(aq) can be prepared from ligand exchange reaction of 1 mol dm3 [Ni(H2O)6]2+(aq) with NH3 and en ligands respectively. (1): [Ni(H2O)6]2+(aq) + 6NH3(aq) ⇌ [Ni(NH3)6]2+(aq) + 6H2O(l) (2): [Ni(H2O)6]2+(aq) + 3en(aq) ⇌ [Ni(en)3]2+(aq) + 6H2O(l) The standard enthalpy change of reaction, ΔHo r, and standard entropy change of reaction, ΔSo r, for the ligand exchange reactions (1) and (2) are as follows. Reaction ΔHo r / kJ mol–1 ΔSo r / J mol–1 (1) 27.5 +4.5 (2) 35.0 +34.0 The ΔHo r for both reactions (1) and (2) are similar as the Ni -O bonds broken and Ni-N bonds formed are similar. Chelate effect refers to the greater stability of complexes formed by pol ydentate ligands (e.g. en) than those formed by monodentate ligands (e.g. NH3). (i) Explain why the ΔSo r for reaction (1) is so different from that of reaction (2). (ii) Calculate the standard Gibbs free energy change , ΔGo r for reactions (1) and (2) to show why [Ni(en)3]2+ exhibit the chelate effect.

© PJC 2014 9647/02/JC2 Prelim/2014 10 (iii) Using your answers from (b)(i) and (b)(ii), comment qualitatively on the statement: “Chelate effect is an entropy effect.” (iv) Stronger field ligands are known to give rise to a larger energy gap between the two sets of d-orbitals in a transition metal complex. The figure below shows a colour wheel with approximate wavelength values (in nm) for different colour lig ht. As wavelength decreases, the energy of the light increases. violet blue green red orange yellow 750 400 630 590 560 480 430 Various complexes of Ni have different colours. The colours of [Ni(CN)6]3–(aq) and [Ni(en)3]2+(aq) are given below. Complex Colour [Ni(CN)6]3–(aq) red [Ni(en)3]2+(aq) violet State whether CN– or en is a stronger field ligand. Explain your answer. [7] [Total: 15]