SAJC H2 CHEM P2 ANS Prelim

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 Preliminary Exam (TUTOR’S COPY) Chemistry Higher 2 9647/02 1 September 2015 Paper 2 2 hours Candidates answer in the spaces provided on the question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. 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. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use: Question Marks 1 2 3 4 5 Total (72 marks) This document consists of 21 printed pages (including this page) and 1 blank page.

2 [Turn Over 1. Planning Calcium hydroxide, Ca(OH) 2, is sufficiently insoluble in water such that it forms a saturated Ca(OH) 2 solution, as shown in the following equation: Ca(OH)2(s) Ca2+(aq) + 2OH-(aq) The following experiments were carried out to determine the solubility product of calcium hydroxide. Different volumes of NaOH and water were added separately to different dry conical flasks according to the following table. Experiment Vol of NaOH / cm3 Vol. of water / cm3 [OH-] in the saturated solution/ mol dm-3 1 5.00 45.00 0.04 2 10.00 40.00 0.03 3 15.00 35.00 0.02 4 20.00 30.00 0.01 5 25.00 25.00 0.001 Excess solid calcium hydroxide was added to each conical flask before it was stoppered and shaken for a few minutes. The solution was filtered and titrated to determine the concentration of hydroxide ions in the saturated solution. (a) On the grid provided, plot a graph using the values from the above table. [1] 0 0.01 0.02 0.03 0.04 0.05 0.06 0 5 10 15 20 25 30 [OH-] in the saturated solution/ mol dm-3 Vol of NaOH / cm3

3 [Turn Over 1(b) Explain why the concentration of hydroxide ions in the saturated solution decreases when more sodium hydroxide is added to the conical flask. [1] [OH-] increases, hence equilibrium to shift to the left . (c) Explain why dry conical flasks needs to be used to prepare the saturated solution. [1] To ensure that the concentration of OH - used is accurate/ dilution will occur/ affect the [OH-] (d) Write the Ksp expression for Ca(OH)2, stating its units. [2] Ksp = [Ca2+(aq)] [OH-(aq)]2 Units: mol3 dm-9 (e) Using the graph plotted, calculate the numerical value of Ksp of Ca(OH)2 [2] Extrapolate the line to the y axis to determine the concentration of [OH-(aq)] when no NaOH(aq) is added. [OH-(aq)] = 0.05 mol dm-3 [Ca2+] = 0.05/2 = 0.025 mol dm-3 Ksp = [Ca2+] [OH-]2 = 0.025 x 0.052 = 6.25 x 10-5 mol3 dm-9 (f) Using the information above, you are required to write a plan to  prepare a saturated solution of calcium hydroxide and  determine the concentration of hydroxide ions in the saturated solution in experiment 1. You may assume that you are provided with:  solid calcium hydroxide  1 mol dm–3 aqueous hydrochloric acid;  the apparatus normally found in the college laboratory Outline how you would carry out experiment 1. Your plan should include details of :  the apparatus you would use, bearing in mind the levels of precision they offer;  Indicate the colour change at the end point with your chosen indicator. [5]

4 [Turn Over 1(f) Preparation of saturated solution of Ca(OH)2 for titration From a burette, run 5 cm 3 of NaOH(aq) and 45 cm 3 of distilled water from a burette into a 250 cm3 conical flask. Add in excess Ca(OH)2(s) until a white solid of Ca(OH)2 remains Stopper the conical flask and shake for a few minutes. Filter using filter funnel and filter paper and collect the filtrate which contains saturated Ca(OH)2 solution. Dilution of HCl to 0.04 mol dm-3 From a burette, run 10.00 cm3 of 1 mol dm-3 HCl into a 250 cm3 volumetric flask. Top up to the mark using distilled water. Stopper and shake well to obtain a homogeneous solution. Pour this diluted HCl solution into a burette. Titration Pipette 25.0 cm3 of the saturated Ca(OH)2 solution into the conical flask. Add 1 – 2 drops of phenolphthalein / methyl orange to the solution and titrate against the diluted HCl in the burette until pink colour decolourises. Repeat the titrations to obtain at least two consistent results within ± 0.10 cm3. [Total: 12 marks] 2 The Cobasys’ NiMH batteries supersede the performance of other rechargeable batteries due to its high performance, lighter weight and longer life cycle. It uses a metal hydride alloy as one of the electrode. Today, NiMH batteries are used pervasively in many application s such as cell phones and hybrid cars. (a) The standard electrode potential of the NiOOH(aq) / Ni(OH) 2(s) system in an alkaline medium is +0.41 V. Draw a fully labelled diagram of how you would determine the standard electrode potential of the NiOOH(aq) / Ni(OH)2(s) system.

5 [Turn Over 2(a) (b) The equation at the anode is MH + OH− M + H2O + e− Construct the half equation that occurs at the cathode in an alkaline medium , and the overall equation for the Cobasys’ NiMH cell during discharging. Cathode: ………………………………………………………………….... Overall: ………………………………………………………………………. Cathode: NiOOH + H2O + e− Ni(OH)2 +OH- Overall: NiOOH + MH  Ni(OH)2 + M [2] 2 (c) The table below shows the standard electrode potential of different types of metal hydride, XH, YH, ZH that could possibly be used as an anode in the Cobasys’ NiMH cell. Metal Hydride, MH Eθ value/ V XH + 0.37 YH + 0.58 ZH - 0.42 Ni(OH)2 [NiOOH] = 1 mol dm-3 e- Cathode (+) Anode (-)

6 [Turn Over (i) State which metal hydride is the most effective to be used in the Cobasys’ NiMH cell. ……………………………………………… ZH. [1] 2 (c) (ii) Explain your answer in (c)(i). Eθ cell = Eθ (cathode) - Eθ (anode) = + 0.41 - Eθ (anode) As E θ(anode) for ZH is the least positive / most negative , the Eθ cell would be the most positive or ZH is most easily oxidized hence strongest battery. [1] (d) Explain the effect, if any, on the e.m.f. of the cell on adding aqueous H2SO4 to the anodic half cell during discharging. M + H2O + e− MH + OH− (1) The Eθ(anode) is less negative/ more positive or decreasing the extent of oxidation as the equilibrium position of (1) shift to the RHS (must come with eqm eqn) [½], to replenish the loss of OH-. Hence, the emf is likely to be more negative. [2]

7 [Turn Over (e) The Cobasys’ NiMH cell can be recharged when the power is depleted. (i) Write the overall equation for the reaction upon charging. Ni(OH)2 + M  NiOOH + MH [1] (ii) To charge the Cobasys’ NiMH cell completely, a continuous current of 2.0 A for two hours is required. Calculate the minimum mass of the Ni(OH) 2 that would have to be present initially. Q = It =2 x 2 x 60 x 60 = 14400 C Ni(OH)2 + OH− NiOOH + H2O + e− No of Faraday = 14400 / 96500 =0.149 F Since 1F deposit 1 mole of Ni(OH)2, 0.149 F will deposit 0.149 mole of Ni(OH)2 Mass of Ni(OH)2 present initially = 0.149 x 92.7 = 13.8 g [3] [Total: 13 marks]

8 [Turn Over 3 Two cations and one anion are mixed together to form a brownish green solution, FA 1. A series of tests is