DHS H2 CHEM P2 ANS

2 BLANK P AGE [Turn Over

3 1(a) The chlorides of sodium to phosphorus (NaC l, MgC l2, AlCl3, SiCl 4 and PCl 5) are separately added to water. (i) Sketch a graph to show the pH of the resulting mixture, indicating the pH of sodium chloride. [2] (ii) Account for the pH value of aluminium chloride. The pH is 3. Aluminium chloride undergoes hydrolysis to give an acidic solution due to the high polarising power of Al 3+. [1] (iii) Write equations for the reactions of silicon chloride and phosphorus pentachloride with water. Silicon chloride: SiCl4 (l) + 2H2O (l)  SiO2 (s) + 4HCl (aq) Phosphorus pentachloride: PCl5 (l) + 4H2O (l)  H3PO4 (aq) + 5HCl (aq) [2] pH of chloride Na Mg Al Si P [Turn Over

4 (b) Under suitable conditions iodine and ch lorine react to give int erhalogen compound, IClx. When a pure sample of IClx was dissolved in an excess of aqueous potassium iodide, the iodine liberated requires 17.1 cm3 of 0.1 mol dm–3 of Na2S2O3 for the titration. On the other hand, the chloride ion liberated requires 12.8 cm3 of 0.1 mol dm–3 silver nitrate for complete precipitation. (i) Determine the value of x. Hence, write a balanced equation for the reaction between IClx and I–. IClx + I-  I2 + xCl- I2 + 2S2O3 2-  S4O6 2- + 2I- No. of mol of iodine = 0.0171 x 0. 1 x ½ = 8.55 x 10-4 No. of mol of Cl- = 0.0128 x 0.1 = 1.28 x 10-3 I2 : Cl- 2:3 x = 3 ICl3 + 3I-  2I2 + 3Cl- [4] (ii) Sketch on the same axes, the variation of pV m / RT against p at 300K, for 1 mol of (i) an ideal gas and (ii) IClx gas. Label your graphs clearly where V m is the gas molar volume. [2] pVm / RT p [Turn Over

5 (iii) Using the same axes as in (b)(ii), show the change at 700K for one mol of IClx. Explain your sketch. At a higher temperature, the average K.E of ICl x increases and the intermolecular forces of attractions between the molecules become less significant. [2] [Total: 13] 2 This question is about a Group II element, barium and the reactions of its compounds. (a) Write a balanced chemical equation, with state symbols, for the decomposition of barium nitrate. Ba(NO 3)2  BaO(s) + 2NO2(g) + ½ O2(g) [1] (b) How would you expect the decomposition temperature of barium nitrate to differ from that of calcium nitrate? Explain your answer. Decomposition temperature of barium nitrate will be higher than calcium nitrate. Ba 2+ has lower polarizing power , the nitrate electron cloud polarized to a lower extent , therefore easier to decompose, hence lower decomposition temp. Ca 2+ higher polarizing power due to smaller size and hence nitrate anion is polarized to a larger extent. [3] (c) Draw the dot-and-cross diagram of the NO3 – anion. [2] [Turn Over

6 (d) The solid product formed in (a) reacts with dilute hy drochloric acid to give BaCl2 and H2O. Write a balanced chemical equation, with state symbols, for the reaction of BaCl2 with concentrated sulfuric acid. BaCi2(s) + conc H2SO4(l)  2HCl(g) + BaSO4(s) [1] (e) Predict the products formed from the reaction of barium bromide, BaBr2 with concentrated sulfuric acid. HBr, SO 2 or Br2 [2] (f) Suggest an explanation for the difference in products formed in (d) and (e). Support your explanation with relevant data from the Data Booklet. Cl2+2e 2Cl- Eo =+1.36V Br2 + 2e 2Br- Eo =+1.07V Br– is a better reduc ing agents than Cl –. Br– can reduce conc. H 2SO4 to SO2 but not Cl–.  Ease of oxidation from X –  X2 increases from Cl – to Br - due to the less positive Eo value. [3] [Total: 12] [Turn Over

7 3 The technology behind Direct M ethanol Fuel Cells (DMFC) is still in the early stages of development, but it has successfully powered mobile phones and laptop computers, the target end uses in futu re years. The fuel and oxygen are pumped continuously to the two electrodes which are made of platinum. (a) Write equations for the reactions which take place for the fuel cell. Anode: CH 3OH (l) + H2O (l)  CO2 (g) +6 H+ (aq) + 6e- Cathode: O2 (g) + 4H+ (aq) + 4e  2H2O (l) Overall: 2 CH3OH (l) + 3O2 (g)  2 CO2 (g) + 4H2O (l) [3] (b) Write the cell diagram. Pt(s) I CH3OH(l) I CO2(g) I H+ (aq) I O2(g) I H2O(l) I Pt(s) [1] (c) Give an advantage of the fuel cell with methanol as a fuel. There is no carbon or CO formed in the fuel cell. Or No poisonous gases evolved. [1] [Total: 5] Acidic electrolyte solution Cathode Anode Methanol Carbon dioxide (–) (+) Water Air (CH3OH) [Turn Over

8 4 Magnesium hydroxide, Mg(OH) 2, is sparingly soluble in water. The solubility product of magnesium hydroxide in water is 5.61 x 10–12 mol3 dm–9. (a) Write an expression for the solubility product, Ksp of magnesium hydroxide. [Mg2+][OH-]2 [1] (b) Calculate the solubility of magnesium hydroxide in g dm –3 for a saturated solution of magnesium hydroxide at 25°C. Let the solubility be x. [Mg2+][OH-]2 = 5.61 x 10-12 x(2x)2 =5.61 x 10-12 x = 1.119 x 10-4 mol dm-3 Solubility = 6.53 x 10-3 g [2] (c) Calculate the solubility of Mg(OH) 2 in 5.00 x 10–2 mol dm–3 aqueous sodium hydroxide. Let the solubility of Mg(OH)2 be x mol dm-3. [Mg2+][OH-]2 = 5.61 x 10-12 x(2x + 0.05)2 =5.61 x 10-12 Assuming that x is so small such that 2x + 0.05 ~ 0.05, x(0.05)2 =5.61 x 10-12 x = 2.24 x 10-9 mol dm-3 [2] (d) Equal volumes of solutions containing 5.0 x 10 –3 mol dm –3 magnesium nitrate and 6.0 x 10 –3 mol dm –3 of sodium hydroxide ar e mixed. Predict if a precipitate will be formed. Explain your answer with the aid of relevant calculations. Ksp = [Mg2+][OH–]2 = 5.61 x 10–12 mol2 dm–6 On mixing aq Mg(NO 3)2 and NaOH: [Turn Over

9 New [Mg2+] = V x 5.0 x 10-3 / 2V = 2.5 x 10-3 mol dm–3 New [OH–] = V x 6.0 x 10-3 / 2V = 3 x 10-3 mol dm–3 At saturation point, Ksp = Ionic product Ionic product = [Mg 2+][OH–] 2 = 9 x 10-8[1] >5.61 x 10-12 [1] Yes there will be a ppt. [3] [Total: 8] [Turn Over

10 5 Complete the reaction scheme below by wr iting the structural formula of the organic products (A-D) and the reagents and conditions (1-6) in the spaces below. O COOH C A 3 B 4 5 C H3 CN C H3 D 1 O O NH CH 3 6 2 C H3 Br OH C H3 OH CN C H3 CH2NH2 O COCl SOCl2 or PCl5 room temp H2, Ni heat Heat at 750 °C Al2O3 (alumina) alc. KCN Br2/H2O Total: [10 marks] [Turn Over

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