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Winter 2000 CH3 1. (a) Explain, with reference to the standard electrode potential for sodium and hydrogen, why sodium is manufactured using this method rather than by the electrolysis of aqueous sodium chloride. Na+(aq) + e– Û Na(s); E = –2.71 V ……………………………………………………………………………………….. ……………………………………………………………………………………….. (2) (b) A sample of the sodium chloride which was to be used in the process in (a)(i) was contaminated with sodium carbonate. The carbonate ion can be removed from a solution in water as insoluble barium carbonate and the remaining chloride ions can be analysed by titration with silver nitrate solution using the reaction Ag+(aq) + Cl–(aq) ➤ AgCl(s) In such an analysis 5.65 g of the contaminated sodium chloride was dissolved in water and made up to 250 cm3. A 25.0 cm3 sample of this solution was taken and mixed with excess barium nitrate solution. After filtering and washing the residue, the filtrate was titrated with silver nitrate solution of concentration 0.430 mol dm–3; 19.7 cm3 of the silver nitrate solution was required for complete reaction with the chloride ions. (i) Calculate the number of moles of Ag+ in 19.7 cm3 of the silver nitrate solution. (1) (ii) Hence determine the number of moles of Cl– ions in the 250 cm3 of the sodium chloride solution. (1) (iii) Hence determine the mass of sodium chloride in the solution. (1) 1 et Winter 2000 CH3 (iv) Calculate the percentage purity, by mass, of the sodium chloride. (1) (v) Suggest an alternative method for finding the amount of sodium carbonate in the sodium chloride. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) (Total 8 marks) 2. (a) Define the term lattice enthalpy. …..…………………………………………………………………………………… …..…………………………………………………………………………………… …..…………………………………………………………………………………… (2) (b) Using the following data, construct a Born-Haber cycle for sodium fluoride and from it determine the lattice enthalpy of sodium fluoride. Process The value of the energy change/kJ mol–1 ➤ Na (g) + e F (g) ➤ 2F(g) F(g) + e ➤ F (g) Na(s) + ½F (g) ➤ NaF(s) Na(s) ➤ Na(g) Na(g) + +494 – +158 2 – –348 – –569 2 +109 (5) 2 et Winter 2000 CH3 (c) The table below gives some information about the hydroxides of the Group 2 elements. salt lattice enthalpy/kJ mol–l hydration enthalpy/kJ mol–l solubility in water/g per 100 g of water magnesium hydroxide –2383 –2380 0.9 × 10–4 calcium hydroxide –2094 –2110 156 × 10–4 strontium hydroxide –1894 –1940 800 × 10–4 barium hydroxide –1768 –1820 3900 × 10–4 (i) Explain why energy is required to break up an ionic lattice. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (1) (ii) Suggest why the lattice enthalpies of the hydroxides of Group 2 metals become more exothermic from Ba(OH)2 to Mg(OH)2. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) (iii) Suggest why the lattice enthalpy of beryllium hydroxide, Be(OH)2, cannot be predicted from the data in the table. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (1) (iv) Explain why energy is released when ions are hydrated. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) 3 et Winter 2000 CH3 (v) Hence, account for the trend in solubilities from Ba(OH)2 to Mg(OH)2 …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (3) (Total 16 marks) 3. (a) Define the term standard electrode potential for a metal/metal ion system …..…………………………………………………………………………………… …..…………………………………………………………………………………… …..…………………………………………………………………………………… (2) (b) What would you expect to happen if a small piece of copper was placed in a aqueous solution of iron(II) sulphate? Give your reasoning. …..…………………………………………………………………………………… …..…………………………………………………………………………………… …..…………………………………………………………………………………… (2) (c) The standard electrode potentials of the reactions involved in the first stage in the rusting of iron are Fe2+ (aq)+ 2e– O2(g) + 2H2O(l)+ 4e– (i) Fe(s) –0.44 volts 4OH– (aq) +0.40 volts Write an overall equation for the first stage in the rusting of iron. …..…………………………………………………………………………….. (2) 4 et Winter 2000 CH3 (ii) Explain how magnesium metal attached to a sheet of iron prevents it from rusting. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) (Total 8 marks) 4. (a) (i) State the oxidation number of iodine in KIO3 and KIO4. KIO3 …..………………………………………………………………………. KIO4 …..………………………………………………………………………. (2) (ii) In the reaction ✲ IO 3 (aq) + 5I– (aq)+ 6H+ (aq) ➤ 3I2(s) + 3H2O (1) iodine is simultaneously oxidised and reduced. Explain why this is not a disproportionation reaction. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (1) (iii) Describe a chemical test you could carry out to show that iodine was produced. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) (iv) Suggest, by reference to the ionic equation in part (ii), the ionic equation for the reduction of KIO4 to iodine in the presence of excess acid and excess potassium iodide. …..…………………………………………………………………………….. (2) 5 et Winter 2000 CH3 (b) Cyanogen, (CN)2, is a gas which is soluble in water to give the weak acid hydrocyanic acid, HCN (CN)2(g) + H2O(1) ➤ HCN(aq) + HCNO(aq) In aqueous alkalis it gives (CN)2(g) + 2OH– (aq) ➤ CN (aq) + CNO (aq) + H O(1) – – 2 It reacts with hot metals to form cyanides (CN)2(g) + 2K(s) (i) ➤ 2KCN(s) It is said that cyanogen behaves as if it were a Group 7 element. Give TWO pieces of evidence in support of this statement. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. …..…………………………………………………………………………….. (2) (ii) Suggest the equation for the reaction between potassium cyanide and concentrated sulphuric acid when heated. …..…………………………………………………………………………….. (2) (c) Suggest why the molar mass of aluminium(III) chloride appears to vary between 133.5 and 267 depending upon the temperature. …..…………………………………………………………………………………… …..…………………………………………………………………………………… …..…………………………………………………………………………………… (3) (Total 14 marks) 6 et