edexcel a level chemistry unit 8 paper jan 2000

8/17/2019 EdExcel a Level Chemistry Unit 8 Paper Jan 2000

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1. (a) Explain, with reference to the standard electrode potential for sodium and hydrogen, whysodium is manufactured using this method rather than by the electrolysis of aqueoussodium chloride.

Na+(aq) + e – Na(s); E = –2.71 V

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(b) A sample of the sodium chloride which was to be used in the process in (a)(i) wascontaminated with sodium carbonate. The carbonate ion can be removed from a solutionin water as insoluble barium carbonate and the remaining chloride ions can be analysed bytitration 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 andmade up to 250 cm 3. A 25.0 cm 3 sample of this solution was taken and mixed with excessbarium nitrate solution. After filtering and washing the residue, the filtrate was titratedwith silver nitrate solution of concentration 0.430 mol dm –3; 19.7 cm 3 of the silver nitratesolution was required for complete reaction with the chloride ions.

(i) Calculate the number of moles of Ag + in 19.7 cm 3 of the silver nitrate solution.

(1)

(ii) Hence determine the number of moles of Cl – ions in the 250 cm 3 of the sodiumchloride solution.

(1)

(iii) Hence determine the mass of sodium chloride in the solution.

(1)

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(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 thesodium chloride.

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(Total 8 marks)

2. (a) Define the term lattice enthalpy .

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(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 energychange/kJ mol –1

Na(g) ➤ Na +(g) + e – +494

F2(g) ➤ 2F(g) +158

F(g) + e – ➤ F– (g) –348

Na(s) + ½F 2(g) ➤ NaF(s) –569

Na(s) ➤ Na(g) +109

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(c) The table below gives some information about the hydroxides of the Group 2 elements.

salt lattice enthalpy/kJmol –l

hydrationenthalpy/kJ mol –l

solubility inwater/g per

100 g of water

magnesium

hydroxide

–2383 –2380 0.9 × 10 –4

calciumhydroxide

–2094 –2110 156 × 10 –4

strontiumhydroxide

–1894 –1940 800 × 10 –4

bariumhydroxide

–1768 –1820 3900 × 10 –4

(i) Explain why energy is required to break up an ionic lattice.

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(ii) Suggest why the lattice enthalpies of the hydroxides of Group 2 metals becomemore exothermic from Ba(OH) 2 to Mg(OH) 2.

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(iii) Suggest why the lattice enthalpy of beryllium hydroxide, Be(OH) 2, cannot bepredicted from the data in the table.

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(iv) Explain why energy is released when ions are hydrated.

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(v) Hence, account for the trend in solubilities from Ba(OH) 2 to Mg(OH) 2

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(Total 16 marks)

3. (a) Define the term standard electrode potential for a metal/metal ion system

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(b) What would you expect to happen if a small piece of copper was placed in a aqueoussolution of iron(II) sulphate? Give your reasoning.

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(c) The standard electrode potentials of the reactions involved in the first stage in the rustingof iron are

Fe2+ (aq)+ 2e – Fe(s) –0.44 volts

O2(g) + 2H 2O(l)+ 4e – 4OH – (aq) +0.40 volts

(i) Write an overall equation for the first stage in the rusting of iron.

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(ii) Explain how magnesium metal attached to a sheet of iron prevents it from rusting.

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(Total 8 marks)

4. (a) (i) State the oxidation number of iodine in KIO 3 and KIO 4.

KIO 3 …..……………………………………………………………………….

KIO 4 …..……………………………………………………………………….(2)

(ii) In the reaction

IO ✲

3 (aq) + 5I – (aq)+ 6H + (aq) ➤ 3I2(s) + 3H 2O (1)

iodine is simultaneously oxidised and reduced. Explain why this is not adisproportionation reaction.

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(iii) Describe a chemical test you could carry out to show that iodine was produced.

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(iv) Suggest, by reference to the ionic equation in part (ii), the ionic equation forthe reduction of KIO 4 to iodine in the presence of excess acid and excesspotassium iodide.

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(b) Cyanogen, (CN) 2, is a gas which is soluble in water to give the weak acid hydrocyanicacid, HCN

(CN) 2(g) + H 2O(1) ➤ HCN(aq) + HCNO(aq)

In aqueous alkalis it gives

(CN) 2(g) + 2OH–

(aq)➤

CN–

(aq) + CNO–

(aq) + H 2O(1)

It reacts with hot metals to form cyanides

(CN) 2(g) + 2K(s) ➤ 2KCN(s)

(i) It is said that cyanogen behaves as if it were a Group 7 element. Give TWOpieces of evidence in support of this statement.

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(ii) Suggest the equation for the reaction between potassium cyanide and concentratedsulphuric acid when heated.

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(c) Suggest why the molar mass of aluminium(III) chloride appears to vary between 133.5and 267 depending upon the temperature.

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(Total 14 marks)

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edexcel a level chemistry unit 8 paper jan 2000

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