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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
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(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)
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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.
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(2)
(Total 8 marks)
2.
(a)
Define the term lattice enthalpy.
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(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)
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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.
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(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.
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(2)
(iii)
Suggest why the lattice enthalpy of beryllium hydroxide, Be(OH)2, cannot be
predicted from the data in the table.
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(1)
(iv)
Explain why energy is released when ions are hydrated.
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(2)
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Winter 2000 CH3
(v)
Hence, account for the trend in solubilities from Ba(OH)2 to Mg(OH)2
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(3)
(Total 16 marks)
3.
(a)
Define the term standard electrode potential for a metal/metal ion system
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(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.
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(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.
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(2)
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Winter 2000 CH3
(ii)
Explain how magnesium metal attached to a sheet of iron prevents it from rusting.
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(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.
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(1)
(iii)
Describe a chemical test you could carry out to show that iodine was produced.
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(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.
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(2)
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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.
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(2)
(ii)
Suggest the equation for the reaction between potassium cyanide and concentrated
sulphuric acid when heated.
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(2)
(c)
Suggest why the molar mass of aluminium(III) chloride appears to vary between 133.5
and 267 depending upon the temperature.
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(3)
(Total 14 marks)
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