Question 1
The chlor-alkali industry is very important in the production
of many chemicals; its principal feedstock is sodium chloride, which is
electrolysed under various conditions.
(a)(i) What is the main impurity in the sodium hydroxide
solution produced from the diaphragm cell, and how is it removed?
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(2)
(ii) There are two products other than sodium hydroxide
obtained from the diaphragm cell. Identify these products and state
one for each in industry.
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(2)
(b) Sodium hydroxide is used in large quantities for the purification of bauxite.
(i) Give an equation for a reaction in which sodium hydroxide
is used in this purification.
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(2)
(ii) Outline how aluminium metal is obtained from purified
bauxite.
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(4)
(iii) Explain in terms of bonding why aluminium is produced
from aluminium oxide rather than from aluminium chloride.
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(2)
Total 20 marks
Question 2
(a) (i) Using the data provided, construct a Born-Haber cycle for magnesium chloride, MgCl2, and from it determined the electron affinity of chlorine.
/\H/kJmol-1
Enthalpy of atomisation of chlorine
+122
Enthalpy of atomisation of magnesium
+148
First ionisation energy of magnesium
+738
Second ionisation energy of magnesium
+1451
Lattice enthalpy of magnesium chloride
-2526
Enthalpy of formation of magnesium chloride
-641
(5)
(ii) The theoretically calculated value for the lattice
enthalpy of magnesium chloride is -2326kJmol-1
Explain the difference between the theoretically
calculated value and the experimental value given in the data in (a)(i),
in terms of the bonding of magnesium chloride.
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(3)
(b) The table below gives some information about the sulphates
of elements in Group 2.
| Sulphate | Solubility/mol dm-3 | Lattice enthalpy/
kJ mol-1 |
Hydration enthalpy of M2+/kJ mol-1 |
| CaSO4 | 4.6*10-2 | -2480 | -1650 |
| SrSO4 | 7.1*10-4 | -2484 | -1480 |
| BaSO4 | 9.4*10-6 | -2374 | -1360 |
(i) Suggest an explanation for the trend in the hydration
enthalpies of the cations.
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(2)
(ii) Comment on the trend in the solubilities of these
sulphates in relation to the lattice and hydration enthalpies given in
the table.
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(4)
(iii) Barium sulphate, which is opaque to X-rays, is used
for the "barium meal" to enable X-ray pictures to be taken of the gut.
Barium ions are very toxic; why is this not a problem here?
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(1)
(iv) Give the equation for the reaction of barium with
cold water.
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(2)
(v) Suggest the practical procedure by which you might
convert the solution of the product in reaction (iv) into a reasonably
pure sample of barium sulphate.
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(3)
Total marks 20
Question 3
(a) Hydrogen peroxide reacts both as an oxidising agent
and as a reducing agent, depending on the conditions. Hydrogen peroxide
reduces potassium dichromate(VI) in acidic solution to chromium(III) ions.
Hydrogen peroxide will oxidise chromium(III) ions back to chromate(VI)
ions in alkaline solution.
Cr2O72-(aq) + 14H+(aq)
+ 6e- <--> 2Cr3+(aq) + 7H2O(l)
Eo = +1.33 V
O2(g) + 2H+(aq) + 2e-
<--> H2O2(aq) Eo
=
+0.68 V
(i) Deduce the overall equation for the reaction between
hydrogen peroxide and dichromate(VI) ions using the data above.
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(2)
(ii) Calculate the standard potential of the cell which
could be formed from these two half cells.
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(1)
(iii) How would you determine in the laboratory the standard potential of the dichromate(VI)/chromium(III) electrode using a hydrogen electrode?
You may answer with a fully labelled diagram or in words.
(5)
(iv) Why is it necessary to operate electrochemical cells
under conditions where virtually no current is drawn from them when finding
the cell e.m.f.?
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(2)
(b) In alkaline solution the reaction of hydrogen peroxide with chromium(III) ions is
2Cr3+(aq) + 10OH-(aq) +3H2O2(aq)
---> 2CrO42-(aq) + 8H2O(l)
This reaction is used to prepare potassium dichromate(VI)
from chromium(III) chloride and hydrogen peroxide in potassium hydroxide
solution. The mixture is boiled until it is bright yellow.
Boiling is continued until excess hydrogen peroxide has been destroyed.
The solution is then cooled, and acidified with ethanoic acid.
(i) The reaction occurring on acidification with ethanoic acid is:
2CrO42-(aq) + 2H+(aq) <--> Cr2O72-(aq) + H2O(l)
Show that this is not a redox reaction.
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(2)
(ii) Hydrogen peroxide on heating reacts as follows:
2H2O2(aq) ---> 2H2O(l) + O2(g)
Suggest how you would know when all the hydrogen peroxide
has been destroyed in the reaction mixture.
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(1)
(iii) Why is it essential to destroy all the hydrogen
peroxide in the mixture before it is acidified?
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(3)
(iv) Derive the two half equations which together give
the overall equation for the reaction between hydrogen peroxide and chromium(III)
ions in alkaline solution.
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(2)
Total 20 marks