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) Under what conditions is sodium metal made from
sodium chloride?
........The electrolysis of molten
NaCl
..................... ...... (1)
(ii) Write equations for the reactions which occur at
each electrode, identifying which is an oxidation and which is a reduction.
......anode Cl- --->
1/2Cl2 + e-.........oxidation
......cathode Na+ +
e- ---> Na..........reduction...............................................................
(3)
(iii) The sodium metal produced can be used for the production
of titanium from purified titanium (IV) chloride. Suggest an equation
for such a reaction.
........
TiCl4 + 4Na ----> Ti + 4NaCl ....................................................
(1)
(iv) Titanium (IV) chloride has a melting temperature
of 248 K and a boiling temperature of 410K. Suggest how it might
be purified industrially.
.......... fractional distillation........................................................................
(1)
(b) Electrolysis of aqueous sodium chloride in a diaphragm cell gives sodium hydroxide.
(i) Explain, in terms of the standard electrode potential for sodium and for hydrogen, why the metal is not discharged from aqueous solutions of sodium ions;
Na+(aq) + e- <--> Na(s)
Eo = -2.71 V
........ hydrogen has a standard
electrode potential of 0.00 V.................................................
..... The standard electrode potential
of sodium is more negative than that of hydrogen...........
...... so hydrogen is more easily
reduced and is discharged in place of sodium ............................
(2)
(ii) What is the main impurity in the sodium hydroxide
solution produced from the diaphragm cell, and how is it removed?
.......... Sodium chloride is the
main impurity..................................................
.......... It is removed by crystallisation.............................................................................
(2)
(iii) There are two products other than sodium hydroxide
obtained from the diaphragm cell. Identify these products and state
one for each in industry.
........ Chlorine; used as a disinfectant
in swimming pool water........
......... Hydrogen; used as a fuel
in rocket engines...................................................
(2)
(c) 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.
......... Al2O3(s)
+ 2OH-(aq) + 3H2O(l) ---> 2[Al(OH)4]-(aq)...............
(2)
(ii) Outline how aluminium metal is obtained from purified
bauxite.
..... The purified bauxite is dissolved
in molten cryolite and electrolysed. The cathode is made of carbon
and aluminium forms here during the electrolysis. At the cathode
the reaction is
Al3+ + 3e-
---> Al .........
............................................................................................................................................
(4)
(iii) Explain in terms of bonding why aluminium is produced
from aluminium oxide rather than from aluminium chloride.
... Aluminium chloride is covalent
and so contains no ions and so cannot undergo electrolysis......
.............................................................................................................................................
(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
Mg2+(g) + 2Cl(g) +2e-
____________________________________
/\
2*Eact[Cl(g)]
Mg2+(g) + 2Cl-(g)
\/_____________________________________
Mg2+(g) + Cl2(g) +2e-
+244
____________________________________
/\
Mg+(g) + Cl2(g)
+ 1451
____________________________________
/\
Mg(g) + Cl2(g)
+738
____________________________________
/\
Mg(s) + Cl2(g)
+148
___________________________________
-2526
- 641
MgCl2(s)
\/
\/
_________________________________________________
Using the Born Haber cycle
(+148) + (+738) + (+1451) + (+244) + 2*Eact[Cl(g)] + (-2526) = -641
Eact[Cl(g)] = -348 kJmol-1
(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.
.... The theoretical value assumes
an ionic model....................................
..... Covalent bonding causes the
difference.........................................................................
..... The small highly charged
magnesium ion polarises the chloride ion...............................
(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.
........ The hydration enthalpy
becomes less exothermic as the cation size increases.................
........ because the charge
density decreases..................... (2)
(ii) Comment on the trend in the solubilities of these
sulphates in relation to the lattice and hydration enthalpies given in
the table.
...... The solubility decreases
down the group.............
....... Solubility depends on enthalpy
of solution and .........................
.......enthalpy of solution = enthalpy
of hydration - lattice enthalpy......................................
... Enthalpy of hydration changes
more quickly than lattice enthalpy so.........
.... Enthalpy of hydration has
a more dominant effect....................... (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?
........ Barium sulphate is insoluble......................................
(1)
(iv) Give the equation for the reaction of barium with
cold water.
........ Ba + 2H2O --->
Ba(OH)2 + H2.......................................
(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.
....... Add sodium sulphate solution
to the product....................................
........ Filter the precipitate
then wash and dry it.....................................................................
....................................................................................................................................
(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.
..... Cr2O72-
+ 3H2O2 + 8H+ = ---> 2Cr3+
+ 3O2 +7H2O.................
...........................................................................................................................................
.....................................................................................................................................
(2)
(ii) Calculate the standard potential of the cell which
could be formed from these two half cells.
........ Ecell = EoRight
- EoLeft.... Ecell
= 1.33 - (+0.68) = 0.65V.. (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.?
........... This avoids a voltage
drop across the internal resistance of the cell.............
..........................................................................................................................................
..................................................................................................................................
(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.
.....chromate and dichromate both
contain chromium (+6)...................
..................................................................................................................................
(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.
........ Effervescence stops when
the hydrogen peroxide has been destroyed.................. (1)
(iii) Why is it essential to destroy all the hydrogen
peroxide in the mixture before it is acidified?
....... Acid converts chromate
to dichromate................
........ The dichromate formed
oxidises hydrogen peroxide..................................................
..........The chromium (III) formed
reduces the yield.......................................... (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.
........CrO42-
+4H2O +3e- ---> Cr3+ + 8OH-.............
........H2O2
+ 2e- ---> 2OH-..........................................................
(2)
Total 20 marks