A level chemistry A2

1. 5.1 Redox Equilibria
Redox Titrations
MnO4¯ (aq) + 8H+ (aq) + 5e¯  Mn2+ (aq) + 4H2O (l)
2S2O3 ²¯ (aq) + I2 (aq)  S4O6 ²¯ (aq) + 2I¯ (aq)
The iodine is obtained initially by the titration of the substance for analysis with KI (aq) – the
balanced symbol equation for this must be determined using the half equations for Iodide and
the substance.
Standard Electrode Potentials
The e.m.f. of a cell in which the left-hand component is a standard hydrogen electrode and the
right-hand component is the electrode system in question. All measurements at 298K, 100kPa
and 1 molar solutions.
You are not expected to be able to, or know how to, set up such apparatus but you should
appreciate the importance of the standard hydrogen electrode as a way of comparing the
relative reducing / oxidising powers of different ions.
Predicting the Feasibility of a Reaction
Rule: The half equation with the most negative electrode potential will make negative progress
(that is, it will be reversed and so the ions/element will be oxidised)
Simply work out the direction in which both half equations go and you can then deduce if the
reaction in question is feasible.
When does a feasible reaction not happen?
1. If the reaction takes a more favoured course than the one thought.
2. The activation energy of the reaction is too high.
3. The e.m.f. is very small (i.e. the values for E are very close together)
4. Something escapes from the system.
Disproportionation
Disproportionation is the simultaneous oxidation and reduction of a species.
It is VERY IMPORTANT when choosing the half equations for working out the e.m.f. that BOTH
HALF EQUATIONS CONTAIN THE SPECIES WHICH IS DISPROPORTIONATING.
Rusting
Rusting is the attack of iron at sites where oxygen and a film of liquid water is present.
1. At sites where there are slight impurities in the iron, the electrode potential is slightly
different.
2. Fe (s)  Fe 2+ (aq) + 2e-
3. This causes ‘pits’ in the iron.
4. The electrons flow through the iron to a point where water AND oxygen are present.
5. 2H2O (l) + O2 (g) + 4e-  4OH- (aq)
6. This reduction is assisted by acidic gases in the water (e.g. carbon dioxide) as it
removes the OH- ions, shifting the equilibrium to the right.

2. 7. The Fe2+ ions (from 2) are oxidised to iron (III) ions when they come into contact with
oxygen.
8. An iron oxide hydrate is formed ( Fe2O3.H2O)
Corrosion of other metals occurs usually when two metals are in contact. Aluminium and
magnesium are good examples. Magnesium alloy rivets which are riveted into an aluminium
alloy (on ships) will corrode.
Corrosion can be prevented by using water repellent or frequent painting.
Sacrificial Protection – Galvanizing
If the metal is coated in a more reactive metal, this metal will be corroded (oxidised) instead of
the metal we want to protect.
Metals are often dipped in molten zinc (galvanizing) which has a very negative E value. This is
called sacrificial protection – since the zinc is being sacrificed to save the metal.
Tinning
Zinc is not a safe metal for some uses (i.e. in food packaging) so tin is used. However, tin has a
less negative E value than many metals (e.g. steel) and so does not provide sacrificial
protection. If the tin coating is damaged, corrosion WILL occur.