Page 94 - A Working Method Approach For Introductory Physical Chemistry Calculations

78 Chapter 6

(a) EOceii = E'RHE - E'LHE
(b) AGO = -vFEocelI
(c) E = Eocell - (RT/vF)lnK(the Nernst equation)
Do not forget that at equilibrium, E = 0, since AG = 0,
i.e. 1nK = (vFE",II)/(RT)

WORKING METHOD FOR GALVANIC CELL PROBLEMS
The following is an outline of the stepwise procedure on how to
approach a problem on a galvanic cell:

1. Read the question very carefully. Remember, if you see the
words 'electrolysis' or 'electrolysed', this refers to an electrolytic
cell and not a galvanic cell. The working method which follows
is only applicable to galvanic cells.
2. From the one-line representation of the cell, the balanced
chemical equation or the two standard electrode potentials,
identify the two types of electrodes involved. For example:
(a) Metal-metal-ion electrode, e.g. Fe(,)[Fe +(as);
(b) Metal ion in two different valence states,
e-g. ~t(s)I~e~ ~e~ + (as);
(as),
(c) Gas-ion electrode, e.g. PtlH2(g)lH +(as);
+
(d) Metal - insoluble salt anion electrode,
3. You should now re-examine the problem. This is probably the
most important or most critical step in the working method.
(a) If the E" values alone are given (i.e. if a balanced chemical
equation is not given in the question), then the more positive
E" value will indicate the electrode acting as the cathode.
Remember, E" values are standard reduction potentials and
reduction in any electrochemical cell (both galvanic and
electrolytic) takes place at the cathode ('CROA').
(b) If a balanced chemical equation is given in the question,
you now have to determine, from the oxidation numbers,
which species is oxidised and which species is reduced,
remembering:
Reduction is a decrease in the oxidation number,
e.g. Fe3 + (aq) + e -, Fez + (as)
Oxidation is an increase in the oxidation number,
e.g. ce3 +(as) + ce4+ (aq) + e
4. Once you have determined which electrode is acting as the