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304 ELECTROCHEMISTRY

Another convention dictates that we write the more positive electrode on the right-
hand side of a cell, so we often see Equation (7.22) written in a slightly different
form:
(7.23)
emf = E (RHS) − E (LHS)

Being a potential, the electrode potential has the symbol E. We must exercise care
in the way we cite it. E is the energy of a redox couple, since it relates to two redox
species, both an oxidized and reduced form, ‘O’ and ‘R’ respectively. We supplement
the symbol E with appropriate subscripts, as E O,R .

Worked Example 7.9 Consider the electrode potentials for metallic lead within a lead–
0
4+
acid battery. The lead has three common redox states, Pb , Pb 2+ and Pb ,sothere are
three possible equilibria to consider:

−
Pb 4+ (aq) + 2e = Pb 2+ (aq) for which E (equilibrium) = E Pb ,Pb 2+
4+
0
−
Pb 4+ (aq) + 4e = Pb (s) for which E (equilibrium) = E Pb ,Pb 0
4+
0
−
Pb 2+ (aq) + 2e = Pb (s) for which E (equilibrium) = E Pb ,Pb 0
2+
We now see why it is so misleading to say merely E Pb or, worse, ‘the electrode potential
of lead’.
We conventionally cite the oxidized form first within each sym-
We cite the oxidized bol, which is why the general form is E O,R ,so E Pb ,Pb 2+ is correct,
4+
form first, as E O,R . but E Pb ,Pb 4+ is not. Some people experience difficulty in decid-
2+
ing which redox state is oxidized and which is the reduced. A
simple way to differentiate between them is to write the balanced
redox reaction as a reduction. For example, consider the oxidation
reaction in Equation (7.1). On rewriting this as a reduction, i.e.
−
Al 3+ (aq) + 3e = Al (s) , the oxidized redox form will automatically
We usually omit the precede the reduced form as we read the equation from left to
superscripted ‘zero’ right, i.e. are written in the correct order. For example, E O,R for
on uncharged redox the couple in Equation (7.1) is E .
3+
Al ,Al
states. We usually cite an uncharged participant without a superscript.
−
Considering the reaction Pb 2+ + 2e = Pb, the expression E
2+
Pb ,Pb
is correct but the ‘0’ in E Pb ,Pb 0 is superfluous.
2+
When choosing bet-
ween two ionic valen- SAQ 7.10 Consider the cobaltous ion | cobalt redox cou-
ces, the name of the ple. Write an expression for its electrode potential.
higher (more oxidized)
state ends with -ic
+
−
With more complicated redox reactions, such as 2H (aq) + 2e =
and the lower (less
oxidized) form ends H 2(g) , we would not normally write the stoichiometric number,
with -ous. so we prefer E H ,H 2 to E 2H ,H 2 ; the additional ‘2’ before H + is
+
+
superfluous here.