Page 147 - Instant notes

Electrochemical thermodynamics 133

and, as with the half-cell potentials, the LH reaction is subtracted from the RH to give the
formal cell reaction. Minus the LH reaction is equivalent to reversing the reaction
(writing it as an oxidation):
−
RH Zn 2+ (aq)+2e → Zn (s)
−
−2LH 2Fe 2+ (aq)→2Fe 3+ (aq)+2e
which is followed by combining species on the same side of the reaction arrows together
and cancelling those which appear on both sides to give:
2Fe 2+ (aq)+Zn 2+ (aq)→Zn (s)+2Fe 3+ (aq)

which is the formal cell reaction.
This should always result in the total charge on both sides of the reaction being equal
and the removal of electrons from the equation. Balancing of this equation can also be
achieved by reducing each half-cell reaction to a one-electron reduction. This results in
the same cell reaction, but with the amount of reaction halved. It is important to
remember this when calculating thermodynamic parameters for the formal cell reaction,
as these are for the formal cell reaction as derived.

Reference electrodes

It is not possible to measure the potential of one half-cell, only the voltage between two
half-cells (see Topic E3). However, changes in the potential of one half-cell (RH) can be
measured by measuring cell potentials and choosing an LH half-cell which maintains a
constant potential despite small changes in the amounts of its redox reagents. This is
termed a reference electrode. Examples are given in Table 1.

Table 1. Examples of reference electrodes
Electrode Half-cell Reaction
−
Silver/silver chloride Ag(s)|AgCl(s)|Cl (aq, c=0.1
M)
−
Calomel (mercury/mercurous Hg(l)|Hg 2 Cl 2 (s)|Cl (aq)
chloride) a
+
Hydrogen electrode b Pt|H 2 (g)|H (aq)
a Saturated calomel (SCE): using KCl(satd): sodium saturated calomel (SSCE): using NaCl(satd).
b
St andard hydrogen electrode (SHE): , p H2 =1 atm.
Reference electrodes have large concentrations of the ions and large reservoirs of the
solid, liquid and gaseous reagents necessary for the redox reaction. Small changes in the
amounts of these species make little difference to their activities and make little
difference to the electrode potential (see Topic E5). The standard hydrogen electrode
(SHE) is not a very practical reference electrode, as the platinum electrode is readily
poisoned, hydrogen gas is explosive and requires bulky cylinders and the electrode