Page 148 - Instant notes

Physical chemistry 134

potential is sensitive to small changes in gas pressure (see Topic E5), so alternative
reference electrodes are used for experimental measurements.

Standard reduction potentials

Variations in the potential of different half-cells can be used to compare the propensity
for oxidation or reduction in the half-cell reactions. To do this, a standard reduction
potential or standard potential is measured for a half-cell. This is the standard cell
potential of a cell consisting of the half-cell as the right-hand electrode and the SHE as
the left-hand electrode, separated by a salt bridge. These are denoted as (ox1,
ox2,../red1, red2..) where ox1, ox2.. are the oxidized species and red1, red2,.. the reduced
species in the half-cell reaction. Values are typically measured and tabulated at 25°C or
298 K. The standard cell potential for any cell can then be calculated, as it is simply the
difference between the RH and LH half-cell standard potentials:
3+
2+
2+
Pt|Fe (aq), Fe (aq)||Zn (aq)| Zn(s)

The electrochemical series
When the two electrodes are connected and current is allowed to flow in a galvanic cell,
the formal cell reaction as written is spontaneous when ∆G is negative (see Topic B6),
which is when reduction occurs at the RH electrode and oxidation at the LH electrode.
This happens when the RH electrode potential is more positive than the LH electrode, or
when . The reverse reaction is spontaneous when and when
, the cell is at equilibrium. This means that the reduced form of a couple with a
low value will reduce the oxidized form of a couple with a higher value. For

and , the spontaneous reaction
will therefore be:
−
−
F 2(g)+2Br (aq)→2F (aq)+Br 2(1)
and values of when tabulated in order give an electrochemical series which shows an
increase in the oxidizing power of the oxidizing agent (and a corresponding decrease in
the reducing power of the reducing agent) in the redox couple as increases.

Thermodynamic data

The change in Gibbs free energy (see Topic B6) is given by the energy change of the
electrons travelling across the cell voltage:
∆G cell=−nFE cell