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INTRODUCING HALF-CELLS AND ELECTRODE POTENTIALS 303

The preferred electrolytes if the solvent is water are KCl and NaNO 3 . If the solvent
is a non-aqueous organic liquid, then we prefer salts of tetra-alkyl ammonium, such
n
+
as tetra-n-butylammonium tetrafluoroborate, Bu 4 N BF 4 .
−
7.2 Introducing half-cells and electrode
potentials

Why are the voltages of watch and car batteries
different?

Relationships between emf and electrode potentials

Being a cell, a battery contains two half-cells separated by an electrolyte. The elec-
trodes are needed to connect the half-cells to an external circuit. Each electrode may
act as part of a redox couple, but neither has to be.
The market for batteries is huge, with new types and applications
being developed all the time. For example, a watch battery is a type An ‘electrode poten-
of ‘silver oxide’ cell: silver in contact with silver oxide forms one tial’ E is the energy
half-cell while the other is zinc metal and dications. Conversely, (expressed as a volt-
a car battery is constructed with the two couples lead(IV)|lead age) when a redox
and lead(IV)|lead(II). The electrolyte is sulphuric acid, hence this couple is at equilibrium.
battery’s popular name of ‘lead–acid’ cell (see further discussion The value of E cannot
be measured directly
on p. 347). and must be calculated
The first difference between these two batteries is the voltage
from an experimental
they produce: a watch battery produces about 3 V and a lead–acid
emf.
cell about 2 V. The obvious cause of the difference in emf are
the different half-cells. The ‘electrode potential’ E is the energy,
expressed as a voltage, when a redox couple is at equilibrium.
Two redox states of the
As a cell comprises two half-cells, we can now define the emf
same material form a
according to redox couple.
(7.22)
emf = E (positive half-cell) − E (negative half-cell)

This definition is absolutely crucial. It does not matter if the
values of E for both half-cells are negative or both are positive:
E (positive) is defined as being the more positive of the two half-cells, It is impossible to
determine the potential
and E (negative) is the more negative.
of a single electrode:
We now consider the emf in more detail, and start by saying only its potential rel-
that it represents the separation in potential between the two half- ative to another elec-
cell potentials; See Equation (7.22). In order for G (cell) to remain trode can be measured.
positive for all thermodynamically spontaneous cell discharges, the
emf is defined as always being positive.