Page 321 - Physical chemistry understanding our chemical world

288 ELECTROCHEMISTRY

Equation (7.11) is important, since it emphasizes how currents flow even at equi-
librium.
But the value of I (net) is only ever zero at equilibrium because I (forward) =−I (backward) ,
which can only happen at one particular energy, neither too reductive nor too oxidative.
The voltage around the legs of the oil rig needs to be chosen carefully.

What is a battery?
The emf of cells

A battery is an electrochemical cell, and is defined as ‘a device
A battery is a device
for converting chemical comprising two or more redox couples’ (where each couple com-
energy into electrical prises two redox states of the same material). An oxidation reaction
energy. occurs at the negative pole of the battery in tandem with a reduc-
tion reaction at the positive pole. Both reactions proceed with the
passage of current. The two redox couples are separated physically
The word ‘cell’ comes by an electrolyte.
from the Latin for ‘small
The battery requires two redox couples because it is a cell. Each
room’, which explains couple could be thought of as representing half of a complete cell.
why a prisoner is kept This sort of reasoning explains why the two redox couples are
in a ‘cell’.
called half-cells. We could, therefore, redefine a cell as a device
comprising two half-cells separated with an electrolyte.
In practice, the voltage of a battery is measured when its two ends are connected
to the two terminals of a voltmeter, one contact secured to the positive terminal of
the battery and the other at the negative. But a voltmeter is a device to measure
differences in potential, so we start to see how the ‘voltage’ cited on a battery label
is simply the difference in potential between the two poles of the battery.
While the voltage of the cell represents the potential difference
The cell’s emf is a pri- between the two ‘terminals’ of the battery, in reality it relates to
mary physicochemical the separation in energy between the two half-cells. We call this
property, and is mea- separation the emf, where the initials derive from the archaic phrase
sured with a voltmeter
electromotive force.An emf is defined as always being positive.
or potentiometer.
We have already seen from Faraday’s laws how a zero current
implies that no redox chemistry occurs. Accordingly, we stipulate
An emf is always that the meter must draw absolutely no current if we want to mea-
defined as being posi- sure the battery’s emf at equilibrium. Henceforth, we will assume
tive. that all values of emf were determined at zero current.

Aside

The term ‘emf’ follows from the archaic term ‘electromotive force’. Physicists prefer
to call the emf a ‘potential difference’ or symbolize it as a ‘p.d.’.
Confusingly, potential also has the symbols of U, V and E, depending on the context.