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294 ELECTROCHEMISTRY

G (cell) is a function of temperature. Ultimately, then, a digital watch loses time in
the winter as a simple result of the cold.

Worked Example 7.6 The emf of a typical ‘lithium’ watch battery (which is a cell) is
3.0 V. What is G (cell) ?

Look at the units and The number of electrons transferred n in a lithium battery is one,
since the redox couple is Li , Li. Inserting values into Equation
+
note how 1 J = 1C ×
1V. (7.15) yields:
G (cell) =−1 × 96 485 C mol −1 × 3.0V

G (cell) =−289 500 J mol −1 =−290 kJ mol −1

Notice how the molar energy released by a simply battery is enormous.

SAQ 7.5 A manganese dioxide battery has an emf of 1.5 V and n = 2.
Calculate G (cell) .
In the absence of any pressure–volume work, the value of G (cell) is equal to the
work needed to transfer charge from the negative end of the cell to the positive. In
practice, G (cell) equates to the amount of charge passed, i.e. the number of charged
particles multiplied by the magnitude of that charge.

Why is a battery’s potential not constant?

Non-equilibrium measurements of emf

A healthy battery for powering a Walkman or radio has a voltage
We define the emf
of about 1.5 V. In the terminology of batteries, this value is called
as having a positive
value and, strictly, it is its open-circuit potential, but an electrochemist talking in terms of
cells will call it the emf. This voltage is read on a voltmeter when
always determined at
equilibrium. we remove the battery from the device before measurement. But
the voltage would be different if we had measured it while the
battery was, for example, powering a torch.
We perform work whenever we connect the two poles of the
battery across a load. The ‘load’ in this respect might be a torch,
The two currents (for calculator, car, phone or watch – anything which causes a current
anode and cathode)
are generally differ- to pass. And this flow of current causes the voltage across the
battery or cell to decrease; see Figure 7.4. We call this voltage the
ent. Neither of them is
related to potential in a ‘voltage under load’.
linear way. A similar graph to Figure 7.4 could have been drawn but with
the x axis being the resistance between the two electrodes: if the