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INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 299

2+
Worked Example 7.8 The Clark cell Zn|Zn , Hg SO 4 |Hg is often employed as a
2
standard cell since its emf is known exactly as a function of temperature. The cell emf is
−1
1.423 V at 298 K and its temperature coefficient of voltage is −1.2 × 10 −4 VK .What
are G (cell) , S (cell) and thence H (cell) at 298 K?
Before we commence, we note that the spontaneous cell reaction is

Zn + Hg SO 4 + 7H 2 O −−−→ ZnSO 4 · 7H 2 O + 2Hg 0
2

so the cell reaction is a two-electron process.
Next, we recall from Equation (7.15) that G (cell) =−nF × emf . Inserting values for
the cell at 298 K gives

G (cell) =−2 × 96 485 C mol −1 × 1.423 V
G (cell) =−275 kJ mol −1

Then, from Equation (7.18), we recall that

d(emf )
S (cell) = nF
dT
p
Inserting values:

−1
S (cell) = 2 × 96 485 C mol −1 × (−1.2 × 10 −4 VK )
S (cell) =−23.2J K −1 mol −1

Finally, from Equation (4.21), we say that H (cell) = G (cell) + T S (cell) .Weagain
insert values:

−1
−1
H (cell) = (−275 kJ mol ) + (298 K ×−23.2J K −1 mol )
so
H (cell) =−282 kJ mol −1

SAQ 7.9 A different cell has an emf of 1.100 V at 298 K.
The temperature voltage coefficient is +0.35 mV K −1 .Cal- Remember:5 mV (5
culate G (cell) , S (cell) and hence H (cell) for the cell at milli-volts) is the same
−3
298 K. Take n = 2. as 5 × 10 V.
Having performed a few calculations, we note how values of G (cell) and for
H (cell) tend to be rather large. Selections of G (cell) values are given in Table 7.4.