Page 343 - Physical chemistry understanding our chemical world

310 ELECTROCHEMISTRY

has the effect of making the eau de Cologne more pungent. Stated another way, the
product requires less ester because the alcohol increases its perceived concentration.
Incidentally, the manufacturer also saves money this way.

Thermodynamic activity a

Every day, electrochemists perform measurements that require a knowledge of the
activity a. Measurements can be made in terms of straightforward concentrations if
−3
solutions are very dilute, but ‘very dilute’ in this context implies c ≈ 10 −4 mol dm ,
or less. Since most solutions are far more concentrated than millimoles per litre, from
now on we will write all equations in terms of activities a instead of concentration c.
The values of activity a and concentration c are the same for
The concept of activity very dilute solutions, so the ratio of a and c is one because the
was introduced in the real and perceived concentrations are the same. If a = c, then
early 20th century by Equation (7.25) shows how the activity coefficient γ has a value
one of the giants of of unity at low concentration.
American chemistry, G. By contrast, the perceived concentration is usually less than the
N. Lewis. real concentration whenever the solution is more concentrated, so
γ< 1. To illustrate this point, Figure 7.9 shows the relationship
between the activity coefficient γ (as ‘y’) and concentration (as ‘x’) for a few sim-
ple solutes in water. The graph shows clearly how the value of γ can drop quite
dramatically as the concentration increases.

1

0.8
HCl [1 : 1]
KCl [1 : 1]
KOH [1 : 1]
NaBr [1 : 1]
0.6

g
0.4

0.2
H SO [1 : 2]
2 4
CuSO [2 : 2]
4
(SO ) [2 : 3]
0 In 2 4 3
0 0.2 0.4 0.6 0.8 1 1.2
Concentration / mol dm −3
Figure 7.9 The dependence of the mean ionic activity coefficient γ ± on concentration for a few
simple solutes