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308 ELECTROCHEMISTRY

7.3 Activity

Why does the smell of brandy decrease after
dissolving table salt in it?
Real and ‘perceived’ concentrations

At the risk of spoiling a good glass of brandy, try adding a little table salt to it and
notice how the intensity of the smell is not so strong after the salt dissolves.
We recall from Chapter 5 how the intensity of a smell we detect
We mention the volatile with our nose is proportional to the vapour pressure of the substance
alcohol here because it causing it. The vapour pressure of ethanol is p (ethanol) , its magni-
is responsible for the tude being proportional to the mole fraction of ethanol in the brandy;
smell. brandy typically contains about 40 per cent (by volume) of alcohol.
Although adding table salt does not decrease the proportion of
the alcohol in the brandy, it does decrease the apparent amount. And because the
perceived proportion is lowered, so the vapour pressure drops, and we discern the
intensity of the smell has decreased. We are entering the world of ‘perceived’ con-
centrations.
Although the actual concentrations of the volatile components
The ‘activity’ a is the in solution remain unchanged after adding the salt, the system per-
thermodynamically per- ceives a decrease in the concentration of the volatile components.
ceived concentration. This phenomenon – that the perceived concentration differs from
the real concentration – is quite common in the thermodynamics
of solution-phase electrochemistry. We say that the concentration persists, but the
‘activity’ a has decreased by adding the salt.
As a working definition, the activity may be said to be ‘the perceived concentra-
tion’ and is therefore somewhat of a ‘fudge factor’. More formally, the activity a is
defined by
c
a = γ (7.25)
c O
where c is the real concentration. The concluding term γ , termed the activity coeffi-
cient, is best visualized as the ratio of a solute’s ‘perceived’ and ‘real’ concentrations.
The activity a and the activity coefficient γ are both dimension-
We only add the term less quantities, which explains why we must include the additional
c O in order to render ‘c ’ term, thereby ensuring that a also has no units. We say the
O
the activity dimension- value of c is 1 mol dm −3 when c is expressed in the usual units
O
−3
less. of mol dm , and 1 mol m −3 if c is expressed in the SI units of
−3
mol m , and so on.
Why does the smell of gravy become less intense after
adding salt to it?
The effect of composition on activity

Gravy is a complicated mixture of organic chemicals derived from soluble meat
extracts. Its sheer complexity forces us to simplify our arguments, so we will