Page 159 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 135
extent of the reaction at equilibrium, from which, when we know the initial
state, we deduce the composition of the medium. If we know that composition,
in the case of solutions, we deduce the activities of the substances, knowing
the relation between the activity and the concentration of the solutions used.
There are numerous possible measuring methods, and ultimately they are
the same as those used to track the kinetics of a reaction [SOU 11].
Traditionally, we distinguish between:

– chemical methods, in which we dose a sufficient number of
components at the end of the reaction, but preventing the addition of a
dosing reagent from disturbing the equilibrium by displacing it by capturing
a component;

– physical methods, in which a physical measurement is characteristic of
the extent of the reaction. These methods are easier to implement, but can
only be used in relatively-simple cases.

In fact, the experimental difficulty is two-fold:
– we need to be certain that the steady state reached is indeed the
equilibrium state, meaning that we achieve the same composition of the
medium regardless of which side of the reaction we begin on;
– the slowness or the rapidity of the reactions often prevent us from
covering a sufficiently broad temperature range. The use of a catalyst
enables us to use broader temperature domains without altering the
equilibrium properties.

A particularly elegant method to determine the equilibrium constants is,
when possible, to use the measurement of electromotive forces (emf) of
galvanic elements or electrochemical cells.

We know that an electrochemical cell works by way of a chemical
reaction, transforming chemical energy into electrical energy, and that the
0
standard emf E of the cell or standard potential (i.e. the emf measured at
standard pressure with zero current – in an open circuit) at a given
temperature is linked to the standard Gibbs energy associated with the
reaction at the same temperature by the expression:

0
0
Δ g =− EzF [4.49]
r