Page 160 - Chemical equilibria Volume 4

136 Chemical Equilibria
In that expression, F is the faraday, which is 96,500 coulombs and z is
the number of electrons exchanged in the stack reaction, which is a redox
reaction. Thus, by measuring the standard potential, we are able to find the
standard Gibbs energy.
The system is given reversibility by counterbalancing the measuring cell
with an electromotive force set up in opposition, in such a way that the
current is minimal. A slight increase or decrease in the strength of the
opposing emf changes the direction in which the reaction takes place.

The main precaution is to ensure that the stack reaction is really that
which has been studied, and that it is not disturbed by secondary reactions.

4.6. Calculation of the equilibrium constants on the basis of
other thermodynamic data

It is tricky to experimentally determine the equilibrium conditions, so as
much as possible, we use the calculation method to determine the values of
the equilibrium constants (or standard Gibbs energies associated with the
reactions). Those calculations must restrict the number of experiments as far
as possible, or eliminate them altogether.

There are five calculation methods which we can envisage, depending on
the data available to us:

– method 1: we know the enthalpy and the standard entropy of reaction at
the desired temperature;

– method 2: we know the standard enthalpy and the standard entropy at a
specific temperature, and the molar specific heat capacities of components at
constant pressure as a function of the temperature;
– method 3: we know a value of the equilibrium constant at a given
temperature, the variations of the specific heat capacities at constant pressure
of the components with varying temperature, and a value of the standard
enthalpy associated with the reaction at a given temperature;

– method 4: we know two values, at two temperatures, of the equilibrium
constant and the molar specific heat capacities at constant pressure of the
components as a function of the temperature:
– method 5: we know other, judiciously-chosen equilibrium constants.