Page 161 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 137
We shall examine each of these five methods in turn.
4.6.1. Calculation of the equilibrium constants – method 1
We suppose the following to be known:

– the standard enthalpy of the reaction at the desired temperature;
– the standard entropy of the reaction at the desired temperature.

This is the simplest case. Indeed, we can immediately calculate the
equilibrium constant at the same temperature, by applying relations [4.1] and
[4.2].

4.6.2. Calculation of the equilibrium constants – method 2

We know:
– the standard enthalpy at a temperature T 0;
– the standard entropy at a temperature T 0;

– the variations of the specific heat capacities at constant pressure of the
components as a function of the temperature and the enthalpies of the state
changes.

Using a relation such as [4.9], we find the value of the equilibrium
constant at the chosen temperature.

This method enables us to calculate an a priori equilibrium constant
without ever needing to take any measurements in equilibrium conditions.
Thus, it will also enable us to find the constant for an equilibrium which is
not practically realizable.

4.6.3. Calculation of the equilibrium constants – method 3

We know:
– a value of the equilibrium constant at a temperature T 0;
– the variations of the specific heat capacities at constant pressure of the
components with the temperature and the enthalpies of the state changes; and