Page 162 - Chemical equilibria Volume 4

138 Chemical Equilibria
– a value of the standard entropy at the temperature T 0.
By applying relation [4.10], we obtain the equilibrium constant sought at
the chosen temperature T.

4.6.4. Calculation of the equilibrium constants – method 4

We know:
– two values of the equilibrium constant of a reaction at two
temperatures T 1 and T 2;
– the variations of the molar specific heat capacities of the components at
constant pressure as a function of la temperature and the state-change
enthalpies.
We can then calculate the equilibrium constant at any other temperature
T 3.

By combining the equations obtained using equation [4.9] for each of the
temperatures T 1 and T 2, we can extrapolate the value of the standard enthalpy
0
at temperature T 0, Δ h and that of the standard entropy at the same
0
r
0
temperature Δ s . Then, we need only use expression [4.9] for the chosen
r
0
temperature T 3.
4.6.5. Calculation of the equilibrium constants – method 5

We can calculate the equilibrium constant on the basis of other known
equilibrium constants, which are appropriate at the chosen temperature. We
use our knowledge of the equilibrium constants of other reactions, chosen in
such a way that a linear combination of their balance equations gives us the
balance equation of the reaction at hand. The unknown standard Gibbs
energy is deduced from the known standard Gibbs energies by the same
linear combination.

Let us look at the example of the reaction of synthesis of water vapor.
The balance equation is written as:

CO 2 + H 2 = CO + H 2O [4R.10]