CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 249
8.4 Calculation of the equilibrium constant
8.4.1 Calculation from the equilibrium composition
If we know the system composition at equilibrium at a given temperature and pressure, we can
also calculate its equilibrium constant.
Note:In order to accurately determine the equilibrium constant of liquid or solid solutions
we need to know the activity coefficients, in addition to the equilibrium composition. For
gaseous mixtures we need to know the fugacity coefficients.8.4.2 Calculation from tabulated data.
Pressure and absolute temperature have natural zero. The entropy of a pure substance in its
most stable crystalline modification is also zero at absolute zero, and hence it may be calculated
absolutely [see3.5.5]. The energy functionsU,H,GandFdo not have natural zero. In tables,
theenthalpy of elements in a stable modification at a pressure of 101.325 kPa and
a reference temperatureof either0 Kor298.15 Kis chosen as a reference state. It is the
same as if the enthalpy of elements in stable modifications under reference conditionsequalled
zero.
At a temperature other than the reference temperature, the enthalpy of elements can be
calculated provided that we know theirCp and the standard enthalpy of their modification
transformations [see3.5.3]. At the reference temperature, the enthalpy of a compound equals
its enthalpy of formation. Other energy functions are calculated from the defining relations.
In order to calculate the equilibrium constant we need to know ∆rGstbecause
K= exp(
−∆rGst
RT). (8.25)
However, in tables we can find only the quantities from which ∆rG◦can be calculated. This
must be then converted to other standard states.
- Calculation fromHm◦,iandS◦m,i
From the given data we may calculateG◦m,iof every component
G◦m,i=Hm◦,i−T S◦m,i (8.26)