How can I recover thermodynamics coefficients from affinity values?

  • Affinity parameters

    • KA is the affinity constant (M-1)
    • KD is the equilibrium constant (M)

  • Thermodynamics parameters

    • ΔG is the free energy change
    • ΔH is the enthalpy change
    • ΔS is the entropy change

It can be calculated from the affinity constant (or from the dissociation constant)

ΔG = R•T•ln(KD)

where R is the Gas Constant, T is absolute temperature (K) and KD is expressed in M.

In order to calculate ΔH and ΔS, the affinity parameters must be determined at several temperatures. ΔG is deduced from theses values over a range of temperature. The non-linear Van’t Hoff equation is fitted to the data to determine ΔH, ΔS

Then, knowing that ΔG = ΔH-T•ΔS, the entropy change can also be evaluated.