Chemical Equilibria

Sources of Thermodynamic Data

Equilibrium constants are needed to create a chemical equilibrium diagram. They can be obtained from thermo­dynamic data. The Gibbs free energy change of a reaction is:

ΔGr° = RT lnK°

where R is the gas constant; the temperature, T, is in Kelvins; K° is the equilibrium constant; and ΔGr° may be calculated from the individual free energies of formation from the elements, ΔGf°, for each species

ΔGr° = RT lnK° = ∑ νi ΔGf°(i)

where the stoichiometric coefficient, νi, for a species i is positive for the product of the reaction, and negative for the reactants. For example:

  

Fe3+ + 2 H2O − 2 H+ = Fe(OH)2+

ΔGr°  = 

ΔGf°(Fe(OH)2+) −ΔGf°(Fe3+) −2ΔGf°(H2O) +2ΔGf°(H+)

Note that by convention ΔGf°(H+) = zero at all temperatures.

You may get information on sources of thermo­dynamic data by searching the WWW. Here are some sites:

References that also contain thermodynamic data: