Definition of Gibbs energy of activation (standard free energy of activation) G (SI unit kJ mol^-1)

The standard Gibbs energy difference between the transition state of a reaction (either an elementary reaction or a stepwise reaction) and the ground state of the reactants. It is calculated from the experimental rate constant k via the conventional form of the absolute rate equation:

G = RT [ln(kB/h) - ln(k/T)]

where kB is the Boltzmann constant and h the Planck constant (kB/h = 2.08358 x 1010 K-1 s-1). The values of the rate constants, and hence Gibbs energies of activation, depend upon the choice of concentration units (or of the thermodynamic standard state). See also enthalpy of activation, entropy of activation.