Definition of Entropy of Activation, So (SI Unit J mol^-1 k^-1)

The standard entropy difference between the transition state and the ground state of the reactants, at the same temperature and pressure.

It is related to the Gibbs energy of activation and enthalpy of activation by the equations

S = (H -G)/T

= H/T - R ln (kB/h) + R ln (k/T)

or, if ln k is expressed as ln k = a/T + b + c ln T + dT,

S = R [b - ln (kB/h) + (c - 1)(1 + ln T) + 2 dT]

provided that rate constants for reactions other than first-order reactions are expressed in temperature-independent concentration units (e.g., mol dm-3, measured at a fixed temperature and pressure). The numerical value of S depends on the standard state (and therefore on the concentration units selected). If entropy of activation and enthalpy of activation are assumed to be temperature-independent,

S = R[b - ln(kB/h)]

Strictly speaking, the quantity defined is the entropy of activation at constant pressure from which the entropy of activation at constant volume can be deduced.

The information represented by the entropy of activation may alternatively be conveyed by the pre-exponential factor A (see energy of activation).