Absolute Rate Theory
The velocity (v) of the reaction S ® P is the rate of formation of P.
v = k [S] where k is the rate constant.
According to absolute rate theory the reaction actually progresses as follows:
S ® TS ® P were TS is the "transition state"
We define the activation energy (DG*) as the difference in free energy between S and TS, i.e.
DG* = GTS - GS
TS always has a higher free energy than S, so DG* is always positive.
As is the case for all DGs, DG* contains enthalpy and entropy components:
DG* = DH* - TDS*
The rate constant (k) for the reaction is a function of DG*:
k = Ce-DG*/RT or alternatively:
ln(k) = ln(C) - DG*/RT
where R and C are constants.