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When a series of structurally related substrates undergo the same general reaction or when the reaction conditions for a single substrate are changed in a systematic way, the enthalpies and entropies of activation sometimes satisfy the relation
H - S = constantwhere the parameter is independent of temperature. This equation (or some equivalent form) is said to represent an "isokinetic relationship". The temperature T = (at which all members of a series obeying the isokinetic relationship react at the same rate) is termed the "isokinetic temperature".
Supposed isokinetic relationships as established by direct correlation of H with S are often spurious and the calculated value of is meaningless, because errors in H lead to compensating errors in S. Satisfactory methods of establishing such relationships have been devised. EXNER (1973); LEFFLER (1955). See also compensation effect, isoequilibrium relationship, isoselective relationship.
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