Theta Rule

Another old mystery in heterogeneous catalysis, dating back to F. H. Constable (1925) and G.-M. Schwab (1929), was the compensation effect or theta rule. The Arrhenius plots for similar reactants on the same catalyst or for the same reactant on similar catalysts would differ in slope across a common point of intersection. Approximately 70 years later, a First Workshop on the Compensation Effect was organized (DECHEMA, Berlin, 1997) to debate this enduring mystery. Werner Haag demonstrated that such an effect must necessarily result from the temperature dependence of reactant adsorption (and hence its site concentration) and that of the reaction rate of the adsorbed species, operating in opposite directions. A second workshop may never follow ... 

This relation is called an isokinetic relation, compensation law, or theta rule. The last name was sometimes applied for this phenomenon in the field of heterogeneous catalysis (cf. Cremer, 6) where it was observed mainly with a series of different catalysts. 

Rules of thumb estimates of miscibility (solubility parameters, theta parameters, etc.)... 

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