On the apparent compensation effect

Abstract

It is shown that by linear extrapolation of low temperature Arrhenius lines to 1 T = 0 a phenomenon resembling the compensation effect is generated because the absolute position of those lines is raised by the entropy of activation energy distribution so that the lines have an intersection point. An equation first derived by O. K. Rice (Activation et Structures des Molécules, p. 304, Paris (1928); O. K. Rice and Ramsberger, J. Amer. Chem. Soc. 50, 617 (1928)) and C. N. Hinshelwood (Kinetics of Chemical Change in Gaseous Systems, 3rd ed., Oxford Clarendon Press (1933)) is used. For a well documented example it is shown graphically that the construction described fits the observations. From the rate equation used an expression is derived which gives for any reaction the number of degrees of freedom contributing to the activation of the critical complex as a function of the limiting activation energies and of the apparent “isocatalytic temperature.” It is applied to a number of catalyses by alloys, salts, supported and inverse supported catalysts for a variety of reactions. In all cases values around 10–20 result for the freedom numbers. The conductivity of several doped semiconductors follows the same rule. The results are briefly discussed.