Eyring , Henry

(1901–1981) Mexican–American physical and theoretical chemist

Eyring, a grandson of American missionaries who had become Mexican citizens, was born at Colonia Juarez in Mexico. He thus first came to America in 1912 as a Mexican citizen and did not take American citizenship until 1935. He was educated at the University of Arizona and the University of California, where he obtained his PhD in 1927. He then held a number of junior appointments before joining the Princeton faculty in 1931, becoming professor of chemistry there in 1938. Eyring moved to a similar chair at the University of Utah, holding the post until his retirement in 1966.

Eyring, the author of 9 books and over 600 papers, was as creative a chemist as he was productive. His main work was probably in the field of chemical kinetics with his transition-state theory. Since the time of Sven Arrhenius it had been appreciated that the rate constant of a chemical reaction depended on temperature according to an equation of the form: k=Ae^–E/RT

The constant A is the frequency factor of the reaction; E_A is the activation energy. The values of A and E_A can be found experimentally for given reactions. Eyring's contribution to the field was to develop a theory capable of predicting reaction rates.

In a reaction, the atoms move – i.e., molecules break and new molecules form. If the potential energy of a set of atoms is plotted against the distances between atoms for chosen arrangements, the result is a surface. Positions of low energy on the surface correspond to molecules; a reaction can be thought of as a change from a low-energy point, over a higher energy barrier, to another low-energy position.

A. Marcelin, in 1915, had shown that reactions could be represented in this way, and in 1928 Fritz London pointed out that it was possible to calculate potential surfaces using quantum mechanics. Eyring, with Michael Polyani, first calculated such a surface (1929–30) for three hydrogen atoms and Eyring later went on to calculate the potential surfaces for a number of reactions. The activation energy of the reaction is the energy barrier that the system must surmount.

Eyring later (1935) showed how to calculate the frequency factor (A). He assumed that the configuration of atoms at the top of the energy barrier – the ‘activated complex’ – could be treated as a normal molecule except for a vibrational motion in the direction of the reaction path. Assuming that the activated complex was in equilibrium with the reactants and applying statistical mechanics, Eyring derived a general expression for reaction rate. Eyring's theory, called absolute-rate theory, is described in his book (with Samuel Glasstone and KeithJ. Laidler) The Theory of Rate Processes (1941).