The exchange reaction between ethylene and deuterium on a nickel catalyst

It was shown that, when deuterium and ethylene are passed over a nickel catalyst, in addition to the formation of deuterium substituted ethanes, an exchange reaction which may be formulated C 2 H 4 + D 2 ⇄ C 2 H 3 D + HD takes place (Farkas, Farkas and Rideal 1934). Horiuti and Polanyi (1934) pointed out that if the exchange reaction involved atoms, and the addition reaction molecules of hydrogen, the two reactions should differ in their kinetics; this inference is only true under those conditions where the rate-controlling mechanisms involve those species in concentrations in equilibrium with the gas phase. It was found that the two reactions differed at least in respect to their relative energies of activation. If the exchange reaction involves an atomic mechanism, this can be formulated in two different ways (Horiuti and Polanyi 1934), either a primary addition of a deuterium atom, or a primary loss of hydrogen atom may take place, the respective reactions being represented diagrammatically as follows: