The combination of nitrogen and hydrogen activated by electrons

Abstract

Although the chemical reactions of active molecules have been used to fix their critical potentials, little attention has been paid to the converse process of analysing the mechanism of a reaction by studying the rates of reaction of molecules activated to the energy-levels already known from physical measurements. The method is analogous to photochemistry, with the advantages that a stream of electrons with a nearly uniform velocity is easier to obtain than quasi-monochromatic light, and that the mechanism is a more general one. The following is an account of experiments on the rate of production of ammonia from nitrogen and hydrogen as a function of the energy of thermions used to activate molecules and atoms. No attempt was made to measure the critical energy increments with great exactness, since that can be accomplished more easily by physical means; the object in the present case was to recognise the critical potentials of significance in the reaction. Previous Work . Heidemann described the production of ammonia even at the lowest voltages, but subsequent work by Andersen and Storch and Olson did not confirm this. They detected no combination until the molecular ionisation potential of N 2 (circa 17 V) was reached, after which the reaction rate increased abruptly every 4-7 V. The mechanism proposed was that H 2 + and N 2 + appearing at 16 V and 17 V respectively gave H and N atoms on collision, and that increased combination was due to the activation of H by 4 V electrons. Later Kwei found that the NH 3 band spectrum was not excited in hydrogen and nitrogen mixtures until 23 V was reached. This voltage corresponds to the second jump in Storch and Olson’s curve. In a subsequent note Olson explained the failure of Kwei to detect ammonia at 17 V by postulating that NH 3 + must be present for the spectrum to appear. Thus at 17 V the reactions were considered to be N 2 + + e → N' 2 , N' 2 + N 2 → N 2 + 2N, the nitrogen atoms then combining with H 2 or H produced by the reaction N 2 + H 2 → 2N + 2H; while at 23 V the voltage at which N + begins to appear, NH 3 + is obtained in the same way.