Open AccessThe combination of nitrogen and hydrogen activated by electrons
Author(s) -
A. Caress,
Eric K. Rideal
Publication year - 1927
Publication title -
proceedings of the royal society of london. series a, containing papers of a mathematical and physical character
Language(s) - English
Resource type - Journals
eISSN - 2053-9150
pISSN - 0950-1207
DOI - 10.1098/rspa.1927.0117
Subject(s) - work (physics) , molecule , atomic physics , electron , hydrogen , ammonia , converse , chemistry , reaction rate , nitrogen , physics , thermodynamics , mathematics , catalysis , quantum mechanics , organic chemistry , geometry
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 N2 (circa 17 V) was reached, after which the reaction rate increased abruptly every 4-7 V. The mechanism proposed was that H2 + and N2 + 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 NH3 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 NH3 + must be present for the spectrum to appear. Thus at 17 V the reactions were considered to be N2 + +e → N'2 , N'2 + N2 → N2 + 2N, the nitrogen atoms then combining with H2 or H produced by the reaction N2 + H2 → 2N + 2H; while at 23 V the voltage at which N+ begins to appear, NH3 + is obtained in the same way.