Kinetics and mechanisms of the thermal decomposition of ethane - II. The reaction inhibited by nitric oxide

Abstract

The pressure-time curves for the decomposition of ethane when fully inhibited by nitric oxide have initially a point of inflexion. The initial rates are proportional to the first power of the pressure at higher pressures, and to the 3/2 power at lower pressures; the rates at the inflexion point are proportional to the pressure to a power which is slightly greater than unity. The acti­vation energy corresponding to the initial rates in the first-order region was found to be 77∙5 kcal/mole, and the frequency factor 3∙12 × 10 15 s -1 . The reaction was slightly inhibited by increasing the surface: volume ratio, and the induction period disappeared on addition of ethylene. The facts are shown to be consistent with a mechanism in which initiation occurs by the reaction NO + C 2 H 6 → C 2 H 5 + HNO, which is estimated to have an activation energy of 52 kcal. At the beginning of the reaction and at lower pressures termination is con­sidered to occur by H + HNO → H 2 + NO; as ethylene accumulates the ratio [C 2 H 5 ]/[H] increases and the termination step becomes C 2 H 5 + HNO → C 2 H 6 + NO. The mechanism is shown to account for the fact that propylene and other inhibitors give rise to the same limiting rate.