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

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 $\frac{3}{2}$ 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 x 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$_6 \rightarrow$ 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 $\rightarrow$ H$_2$ + NO; as ethylene accumulates the ratio [C$_2$H$_5$]/[H] increases and the termination step becomes C$_2$H$_5$+HNO \rightarrow 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.