The kinetics of the thermal decomposition of normal paraffin hydrocarbons III. Activation energies and possible mechanisms of molecular reactions

In part I it was concluded that the nitric oxide-inhibited decomposition of paraffins probably represents a molecular reaction. Further experiments in which the presence of hydrogen causes a marked increase in the normal reaction but not of the inhibited reaction strengthen this conclusion, by diminishing still further the likelihood that the inhibited reaction is a chain process not suppressible by nitric oxide. Experiments on variation of the surface/volume ratio and on the coating of the vessel surface with potassium chloride have been made for the normal reaction and for the reaction inhibited by nitric oxide and by propylene respectively. The effect of the surface change is either negligible or, in certain cases, to accelerate a condensation reaction which may vitiate the measurement of the true decomposition rate. Over limited ranges the rate of reaction, r$_{\infty}$, is connected with the pressure by the relation r$_{\infty}$ = Ap$_{0}$ + Bp$_{0}^{2}$, but this is probably an approximation for an expression of the form r$_{\infty}$ = $\frac{ap_{0}^{2}}{1+a^{\prime}p_{0}}+\frac{bp_{0}^{2}}{1+b^{\prime}p_{0}}$, the reaction mechanism being composite. A reaction nearly of the first order predominates at lower pressures and one nearly of the second order at higher pressures. The activation energy of the nitric oxide-inhibited reaction with n-pentane and n-heptane increases steeply as the pressure falls, and by suitable extrapolations the values corresponding to the two assumed components can be estimated. With ethane and propane the activation energy varies very little with pressure while n-butane is an intermediate case. The different modes of reaction appear not to be connected with the different possible positions of rupture of the molecule. Their explanation probably demands an extension of the usual theory of unimolecular reactions. In a complex molecule a large amount of energy distributed in many degrees of freedom eventually causes decomposition by the interference of normal vibration modes, while alternatively a much smaller amount can cause reaction if it is so communicated that the critical bond breakages can occur before the energy has been shared throughout the molecule.