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∞, is connected with the pressure by the relation r∞ = Ap0 + Bp20, but this is probably an approximation for an expression of the form r∞ = ap20/1 + a'p0 + bp20/1 + b'p0, 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.