The kinetics of the thermal decomposition of normal paraffin hydrocarbons - V. Order of reaction over extended ranges of pressure

Abstract

Previous papers have provided evidence that the decomposition of a paraffin reduced to its limiting rate by nitric oxide is a molecular reaction. This reaction has an unusual pressure dependence, which has now been studied over a wider range, namely, up to 1600 mm, and down to 0⋅1 mm. In the higher part of the pressure range the reaction order changes from nearly the second to the first with increase in the initial pressure, as now confirmed by extended measurements on n -butane and n -pentane. In the lower part of the range another transition from second order to first with rise of pressure can be found, and is here shown by measurements on ethane, propane, n -butane, n -pentane and n -hexane. Thus with higher normal paraffins the reaction order is the second at the lowest pressure, decreases to the first, increases again towards the second and finally returns to the first at the highest pressure. Where this behaviour is shown the activation energy is also a function of the pressure. The activation energy for ethane is not a function of the pressure and, correspondingly, there is here a single transition from the second order at the lowest pressure to the first order at about 400 mm and up to at least 1600 mm. Over the range of pressure where the composite behaviour is observed, mass spectrometer analysis of the butane products reveals no important change in the relative probability of rupture at the C 1-2 and C 2-3 linkages respectively. In the region of the lower pressures, however, some of the primarily formed ethane splits to give ethylene and hydrogen more rapidly than can be accounted for by the decomposition rate of ethane as directly determined for comparable conditions.