I. The combination of hydrogen and oxygen in contact with hot surfaces

Abstract

A large number of observations on the rates of chemical reactions in homogeneous systems, e. g. , in solutions, has proved that the velocity of a given reaction is determined by its order in accordance with the principle of mass action. It has been frequently assumed that the velocities of gaseous reactions are governed by the same law, although the few measurements which have been so far made hardly justify the assumption. Take, for example, the decomposition of arsine, the velocity of which was determined by enclosing the gas, at atmospheric pressure, in a glass bulb maintained at 310° and recording the pressure at the end of successive time intervals. The results showed that the velocity of decomposition is directly proportional to the concentration of the arsine. On the assumption that the rate of change was governed by the law of mass action, it was concluded that the decomposition was a monomolecular one. But it is known that at temperatures below about 1000° the vapour of arsenic consists of tetratomic molecules, and, therefore, the equation for the decomposition of arsine at 310° must be written 4AsH 3 = As 4 + 6H 2 . If so, then assuming that the decomposition takes place in a homogeneous system, its velocity should, according to theory, be proportional to the fourth power of the arsine concentration, and not to the first power as the experiment proves it to be. The discrepancy here indicated has been explained by the further assumption that the reaction really proceeds in stages, as follows:— ( a ) The monomolecular decomposition of arsine at finite velocity AsH 3 = As + 3H; ( b ) The subsequent combinations of (1) four arsenic atoms forming the complex As 4 , and (2) two hydrogen atoms forming H 2 , both combinations taking place with relatively infinite velocities.