Search

THE kinetics of the pyrolysis of 1.2 dichlorethane have recently been described by Barton and Howlett1. Over the temperature-range 360°–480° C. they find that the reaction is first order and proceeds by a radical chain-mechanism in vessels coated with a carbonaceous film (k = 1010.81 exp − 47,000/RT). Strong evidence for the radical nature of the decomposition is provided by the powerful inhibiting effect of propylene. The maximally inhibited rate, which is independent of further increase in propylene concentration, is shown to be partially heterogeneous in character.

Kinetics of the reaction between dichromate and iodide in acid solution have been measured under a variety of conditions. By taking account of the equilibria present in the system we find that the initial reaction is accurately of the first order in acid chromate ion and of second order in both iodide and hydrogen ion. At a relatively early stage however the rate falls below that indicated by the initial kinetics. The redox potentials for the CrVI/CrIII and l0/l–1 couples show that there can be no overall detectable position of equilibrium; and the fall seems to be due to a reversible step in the mechanism involving l2 and CrIV. A satisfactory complete rate equation is thereby obtained.

THEORIES of gaseous unimolecular reactions have often been discussed1 ; but very few examples of processes occurring virtually completely by this mechanism have been used to compare theory and experiment. Thus the decompositions of azomethane, nitrous oxide and nitrogen pentoxide, which have been exhaustively examined from this point of view, are not now considered to be simple unimolecular reactions2. It is desirable, therefore, to have authenticated examples, and some evidence has already been presented that the homogeneous, first-order pyrolyses of ethyl, isopropyl and tert. butyl chlorides and of ethylidene dichloride are of this type3. This evidence is found mainly in comparisons with the obvious radical chain-decompositions of 1.2 dichloroethane and the tetrachloroethanes4. In contrast with the latter, the former show no induction periods, no inhibition by propylene, little sensitivity to oxygen and chlorine, and, in the case of ethyl chloride, there is reasonable agreement between the experimental activation energy and that calculated by the semi-empirical method.

Kinetics of the initial-stages of the reaction 2Fe(CN)63–+ 21–= 2Fe(CN)64–+ l2 have been measured. The reaction normally observed is a cation-catalysed process, although specific ion effects caused by both cations and anions may be present. The rate of reaction is relatively insensitive to general ionic-strength effects. The suggested mechanism is in agreement with the observed kinetic form, and we are able to include the interpretation of previously published data.