Chemical reactions. Electron transfer and electronic defects

2 CHEMICAL REACTIONS. ELECTRON TRANSFER AND ELECTRONIC DEFECTS 

Reactions of KI with Cl2 and of NaCl and KC1 with F2 have been studied [92, 93, 103, 104] and a variety of properties have been measured as a function of the extent of reaction in the conversion of CuCl to CuCl2 by reaction with Cl2 [89, 105, 106]. A consistent feature of all these studies is that, together with or sometimes preceding nucleation and growth of new solid phases, the solid reactant is damaged by electron withdrawal, leading to the formation of various types of electronic defect of the trapped-hole or V-centre type. Studies of electrical conductivity, electron spin resonance spectra and UV absorption are useful in elucidating the behaviour of these defects. 

In the KI/C12 reaction, an interesting effect of high defect concentration in preventing growth of nuclei of a new solid phase was disclosed [103]. Solid samples differed widely in their initial ionic conductivity and hence in their initial concentration of cation vacancies. The ones with high vacancy concentration reacted very slowly with chlorine and the colour of iodine was seen uniformly as a pale coloration of the whole solid, which had reacted to the extent of only a few percent after several days. Samples with a low vacancy concentration reacted more rapidly and a sharp boundary developed between an outer, iodine-blackened completely reacted region and a pale-coloured inner region. In the un-reactive samples, electronic (presumably positive hole) conductivity appeared transiently for a few hours and then decayed. In the reactive 

Direct determination of the activities of reacting species at the interface 

One of the most serious drawbacks in studies of gas—solid reactions is that, in general, the activities or concentrations of the reacting species at the interfaces cannot be measured directly and hence the rate equation (if successfully determined) is usually expressed in terms of gaseous 

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