The kinetics of exchange of oxygen-18 between enriched oxygen gas and 38 inorganic oxides and 1 oxy-acid salt has been examined in detail. Most of the exchange reactions occur by a dissociative atomic mechanism confined to the surface layer of oxygen ions. For these there is a strong compensation effect between A,, and E in the rate expression : E shows a systematic fall with increasing size of the unit-crystal cell and related crystal parameters : the plots separate the oxides into structure-dependent groups. The slow stage is the desorption of oxygen. The oxides PbO, PdO, Ago, and CuO exchange the surface layer by a molecular reaction which also occurs to an appreciable extent, together with the atomic mechanism, on a number of other oxides. Na,W04, V,05, MOO,. and WO, exchange the whole of the bulk oxygen with Bhe gas phase by a combination of both mechanisms. SiOe and GeO, are inactive. Possible mechanisms for the twomain reactions are discussed.
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