Ref. 1, Fig. 1, is wrong, and should be ignored.

I. IntroductionTHE study of the kinetics of heterogeneous reactions covers a wide field.In one system the problem may be purely chemical iii nature, but in another the characteristics which are of interest to the chemist may be entirely determined by non-chemical factors. In this Review an account will be given of systems in which the observed rate of reaction is either wholly or partly determined by the rate of a transport process, i.e., by the rate of mass-transfer of a species by convection and diffusion. In general, attention will be confined to systems subject to forced convection, although many of the considerations apply also to mass-transfer under natural convection. The subject has been reviewed as a whole, or in part, by Centnerszwer,l Taylor,2 Bowden and Agar,3 T o ~b i n , ~ and by Moelwynhug he^.^ Hixson and Crowell,6 and Hixson,' have reviewed the topic from the standpoint of the study of agitation.The general case of a reaction between a solid and a solution, resulting in soluble products, will be taken as a model. The overall process may comprise as many as five primary steps : ( a ) Transport of solute molecules to the interface. ( b ) Adsorption a t the surface. (c) Reaction a t the surface. ( d ) Desorption of the products. ( e ) Recession of the products from the interface.Of these, steps ( b ) , ( c ) , and ( d ) are characterised by an interaction between the solid and solute and, quite generally, will be termed chemical processes. Complications may arise in other systems ; for instance, the products may be gaseous, or may form an insoluble layer on the solid surface, or may undergo further reaction in the bulk of the solution. For a liquid-liquid system in which reaction takes place between two solute species, one in each phase, step ( a ) will comprise two transport processes, etc. By con-

When copper disks rotate in sulfuric acid solutions, the observed rate of dissolution is first order with respect to the concentration of dichromate ion used as a depolarizer. By studying the effect of temperature and stirring speed at high and low acid concentrations, the process is shown to be determined completely by the rate of transport of dichromate ion to the reacting surface. This has been confirmed by studying the effect of different surface preparations, by measuring the rate of reaction of brass and amalgamated copper with these solutions, and by comparing the observed values with the theoretical rates. The effect of blocking off progressively larger areas of the working surface is reported.

No abstract

Three paths for the iodine redox process at platinum electrodes are discussed with reference to stationary current/potential curves obtained for the cathodic reduction of the tri‐iodide ion in potassium iodide solutions. It is shown that the process follows the path originally proposed by Vetter, the slow stage being the transfer of an electron from the platinum to an adsorbed iodine atom, and that the degree of reversibility of the process increases as the concentration ratio of tri‐iodide to iodide is decreased. The apparent energy of activation for the over‐all process is 5.3 kcal·mole−1 for solutions containing only potassium iodide and tri‐iodide. The results, and those of other workers, indicate that the electrode surface is nonuniform.