CATALYSIS AND BIOCHEMICAL PROCESSES 223 



active in catalyzing chemical reactions; this is especially 

 true of certain forms of carbon anrl of metals like 

 platinum. The action of platinum is especially well 

 known; it increases with the state of subdivision, i.e., 

 the extent of surface, hence colloidal platinum is a very 

 effective catalyzer. Other metals have similar proi)er- 

 ties, although usually less marked. 



Usually in such heterogeneous catalyses the accelera- 

 tion of reaction- velocity is regarded as a result of in- 

 creased concentration at the surfaces. Faraday (1839) 

 suggested this explanation for the action of platinum 

 in catalyzing the combination of hydrogen and oxygen. 

 In general, when considering any special case of hetero- 

 geneous catalysis, three independent processes with 

 different rates are taken into account: (i) the rate of 

 diffusion of the dissolved substrate to the active surface; 

 (2) the rate of adsorption at the surface; and (3) the 

 rate of chemical combination. The rate of reaction is 

 limited by the rate of the slowest of these interdependent 

 processes. In most cases the reaction- velocity (F) is 

 regarded as determined by the concentration (C) 

 attained at the surface and by the specific velocity- 

 constant (K) of the reaction (i.e., V = KC), since adsorp- 

 tion is rapid and also diffusion (when the distances are 

 small). The rate of chemical change is increased (cata- 

 lytic effect) because the concentration of the reacting 

 molecules is increased in this part of the system.^ 



It is evident, however, that other factors frequently if 

 not usually enter dependent on the special chemical 

 nature of the reacting compounds. Many inorganic 



^ Cf. Hober's Physik. Chemie der Zelle, pp. 702 flf., for an account of 

 the general conditions of catalysis in heterogeneous systems. 



