Catalysis or the Speeding-up of Chemical Reaction 37 



Of the metals, platinum and nickel are the most effective 

 catalysts, and their efficiency is greatly increased as their fineness 

 of division increases. Thus colloidal platinum and platinum 

 black are incomparably more effective than platinum wire or 

 platinum foil. 



Theories of catalytic action. — Many views have been pre- 

 sented to account for the action of catalysts, and of these two 

 deserve mention. The first — which may be regarded as the 

 chemical view — supposes the catalyst to form transitional 

 chemical compounds. Here the catalyst acts as a kind of middle- 

 man, inducing reaction between the two main reagents. The 

 hypothesis may best be supported by reference to the reaction of 

 sulphur dioxide witlj oxygen to form sulphur trioxide. This 

 addition takes place but slowly in the absence of a catalyst. In 

 the presence of a trace of nitrogen peroxide the difficulties 

 disappear and the reaction proceeds quickly. This is explained 

 by the nitrogen peroxide first parting with some of its oxygen 

 to the sulphur dioxide to convert it into the trioxide. The 

 resulting nitric oxide quickly takes up oxygen and is again 

 converted into the peroxide, which is available for further 

 reaction with sulphur dioxide. Thus a small amount of nitrogen 

 peroxide is sufficient to bring about the union of large amounts 

 of sulphur dioxide and oxygen. The transition of the catalyst 

 into a lower oxide during the course of the reaction supports the 

 view of catalytic action mentioned above. 



There are many cases in which the intermediate compound 

 view is untenable, notably in reactions which are accelerated by 

 catalysts such as platinum or other finely divided metals. In 

 such cases it is assumed that the catalytic power of the substance 

 is connected with some physical property. The chief property 

 that has been emphasised is that known as adsorption. Most 

 substances have the power of condensing other compounds on 

 their surface, and this is known as adsorption. Charcoal, for 

 example, can take up about 170 volumes of ammonia gas ; it can 

 also remove substances from solution, as is seen in the case of the 



