CATALYSIS 117 



ratio of the new surface a^ to the original surface a would be 



47rr^ ( >JA^ X 100 



^^^ = ^ ^^' 



i.e. the surface would be increased over four and a half times. If 

 the subdivision were carried still further till there were 10^" small 

 shot, then the total adsorbing surface would be increased 

 10,000,000,000 times. The intensity of adsorption is chiefly 

 dependent on the area of adsorbing surface (cf. Table IX.). In 

 other words, contact catalysis is indicated where the specific 

 surface of the catalyst comes within the colloidal range. Charcoal 

 is used as an adsorbent in the clarification of sugar. A cubic 

 metre of charcoal consisting of particles 1 mm. in diameter has a 

 surface of about 600 sq. metres. If the particles are reduced to 

 colloidal dimensions, say to O-l^Lt diameter, then the adsorbing 

 surface becomes 60,000,000 sq. metres. Capillary active substances, 

 e.g. anaesthetics, by being themselves adsorbed to the surface of the 

 catalyst prevent contact catalysis. 



2. Carriers. In some cases the catalytic agent combines 

 chemically with one of the reacting substances to form an unstable 

 intermediate compound. This, in turn, breaks up, regenerates 

 the catalyst, and liberates the reagent in the active atomic state — 

 so called nascent. Many oxidations and reductions are brought 

 about in this way. That is, if a reaction of the type 



A -\-B^AB 

 takes place very slowly under ordinary conditions, a catalyst C 

 which interacts with A {e.g.), thus A -\- C = AC, and AC itself is 

 acted on by B, 



B + AC = AB + C 



may materially alter the rate at which the whole reaction proceeds. 



3. Ionic Catalysts. Hydrogen and hydroxyl ions act as cata- 

 lysts for many reactions which occur in aqueous solution. The 

 velocity of such a reaction in dilute solution is pro])ortional to 

 the concentration of the ions in question, provided the thermo- 

 dynamic environment remains constant. The ion probably acts as 

 a carrier, forming an unstable perhydrate as intermediate product. 



The following statements are a brief survey of the characteristics 

 of catalysts : 



(a) A very small amount of catalyst can produce a considerable 

 alteration in the rate of reaction. 



(6) No amount of catalyst can start a reaction that would not 

 otherwise take place. 



(c) A catalysed reaction reaches the same final state as ultimately 

 it would reach if no catalyst were present. 



