CATALYSIS 3 



ctemical changes take place in nature they may be compared 

 in the first place with ordinary chemical changes, which can 

 be effected in the laboratory with a minimum of assistance 

 from external chemical or physical energy. Examples of 

 such changes are frequent in the category of so-called catalytic 

 actions. 



The little cigar lighter, a smoker's toy which is often to be 

 seen in tobacconists' shops, is a good illustration of the chemical 

 action brought about by catalysis. In this case the warmth 

 of the hand causes a little alcohol vapour to evaporate from 

 the metal box and to impinge on a small knob of spongy 

 platinum which acts as the catalyst. Its precise mode of 

 action is not fuUy known, but it has the effect of bringing 

 about the union of the alcohol vapour with the oxygen of the 

 air, with the result that the alcohol bursts into flame. Platinum 

 in a state of fine division, such as may be obtained, for example, 

 by soaking asbestos in platinum chloride and driving ofi 

 the chlorine by heat, is capable of bringing about a number 

 of changes at temperatures much below those at which they 

 would normally take place, and in some cases these changes are 

 such as would not otherwise occur. If a thread of asbestos, 

 covered with platinum in the manner above described, is 

 warmed and then held in a stream of coal-gas escaping, e.g., 

 from an unlit Bunsen burner, the platinised asbestos will glow. 



A technical process of importance, viz., the manufacture of 

 highly concentrated sulphuric acid, consists in passing sulphur 

 dioxide (SO3), obtained by burning pyrites or sulphur, together 

 with oxygen, or air, over heated platinum in a fine state of 

 division. The two gases then combine in accordance with the 

 simple equation : — 



SO2 + = SO3 



But this change does not take place directly, i.e., without the 

 presence of a substance like the platinum, which acts as a 

 catalyst. 



