210 SCIENCE PROGRESS 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc, 

 University, Liverpool. 



Thermal and Photochemical Change. — Although very numerous 

 reactions are known, which belong, on the one hand to the 

 thermal type, and on the other to the photochemical type, 

 very few are known in which the chemical change takes place 

 at a measurable rate thermally as well as photochemically. 

 Hydrogen peroxide decomposition is an instance of the latter 

 behaviour ; the oxidation of sodium sulphite is another case 

 in point, and as this reaction possesses particular interest in 

 view of the effects of various catalysts upon it, both positive 

 and negative, it is proposed to consider the reaction briefly. 



Bigelow ( i ) in 1 898 carried out a very extensive investigation 

 of the influence of catalysts upon the reaction occurring in 

 aqueous solution at 20 C, the reaction proceeding thermally. 

 Numerous difficulties were encountered, into the details of 

 which it is impossible to go, in connection with the non-repro- 

 ducibility of many of the results. For this reason many of the 

 numerical values possess a relative and not an absolute sig- 

 nificance. Marked effects were finally discovered to be attri- 

 butable to the solvent, the water itself. Exactly analogous 

 behaviour, it may be remarked, has been observed in the 

 decomposition of hydrogen peroxide. By working with a 

 given stock of water, however, it was possible to examine 

 systematically the influence of a number of substances upon 

 the rate of the reaction. All the organic substances examined 

 by Bigelow exerted a negative catalytic effect, that is to say, 

 they diminish the rate of the reaction. Ketones and esters of 

 the aliphatic series were found to exert an almost inappreciable 

 effect. On the other hand, benzyl alcohol, benzaldehyde, 

 butyl alcohol and other alcohols, mannitol, aniline and other 

 substances exert a marked anti-catalytic effect, benzyl alcohol 

 being the most remarkable in this respect. The striking fact 

 in connection with these catalysts is the minute amount which 

 is capable of producing a measurable effect. Thus in the case 

 of mannitol, a quantity so small that the solution was only 

 1 1 1 60000 normal with respect to the catalyst produced a detect- 

 able retardation. In a particular case in which there were 

 800 moles of sodium sulphite to one mole of mannitol, the 

 velocity was reduced to one half of its value in water alone. It 

 is inconceivable that such effects as these are brought about 



