A system of physical chemistry - Index of

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A system of physical chemistry - Index of

138 .4 SYSTEM OF PHYSICAL CHEMISTRY

solvent. Baly's argument is, therefore, that in the case of a dissolved

substance reacting photochemically, less energy is required per molecule

than is required for the same substance in the gaseous state ; and

hence Einstein's law does not apply quantitatively in such a case. In

other words, one quantum can apparently activate numerous molecules.

Baly suggests that this affords an explanation of the results obtained by

Henri and Wiirmser {Journ. de Physique^ [5], 3, 305 (19 13))

in connection

with the decomposition of hydrogen peroxide and of acetone in

aqueous solution. In both of these cases the amount of energy actually

required is considerably less than that demanded by Einstein's law, in

fact, only i to o-i per cent, of the "theoretical" value. As Henri and

Wiirmser point out, the light in these cases acts as a catalyst, accelerat-

ing but not initiating the reaction. Baly's view, which gives the possible

mechanism of such photo -catalysis, is further borne out by the

statement of Henri and Wiirmser that the above reactions will occur in

the dark, i.e. the solvent itself is evidently capable of bringing them

about. On the other hand, it is precisely

in those cases in which solvent

is absent that we find the closest approximation to Einstein's law.

At the present time we do not possess as satisfactory experimental

evidence for Einstein's law as we could wish, but the law may be re-

garded as substantiated at least approximately.

Einstein's law depends upon Wien's radiation expression. It is

therefore restricted to regions in which the wave-length is not very long.

It will be shown in the next section that it is justifiable to regard

Einstein's law as applicable down to the short infra-red region, i.e. the

region which is responsible for so-called thermal reactions at ordinary

temperatures.

r

Thermal Reactions. Reaction Velocity from the Standpoint

OF THE Quantum Theory.

By the term thermal reactions is meant reactions of the "ordinary"

kind which proceed without being exposed to any definite external

source of short-wave radiation such as we meet with in the so-called

photochemical

reactions. The fundamental difference between the

two kinds of reaction is, as we shall see, the fact that in photochemical

reactions, as ordinarily carried out, the temperature of the active

radiation is very much higher than the temperature of the material

system affected, whilst in the case of thermal reactions, the temperature

of the effective radiation is identical with that of the material system

itself.

Photochemical phenomena have demonstrated the fact that

chemical change can be brought about as a result of absorption of

radiation in the short-wave region. We are led to generalise this and

to seek in radiation of longer wave-length the ultimate cause of ordin-

ary or thermal reactions as well. It is obvious that every material

system, in virtue of its temperature, contains radiation at the same

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