A system of physical chemistry - Index of

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A system of physical chemistry - Index of

LAW OF THE PHOTOCHEMICAL EQUIVALENT 135

to the ordinary mass action law, we shall meet with photochemical

stationary states instead, such as those realised in the anthracene-dianthracene

reaction.

Einstein's general argument is as follows. Consider a gas mixture

containing three different species, whose molecular weights are ;«^, m.^, and

m^, and suppose there are n^, n^, and n.^ gram-molecules of each of these

present, the reaction considered is one in which i molecule of mi

decomposes photochemically into i molecule each of ;'«2 and m-^, an

equilibrium point finally being reached. The first assumption made is

that the decomposition of m^ proceeds (owing to the absorption of

radiation) independently of the presence of the other species. The

second assumption is that the probability that an m^ molecule decomposes

in a given time is proportional to the density p of a given monochromatic

radiation to which the system is exposed. From these two

assumptions it follows that the number Z of m-^ molecules decomposing

per unit time can be written—

Z = KpUi

where A is a proportionality factor which depends only upon the temperature

T of the gas mixture.

As regards the recombination process, Einstein assumes it to be an

ordinary bimolecular reaction (which emits ^ the radiation first absorbed

If Zi denotes the number of m^ molecules thus

by the Wj molecules).

re-formed per unit of time, we can write—

Zi .

= A \

- . . -

where V is the volume of the f system ^ and -^ denoting the concen-

trations of each of these species).

A' likewise only depends upon the

temperature T of the gas, not upon p (assumption 3),

and if the tem-

perature be kept constant, A and A' are constant. Wheii thermodynamic

equilibrium is reached, i.e. equilibrium between the matter

involved and the radiation itself, we can equate the two qu mtities,

supposing Z and Z^ to refer to this case. That is, we obtain the

relation—

«2 fh

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