A system of physical chemistry - Index of

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A system of physical chemistry - Index of

THERMAL REACTIONS 143

The term N/zi' is a constant, characteristic of a given substance, and

independent of the temperature. It may therefore be denoted by the

symbol A, whence we obtain—

d log Kb,ldi: = . . . . A/RT2 (6)

This expression is identical in form with the empirical relation of

Arrhenius which is known to be in agreement with experiment for a

great variety of reactions. The fact that this relation can be deduced

on the basis of the radiation hypothesis of chemical reactivity con-

stitutes strong evidence in favour of the hypothesis itself.

Let us now compare equation (5) with that obtained by Marcelin

and J. Rice, — namely

d log kldT = E/RT2.

On equating like terms it follows that—

= hv.

E/N

But E/N represents the amount of energy which has to be added to

a single molecule to make it react, and by the above relation this is just

one quantum of the absorbable radiation. This is simply a statement

of the Einstein law of the photochemical equivalent, which is now shown

to be applicable to ordinary or thermal reactions caused by radiation of

the short infra-red type. That this result is to be expected follows from

the fact that for radiation of this type Planck's expression reduces to

that of Wien, upon which Einstein founded his law.

The radiation hypothesis has been extended to solute catalysis. It

is not proposed to enter into this question in the present instance ex-

conclusion is arrived at : the

cept to point out that the following general

presence of a positive catalyst dimmishes the temperature coefficient,

i.e. decreases E, whilst a negative catalyst increases the temperature

coefficient.

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