A system of physical chemistry - Index of

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A system of physical chemistry - Index of

THERMAL REACTIONS 139

tempearture. Under ordinary conditions, i.e. at ordinary temperatures,

this radiation is mainly of the infra-red type. It is well known that

electro-magnetic properties of substances, e.g. the dielectric constant,

are intimately connected with the radiation density in the material, and

further, such properties have a marked effect upon the chemical reactivity

of molecules, e.g. the electrolytic

which possess high dielectric constants.

dissociating power of solvents

It will be readily accepted,

therefore, that infra-red radiation is the source of the energy which is

required to bring about ordinary or thermal reactions. The problem

before us is to discover how this concept may be applied in as quantitative

a manner as possible. An attempt in this direction, made by

the writer [inter alia,

sidered briefly here.

Trans. Chem. Soc, 109, 796 (1916)), will be con-

For our present purpose, the most significant phenomenon associated

with thermal reactions is the influence ot temperature upon the

reaction velocity, the velocity constant increasing three or four fold for

a rise of 10° in the neighbourhood of room temperature. It has long

been known that this very great effect cannot be accounted for simply

on the basis of an increase in the kinetic energy of translation of the

molecule as a whole, for in the most favourable case, this mcrease could

not account for more than i or 2 per cent, of the observed effect, and

it is doubtful whether the change so produced would be even in the

right direction. It is evident, therefore, that the effect of temperature

upon the velocity constant has to do not with the kinetic energy of

translation but with the internal energy of the molecule.^

By treating the problem of reaction velocity from the point of view

of statistical mechanics it is possible to correlate the velocity constant

with the temperature, as has been done by Marcelin, and later in a

more exact manner by J. Rice {Brit. Ass. Rep., 1915, p. 397). The

relation between the observed velocity constant k and the absolute

temperature T for a simple type of gaseous reaction is shown by Rice

to be—

d\ogk\d'X: = E/RT2.

This expression is, as a matter of fact, a slightly simplified form, a

small term having been neglected. In deducing this expression it has

been assumed that a molecule reacts when its internal energy has been

raised to a certain critical value which is in general large compared with

' From measurements of specific heat it is known that, on the average, the in-

crease in internal energy per degree rise in temperature is not large. In the simplest

cases the increase in internal energy varies directly as the absolute temperature,

and hence a rise of 10° would cause an increase of approximately 3 per cent, in the

average internal energy at ordinary temperatures. The influence of temperature on

velocity cannot be ascribed therefore to the average increase in internal energy.

When heat is added to a body, however, the energy does not distribute itself evenly

among the molecules; instead, some receive no energy, others a moderate amount,

and a very few receive excessively great amounts of energy. It would seem therefore

that the influence of temperature on reaction-speed is to be sought in the fact

that a very small number of molecules receive an abnormally large quantity of internal

energy.

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