A system of physical chemistry - Index of

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A system of physical chemistry - Index of

THERMAL REACTIONS 141

radiation and to the material), Planck's theory may be applied with

some degree of certainty. In this connection the following statement

made by Planck {Theory of Heat Radiation, Eng. ed., p. 7)

is of importance

: "A special case of temperature radiation is the case of the

chemical nature of the emitting substance being invariable. Neverthe-

less, it is possible, according to what has been said, to have temperature

radiation while chemical changes are taking place, provided the chemical

condition is completely determined by the temperature."

Conditions for Reactivity on the Radiation Hypothesis.

According to the law of mass action the reactivity of a substance is

measured by its active mass. Since the time of Guldberg and VVaage

it has been considered sufficient to regard the concentration of the substance

as an accurate measure of the active mass, and this has been

borne out by an accumulation of experimental results, which serve to

demonstrate the general truth of the hypothesis. At the same time

numerous facts are known which indicate that concentration per se is

not the only factor determining reactivity. To attempt to extend the

principle of mass action so as to allow for environment, it is necessary to

take into account the energy exchanges which accompany, or rather

precede, molecular change, for as the fact of a^ finite velocity demonstrates,

all the molecules of a system are not at any one moment in the

same reactive state. From what has already been said regarding the

absorption of energy by an average molecule to bring its energy content

to the critical value, it will be evident that the amount and nature of

the radiation present in the system must play a definite part in the

reactivity of a given species. From this point of view, therefore, the

active mass of a species is not simply proportional to its concentration,

but is simultaneously dependent on the radiational environment, the

observed reactivity being due to the product of these two effects. The

active mass is, in the case of unimolecular decomposition, identical

with the number of molecules brought per second into the active state

by means of the radiation present in the system. The fundamental

assumption is that the radiational environment can be identified with

the radiation density of the absorbable type present throughout the

system. That is, the active mass of a species, spontaneously decomposing,

is proportional to its concentration, multiphed by the radiation

density of the type which the substance can absorb, the radiation being

present in virtue of the temperature of the system. The radiation

density, u^} is the amount of radiation made up of quanta corresponding

with a certain frequency v, which is present in unit volume of the

system, when the radiation and the matter are in thermal equilibrium.

If the radiation density is too small the substance will not react, howsoever

great its concentration may be. On this view the function of the

1 As already pointed out iiv has not the dimensions of energy density the

;

actual expression for energy density is Uvdv. The term Uv may be called the radiation

density /acioy.

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