A system of physical chemistry - Index of

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A system of physical chemistry - Index of

154 '4 SYSTEM OF PHYSICAL CHEMISTRY

in the dielectric constant apparatus.

In the case of water it is considered

that the high value obtained is partly due to the fact that water

possesses a band for the electro-magnetic waves usually employed.

Further, Kriiger's assumptions are possibly not correct in all cases.

Thus, it is very surprising that the results quoted should be as satisfactory

as they are, for the electrolyte is strong. Nevertheless the

deduction, so far as it goes, indicates that radiation plays a rdle in

ordinary chemical processes. Kriiger shows further how an empirical

relationship of Walden, according to which " saturated solutions of one

and the same electrolyte in various solvents possess the same degree of

ionisation," can be deduced on the radiation basis. For details regarding

this and other matters the original paper of Kriiger must be consulted.

The Heat of Reaction. Haber's Relation.

In Chap. VI. we have dealt briefly with thcrate of reaction and the

velocity constant from the point of view of the quantum theory. In

the case of a monomolecular reaction in a dilute gaseous system the

variation of the velocity constant with the temperature is given by the

expression —

d log klcll = NA.//RT^

where v is the characteristic vibration frequency, or head of the absorption

band, of the decomposing substance. If the reaction be a reversible

one, the resultant being characterised by the frequency v', the

corresponding velocity constant being k', we have—

Hence

d log k'jdT = NAv'/RT-^.

d log kjk' NA(v - v')

— -p— =

Rp

But>^//^' = K, the equilibrium constant. Hence we obtain the relation—

d log K/^T = N^(v - v')/RT2.

At the same time the statistical mechanical expression of Marcelin

and Rice gives the relation—

d log K/^T = (E - E')/RT2

where E is the critical increment of the reactant, E' that of the resultant.

On comparing these expressions with the van 't Hoff isochore, viz.—

d log YildT = - Q„/RT2

where -

Q^, is the heat absorbed per stoicheiometric quantity of the

reactant transformed or decomposed, we obtain the relation—

- Qj; (heat absorbed) = N>^(v - i/') = E - E'

or, heat evolved in the reaction = critical quantum of the resultant

minus the critical quantum of the reactant.

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