A system of physical chemistry - Index of

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A system of physical chemistry - Index of

146

A SYSTEM OF PHYSICAL CHEMISTILY

for the quantity U, which involves the algebraic sum of a number of such

terms, we can write-^

T

U = Uo + 'S^ucd^T

where Uq is the change in internal energy accompanying the reaction

at absolute zero, and n denotes the number of moles of reactants and

resultants (with proper sign) which take part in the reaction.

We have now to express the molecular or atomic heat c of each reacting

substance in terms of the Nernst-Lindemann equation. Having

done this we find that the above integral may be evaluated thus—

Hence—

The final term is an empirical correction, which has to be added to

allow for the fact that the experimental measurements of molecular or

atomic heats refer to constant pressure, whilst the quantities to which

the Nernst-Lindemann expression refer are molecular or atomic heats

at constant volume. {Cf. Magnus and Lindemann {Zeitsch. Elektrochem.,

16, 369 (1910)) ; a more exact correction is given by Nernst and

Lindemann {Zeitsch. Elektrochem., 17, 817 (191 1)).)

We have now to consider the free energy term A. On the basis of

the first and second laws of thermodynamics we have already seen that—

Further, Nernst's theorem leads to the conclusion that a = 0, and

therefore we can write—

Again writing—

A= -

tJ^^../T.

U = U„ + ^\„cdT,

and substituting this in the above integral, and also substituting the

Nernst-Lindemann expression, we obtain—

A = -

U„ RS - _ .

log,(^3oWT i)

I log,(^^0W2T

[|03ov)

_ 1)1

-

,^ S«T5/^ .... (2)

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