A system of physical chemistry - Index of

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A system of physical chemistry - Index of

NERNSTS HEAT THEOREM ,47

Equations (i) and (2) express the Nernst heat theorem in terms of the

quantum theory.

[Note.—The mode of obtaining equations (i) and (2) is given in detail in Pollitzer's

article, Die Berechming chemischer Affinitdten nach dem Nernstchen Theorem,

Ahrens Sammlung, 17, 1912.

To facilitate calculation Pollitzer has included as an

appendix a number of tables giving the Einstein and Nernst-Lindemann functions

for various values of j3o and j/.]

We now proceed to consider an actual case, namely, the affinity of

silver for iodine investigated by U. Fischer {Zeitsch. Elektrochem., 18,

283, 191 2). The treatment of a number of difificulties met with is

instructive and is therefore given in some detail.

The first question taken up by Fischer is the heat of the reaction—

2Ag + I2 = 2AgI

(solid) (solid) (solid)

The direct measurements of Thomsen and of Berthelot being unsatisfactory,

recourse was had to the Gibbs-Helmholtz equation—

E - U - T^-|

as applied to the measurement of the e.m.f. of the cell—

Ag I

solution, saturated with Agl |

KI Ij

at several temperatures.

The results were as follows :—

N

Employing — KI solution, EvoUs = 0-6948 + 0-000305/

N

3

N

10

Evolts = 0-6932 + 0-000305/

Evolts = 0-6916 + 0-000305/

the temperature / being given in degrees centigrade.

These values for the e.m.f. require a correction, however, owing to

the fact that some of the iodine from the right-hand electrode dissolves

in KI, giving tri-iodide (and other polyiodides, the quantity of which

may, however, be neglected). This causes the concentration of the

iodine ion I' on the iodine side of the cell to be diminished, and the

concentration difference of I' on the two sides of the cell give rise

an e.m.f. e, where—

to

e = RT log, ^

- being the ratio of the I' ion concentrations. With the help of the

data of von Ohlendorff, and some experiments of his own on the solulO

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