A system of physical chemistry - Index of

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A system of physical chemistry - Index of

NERNST'S HEAT THEOREM 149

The difference of the heat effects of the reactions (i) and (2) gives

the heat effect desired, viz.—

Ag + I-^Agl.

The mean value thus obtained was U = 14,280 cals., which agrees

pretty closely with the result of the first method. Fischer made use of

still another calorimeter method, in which some of Thomsen's data on

other salts were employed, the results being less trustworthy. [Note

that this is a heat effect at constant volume, since the system does not

alter appreciably in volume.]

Now, if we consider the cell—

Ag I Agl I I2

(solid) (solid) (solid)

this represents a condensed system, each phase being a single substance

[no solution or vapour being present), and we can apply Nernst's heat

theorem directly in the form given in equations (i) and (2) above.

[It should be noted that the A or E of this cell is not quite the

same as the E or A' of the cell in which KI solution is present. The

heat effect in both cases is, however, the same, for, as we have seen,

the heat effect as calculated (say by the first is method) independent

of the KI concentration, being simply the heat effect of the reaction

Ag + I = Agl.]

The values of /Jqi/ and of a are known in the case of the three substances

Ag, 1, and Agl from the atomic and molecular heat measurements

of Nernst.

By making use of the values of fi^v and a obtained in this manner,

Fischer calculated U for a*temperature T = 288, finding the value

15,079 calories, a quantity which agrees extremely well with the former

values which were obtained for U quite independently of the Nernst

heat theorem, or the Nernst- Lindemann atomic heat expression. It is

to be remembered that the U and A terms here refer to the reaction

between pure solid Ag and solid I2, giving solid Agl. A is therefore

the affinity which we set out to calculate. The following table contains

the values of A and U thus obtained :—

Affinity of Silver for Iodine at Various Temperatures.

Temperature,

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