Page:Text-book of Electrochemistry.djvu/224

 XIII,

��FREE AND BOUND ENERGY.

��temperature — ^and others are known which give up heat to the surrounding medium, and their electromotive force decreases as the temperature rises.

Helmholtz's deduction has been fully confirmed by- experiment, most thoroughly by Jahn (5), who measured the heat evolution by means of an ice calorimeter. The following table gives the results of his experiments. In the column headed Calc. are given the values of 23,070 x P — fT, calculated by Helmholtz's method from the observed tempera- ture coefficients, and in the column headed Ohs, the calori- metricaUy observed values are given. The experiments were made at 0° (273° absolute).

��DlemeDt.

��Ca I Ca.S04 + lOOHgO | ZqSO^ + IOOH2O | Za.

+ lO€HaO I Pb

AgCl I ZnClg + lOOHoO | Zo

Aga Z0CI2 + ftOUaO I Zn

AgCl I ZaCi% 4- 25H«0 | Zq

AgBr I ZnBro + 25H2O | Za

AgNOs I Pb(3J08}a I Pb

AgNO. |Cu(N0sJ2 Cu

Hg20 I KOH I m I HgCl I Ug (BagMBzky^

��P. 23070P.

I

��3307 OP -W.

��Calc.

��Ag Ag Ag Ag Ag Ag Hg

��w.

� � �ObB.

��From the numbers quoted, it can be seen that the value of 23,070P is sometimes greater and sometimes smaller than W, the diflTerence amounting to even as much as 50 per cent. Indeed, in Bugarszky's element these two values have diJBFerent signs. It is noteworthy that such a small addition of water to the zinc chloride in the element Ag — Zn is able to produce such a great change in its behaviour.

Free and Bound Energy. — As already mentioned, the view was previously entertained that the whole heat energy of an element might be transformed into electric energy. Helmholtz, however, showed by the above reasoning that this is not always the case, and he therefore introduced the idea of free energy as that part of the total energy which can be completely transformed into mechanical work. The energy 23,070 . P in the above case, is evidently of this kind, for

p

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