Page:Outlines of Physical Chemistry - 1899.djvu/204

 186 OUTLINES OF PHYSICAL CHEMISTRY

silver, or the same fraction of the equivalent of any other element or radical. The electrolytic transportation of one gram-equwalent corresponds, therefore, to the passage of 96,540 coulombs.

Bemark. — Faraday's law only takes one of the factors of electrical energy into account, namely, the quantity, and leaves the other factor — the fall of potential — entirely oat of the question.

This relationship between the current and the amount of chemical decomposition holds not only for cases of electrolysis, but is also true for those reactions by which electricity is produced, and gives us a ready means of calculating the electromotive force of batteries. In DanielVs element, for instance, we have a zinc plate immersed in a solution of zinc sulphate and a copper plate immersed in a solution of copper sulphate, the two solutions being separated by a porous pot. The ultimate chemical reaction which takes place is the replacement of copper by zinc : CuS0 4 + Zn = ZnS0 4 + Cu, and for each gram-atom of zinc dissolved there is an evolution of 50-5 Cal. :

(Zn, S, 4, aq) - (Cu, S, 4 , aq) = 248-0 - 197*5 =

50-5 Cal.,

and for an equivalent of zinc dissolved 25,250 small calories are evolved.

If this energy were entirely converted into electricity we should obtain ^ = 105,204 units' of electrical energy (because 0*24 Cal. = volt x coulomb). But we know from Faraday's law that the quantity of electricity produced in this case is 96,540 coulombs. The energy developed by the battery will, therefore, be 96,540 x e, e being the electromotive force, and so we get the equation :

from which we deduce

e = 1-09 volt,

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