Page:Text-book of Electrochemistry.djvu/218

 of which is the same as the metal of the electrode. The commonest of these elements is the Daniell cell —

Zn I ZnSOi | CUSO4 I Cu,

in which zinc is dissolved with formation of zinc sulphate; and copper is deposited from the copper sulphate. When a current is passed in the opposite direction, i.e. from the copper to the zinc through the solutions, the deposited copper is dissolved and zinc is separated, so that the original condition can be re-established. Combinations of the type of the Clark cell (see p. 124) are also reversible. The commonly used Bunsen and Leclanche cells belong to the group of irreversible elements.

In order to express electric energy in the ordinary units, we recall what has already been said (pp. 6 and 11). Electrical work is expended when a given quantity of positive electricity, g' coulombs, is brought from a place of lower potential, Vq volts, to a place of higher potential, Vi volts. The work done then amounts to —

q(Vi — Vo) volt-coulombs.

The same work has to be done to bring the quantity q of negative electricity from the higher potential Vi to the lower potential Vq, and the calculation for the simultaneous trans- port of the two electricities (positive and negative) can be made in an analogous manner.

Now, according to definition, the value of a volt-coulomb is —

1 volt-coulomb = 10'' ergs = 01018 kilogram-metres

= 0-239 cal.

To separate a gram-equivalent of a metal 96,500 coulombs are required; the work done for such a quantity of electricity is therefore —

23,070(.ro - Fi) cal.

Transformation of Chemical into Electrical Energy in

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