Page:Outlines of Physical Chemistry - 1899.djvu/205

 a number which does not differ very much from that found for this element.

The energy of a battery is necessarily proportional to the quantity of heat evolved in the chemical reaction. But the agreement between the calculated electromotive force and that found by experiment is not always very close. Certain elements when in action become hot, because the chemical energy is not completely converted into electricity (their electromotive force diminishes when the temperature is raised). Other elements which become cold when in action absorb heat from the surrounding medium and give a higher electromotive force than the calculated one (their electromotive force increases as the temperature is raised (von Helmholtz)). "We even know some elements in which no real chemical transformation and the following are three examples :

Concentrated zinc

amalgam Solution of zinc

sulphate Weak zinc

amalgam Concentrated cadmium

amalgam Solution of

cadmium iodide Weak cadmium

amalgam

8ilver Concentrated (O'l

normal) solution of

normal) solution

of silver nitrate Silver

According to Nernst the electromotive force of this last-mentioned cell at 18° is 0*055 volt. Its activity is due to the tendency of the two solutions to equalise their con- centrations. The dilute solution dissolves some metal from one electrode, and the concentrated solution deposits an equal quantity on the other electrode. Whilst the Ag ion, carrying the positive electricity, passes in the direction from the dilute to the concentrated solution, a corresponding

1 The electrical energy furnished by these cells is generally only a transformation of heat energy derived from the surrounding medium.

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