Page:Text-book of Electrochemistry.djvu/219

 204 ELECTROMOTIVE FORCE. chap.

a Daniell Cell. Thomson's Rule.—We may now go more fully into the processes which take place in a Daniell element. If this developes 96,500 coulombs, then at the same time a gram-equivalent (32-7 grams) of zinc is dissolved, and a gram-equivalent (31*8 grams) of copper is deposited. A certain amount of heat is evolved during this change, and can be determined calorimetrically ; it amounts to 25,065 cal. If the electric work done by the element were exactly equal to the quantity of heat evolved, or, in other words, if the heat evolved in the Daniell cell were completely changed into electric work, it could move the 96,500 coulombs against an electromotive force Vq — F\, which is given by the relationship —

23,070(F"o - Fi) = 25,065.

The value of Vo - Vi would then be 1'086 volts, i.e. the coulombs from potential to potential 1086 volts. If we connect the zinc pole of the cell with the earth, and the copper pole with a condenser at potential P, the elec- tricity can pass to this condenser, i.e. the condenser can be charged, so long as F is smaller than the electromotive force of the element. The greatest work which an element can do in this way is to charge a condenser to the same potential as the electromotive force of the element.

If our assumption be correct, that the Daniell element changes the whole of the chemical energy used up into volts. Experiment shows that the electromotive force almost reaches this value (1 dll. = about 110 volts, see p. 124), and on this account it was for a long time believed that the electromotive force of an element could be calculated from the heat value (per gram-equivalent) of the chemical process taking place by simply dividing this (expressed in gram- calories) by the number 23,070. According to this, if £ is the electromotive force and H the quantity of heat developed by the chemical reaction per gram-equivalent, then —

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