Page:Text-book of Electrochemistry.djvu/254

 The above formula is valid for the condition that q = 96,500 coulombs, i.e. for an equivalent, so that the heat of ionisation obtained refers to a gram-equivalent of copper ( = 31*8 grams). For a gram-ion (63*6 grams), the heat is, grams of copper from the metallic to the ionic condition, 2 X 8718 (= 17,436) cal. are required.

As a rule, heat is evolved when ions are formed ; that is to say, the heat of ionisation is negative. If v^e know the heat of ionisation of one metal, that of any other metal can be calculated from the thermochemical data. For instance, when copper is displaced from copper sulphate by zinc, 25,055 cal. are evolved per equivalent. This process consists partly in the transformation of an equivalent of zinc from the metallic to the ionic condition, and partly in the trans- formation of an equivalent of copper from the ionic to the metallic condition. The heat evolution for the latter has been shown to be 8718 cal. For the former, therefore, there remain 16,337 cal. Now, since the heat changes which occur when one metal displaces another from its salts are known from thermochemical measurements, it is easy to calculate in the above manner the heats of ionisation, as has been done by Ostwald (10), whose values for equivalent quantities are contained in the following table. The heat of ionisation of hydrogen is almost zero; it amounts to 550 cal. The heat of ionisation of a metal is, therefore, equal to the negative heat of solution of the metal in an acid less 550 cal.

Heats or Ionisation in Calobies (small).

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