Heat of Combination of Metals in the Formation of Alloys

Extract

In the course of his valuable researches on the heats of combination and decomposition of bodies, the late Professor Thomas Andrews, of Belfast, arrived at the conclusion that “If three metals, A, B, C, be so related that A is capable of displacing B and C from their combinations, and also B capable of displacing C; then the heat developed in the substitution of A for C will be equal to that developed in the substitution of A for B, added to that developed in the substitution of B for C; and a similar rule may be applied to any number of metals similarly related.” A tabular statement of experimental results is given, and from them illustrations of the above conclusion may be obtained. For example, let A represent zinc, B lead, and C copper; then, for equivalent quantities of the metals, it was found that the number of (grammewater) heat units centigrade developed when zinc displaces copper is approximately equal to the number developed when zinc displaces lead, added to that obtained when lead displaces copper. But, in contact electricity, the electromotive force between zinc and copper is equal to that between zinc and lead added to that between lead and copper. These facts led Andrews to make the following important remark:—“Electromotive forces which are really due to contact of dissimilar bodies are also the very forces which cause heat when chemical combination ensues, potential energy being converted into kinetic energy by the rushing together of the particles under attracting forces.”