CHEMICAL REACTION AND HEAT. 107 



substances which occasion the greatest development of heat.'" 1 And there- 

 fore, "other things being equal, there is the more chance that a substance 

 can be formed, the greater its heat of condensation." 1 ' While these are the 

 usual rules, they are not broad enough to cover the reactions of meta- 

 morphism under all pressures and temperatures. A more general statement 

 of the law as to the relations of heat and chemical reactions is that of van't 

 Hoff: "On the whole, the preponderating chemical reactions at lower tem- 

 peratures are the combinings (associations) which take place with a devel- 

 opment of heat, while the reactions preponderating at higher temperatures 

 are the cleavings (dissociations) which take place with the absorption of 

 heat."" The meaning of this law may be illustrated by the following 

 reactions: At ordinary temperatures CO combines with 0, producing C0 2 , 

 with great liberation of heat; at very high temperatures C0 2 dissociates 

 into CO and O, with very great absorption of heat. This illustration 

 makes it clear, as stated by Nernst, that to cover all cases van't Hoff's 

 law must replace that of Berthelot, above given. Still more general laws 

 as to the relations of heat and chemical reactions are the following: "If 

 we heat a chemical system, at constant volume, then there occurs a 

 displacement of the state of equilibrium, and in that direction towards 

 which the reaction advances with absorption of heat." d "Those chemical 

 forces which condition a development of heat will always be weakened by 

 an increase of temperature; and, conversely, those which condition an 

 absorption of heat will be strengthened by such an increase in tempera- 

 ture; and it is this fact which, primarily, gives the preceding proposition 

 its universal validity."' 2 "If we heat the system, therefore, the reaction 

 which takes place will be accompanied by absorption of heat; if we cool 

 the system, the corresponding reaction will develop heat." 6 



Now that the general laws covering the mutual influences of heat and 

 chemical action have been given, we may consider in more detail their 

 meaning. The speed of the reaction is commonly increased much more 

 rapidly than the increase in absolute temperature. Thus, the speed of 

 reaction of two similar solutions, one of which is at higher temperature 



« Nernst, cit., p. 5S1, quoting Berthelot. 

 b Nernst, cit, pp. 585-586. 

 <-' Nernst, cit., p. 583. 

 <? Nernst, cit., p. 566. 



"Ostwald, W., Outlines of general chemistry, translated by James Walker, 2d ed., Macmillan & 

 Co., London, 1895, p. 312. 



