ACTION, CHEMICAL) (3) the influence of perature on chemical equilibrium; (4) the de termination of the maximum mechanical work that can be performed by the chemical energy of reactions. We will briefly consider these ques tions in their consecutive order.
With reference to the dependence of the energy-change of a reaction upon the tempera ture, the verdict of thermodynamics is, first of all, that in case the heat-capacity of the reacting substances is the same as that of the products of the reaction, the energy-change of the reaction does not vary with the temperature. This is generally the case when the reacting substances and the products of the reaction are all solids. Thus the combination of silver and solid iodine into silver iodide sets free practically the same amount of heat (about 14.000 calories), no matter at what temperature the combination is caused to take place. On the other hand, if a
given reaction does cause a change in the heat capacity, then, if t denotes the difference be tween two temperatures at which the reaction may take place. u, the difference between the energy-ehange at the first and that at the second temperature, and c the difference between the heat-capacity of the reacting substances and that of the products of the reaction, u, — ac, = Ct.
By measuring calorimetrically the heat-capaci ties in question and the energy-change of the reaction at some one temperature, it is therefore easy to calculate the energy-change of the re action at any other temperature—a result of con siderable importance, because direct calorimetric measurements of the energy-change can by no means be readily carried out at any desirable temperature.
As to the velocity of reactions (see REACTION,