96 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
the quantity of the vapour must increase at the expense of the water when the temperature is raised. Most substances dissolve in water with an absorption of heat, and in these cases the solubility must increase as the temperature rises. Lime and many of the calcium salts, amyl alcohol, and other substances, behave in the opposite way, because heat is evolved when they are dissolved in water. As the decompo- sition of ammonium chloride into ammonia and hydrochloric acid is accompanied by an absorption of 44,500 cal., the dissociation of this must increase with the temperatui*e. If in the above formula we set T = 0, we find that In -S" = oo , i.e. one of the concentrations, Cs, C^, or C^ must be zero. At the absolute zero, therefore, the substances must so react that the system which is formed with absorption of heat passes as completely as possible (until one of the reacting substances is fully used up) into the opposite system. At the absolute zero, therefore, the assertion of the older thermochemists, that that reaction occurs which is accompanied by an evolu- tion of heat, is quite correct. For the ordinary temperature, which indeed does not lie very high above the absolute zero, most of the reactions examined do take place according to that view, but numerous exceptions are known. The assump- tion may, therefore, be of use to a certain extent in indicating what direction a reaction will probably take at normal temperature.
Maxima and Minima in Equilibria. — It is to be observed that the heat of transformation fi often changes with the temperature. The connection between the heat of vaporisation of water at constant pressure, Q^ and the tem- perature can be represented, according to Eegnault, by the formula (for 1 gram) —
Q^ = 606-5 - 0-695 t cal.
If we transform this formula so as to make it apply to a gram-molecule, and to the absolute temperature, we obtain —
fji^ = (10,917 - 12-51 t) = (14,332 - 1251 T) caL
Let us assume that the vapour is evolved in a closed
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