Page:Outlines of Physical Chemistry - 1899.djvu/197

 H 2 S.aq + I 2 = S + 2HI.aq takes place, because it evolves about - 9-5 + 26-4 = 16-9 Cal.

(2.) The preparation of the oxygen compounds of chlorine by direct union of the elements would give rise to an absorption of heat. These substances are only formed conjointly with the metallic chlorides by the action of chlorine on the oxides or hydroxides of certaip metals. The reaction 2KOH + Cl 2 = H 2 + KC1 + KOC1 is exothermic and produces potassium hypochlorite.

These examples will suffice to demonstrate that the existence of certain endothermic compounds is not incom- patible with the law of maximum work : they are formed in reactions where the total effect is exothermic. Besides, these compounds are very unstable, and if their formation only succeeds by indirect means their decomposition is generally easily brought about and is often even spon- taneous.

The difficulty becomes more serious when we try to interpret those reactions which in their entirety are endothermic. Amongst those there are some which do not fall within the application of BertheloVs principle, because they can only be brought about at very high temperatures or under the influence of a foreign energy. Such, for instance, are the preparation of carbon disulphide, the formation of acetylene in the voltaic arc ; further, those syntheses by reduction which take place under the influence of light in the green parts of plants (formation of carbohy- drates).

But there are also endothermic reactions which can take place spontaneously and without the aid of any external energy. For example :

(a.) Many heats of solution are negative. Sodium

chloride, notably, absorbs 1*1 Cal., and its affinity for water

is such that a mixture of salt and ice liquefies by absorbing,

besides the heat of solution of the salt, the latent heat of

fusion of the ice. The affinity which brings about the

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