178 OUTLINES OF PHYSICAL CHEMISTRY
when it reacts on chloric acid, because the heat of forma- tion of this acid is less than that of iodic acid.
We cannot consider as being in opposition to this rale, whose study we are pursuing, certain endothermic trans- formations, realised by a reaction where the whole produces a positive thermal effect. Examples :
(1.) The transformation of silver iodide into the chloride by direct action of chlorine evolves 14*8 Cal. Yet the reverse transformation according to the reaction AgCl + HI = Agl + HC1 is possible, because this double decomposition corresponds to an evolution of
- (290 - 6-4) + (14-2 + 22) = 18-6 Cal.
(2.) The preparation of silicon tetrachloride by the action of chlorine on silica is impossible, because the reaction
would correspond to a thermal effect of — 179*6 + 121*8 = — 67*8 Cal. But the preparation succeeds if we work at a high temperature and mix the silica with carbon. It becomes slightly exothermic if the oxygen of the silica passes into carbonic oxide. The formation of aluminium
In these reactions the endothermic transformation is possible owing to an excess of heat furnished by another transformation proceeding at the same time. By a com- pensation of this kind certain endothermic combinations may be made to take place. Examples :
(1.) We have already seen that (H, 1) = — 6*4 Cal. whilst (H 2 , S) = 4*8 Cal. Therefore H 2 S and I 2 have no direct action on each other. But it is otherwise if we work in dilute aqueous solution. The heats of solution may then furnish an assisting energy, and the reaction
1 It would be necessary besides to take into account the dis- integration which the substances undergo at the high temperatures at which these operations are carried out.
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