Page:Outlines of Physical Chemistry - 1899.djvu/175

 Heat of solution 7*15 Cal.

Heat of dilution 3*84 „

>! »» 2*32 ,,

1*42 „

»» »> 0*79 „

0-42 „

0-20 „

-004 „

-003 „

For sulphuric acid, Thomsen has established that at 18° the addition of nR 2 to the acid H 2 S0 4 evolves

n x 18 Cal.

��HNO s.

HNO3 + 2HP

HNO, + 4HP

HNO, + 6HP HNO, + 100H 2 O

The methods (direct and indirect) are the same for solids as for liquids.

According to Berthelot, caustic potash (KOH) and its solid di-hydrated derivative (KOH.2H 2 0) give out on dis- solving 12-46 and - 0-03 Cal. respectively. The heat of combination of the 2H 2 is consequently 12-49 Cal.

The dilution of concentrated caustic potash solutions, however, gives rise to a considerable thermal effect :

KOH + 3H 2 evolves on dilution 2-55 Cal. KOH+ 4H 2 „ „ 144 „

KOH + 10R.fi „ „ 0-23 „

KOH + 15H 2 „ „ -003 „

In presence of 15H 2 the hydration of caustic potash has progressed as far as possible.

Let us pass now to the study of salts.

Firstly, it may be stated that all salts, with very rare exceptions, which do not take up water of crystallisation absorb heat on dissolving. In fact, the solution is ac- companied by a breaking down of the molecular grouping of the solid. The solid must suffer a quasi-liquefaction in order that its molecules may become distributed amongst

��1 We shall see later that the solution of salt molecules is probably attended by a dissociation phenomenon (whereby the acid and the base are set at liberty).

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