Page:Outlines of Physical Chemistry - 1899.djvu/178

 160 OUTLINES OF PHYSICAL CHEMISTRY

litres of water for acting on the insoluble metallic

We see that certain acids in combining with the same base evolve almost the same quantities of heat, and in the same way certain bases are isodynamic towards the same acid. Thus the strong monobasic acids, such as HCl, HBr, HI, HNO 3, HCIO3, HBrO 3 , HIO3, HClO4, and the organic sulphonic acids, on neutralisation with caustic soda evolve quantities of heat which vary only within narrow limits (from 18.4 to 14 Cal.). To this list the acids of the acetic acid series may be added, as far as their action on a strong base is concerned. The weak acids, such as HClO, HON, &c., have a relatively low heat of neutra- lisation (9.8 and 2.9 Cal.).

The alkaline and alkaline-earth bases, and, we may also add, the tetra-substituted ammonium- and tri-sub- stituted sulphonium-hydroxides, form an isodynamic group wherein the heat of neutralisation by hydrochloric acid varies from 13.6 to 14 Cal. With ammonia the evolution of heat is much smaller. And the thermal effect of the neutralisation of the oxides of the heavy metals and of the sesquioxides of the aluminium group is still smaller (in this connection we may recall the feeble stability of the salts formed from these sesqui- oxides).

Concerning the action of a weak acid on a weak base

An equivalent of hydrochloric acid is equal to one gram-molecule.

That quantity of a base or of an acid which is capable of saturating or replacing one gram-molecule of hydrochloric acid is its equivalent.

The following are equivalents :

1 gram-molecule of hydrochloric

1 ,, ,, caustic soda

1/2 ,, ,, hme

1/2 ,, ,,, sulphuric acid

and so on.

We even speak of an equivalent of a salt : NaCl, AgNO3, 1/2Na2SO4 1/3Na?PO?, &c.