Page:Outlines of Physical Chemistry - 1899.djvu/181

 second. In this case the addition of an excess of soda does not give rise to a supplementary evolution of beat.

On the contrary the addition of dilute acid to a dilute solution of the neutral sulphate causes an absorption of heat, which Thomsen expresses by the equation :

£[Na 2 S0 4 aq, wH 2 S0 4 aq] = %— 1*65 Cal.

Therefore when n = 1, i[Na 2 S0 4 aq, H 2 S0 4 aq] == - 0*92 Cal.

We here meet with an endothermic reaction to which we shall refer later.

Organic Substances

The application of thermo- chemistry to the study of organic compounds constitutes a problem the solution of which obtrudes as much on the industrial as on the physiological point of view. In the industries fuel is burned to give motion to the machinery, and animal life has its only source of energy in the oxidation phenomena which take place in the various tissues. Unfortunately, organic compounds, as a rule, cannot be synthesised by rapid and complete reactions which lend themselves to calorimetric examination. We know only one method of investigation — an indirect method based on the principle of the initial and final states.

A complete combustion of the organic material is carried out in the calorimeter, and the heat evolved corre- sponds to a final state, represented by a certain number of molecules of liquid water and of gaseous carbon dioxide. (In some cases nitrogen, nitric acid, sulphurous acid, sulphuric acid [more or less hydrated], phosphoric acid, &c., may be formed.) Now we can attain the same final stale by burning the requisite quantity of carbon (diamond) and of free hydrogen (taking into account also the requisite amount, if any, of nitrogen, sulphur, phosphorus, &c.).

But different heats will be evolved in these two operations.