142 OUTLINES OF PHYSICAL CHEMISTRY
stirrer. The caloric capacity of the reacting materials is equal to their weight multiplied by their specific heat. A rigorously exact calculation necessitates therefore a know- ledge of these specific heats. In practice, satisfactory results can be obtained (for reactions which take place in very dilute solutions, 1 equivalent of acid or base in 100 or 200 molecules of water), if we assume, as J. Thomsen does, that the caloric capacity of a dilute aqueous solution is equal to that of the water which it contains, or if we accept, as Berthelot does, that 1 cubic centimetre of each of the liquids is oalorimetrically equal to 1 gram of water.
Example. — Determination of a Heat of Neutralisa- tion. Thomsen has studied the action of NaOH (in 100 H 2 0) on HG1 (in 100 H 2 0) using a quarter of these quantities.
The solution in the metal reservoir was at the tempera-
(1800\ —j— J.
The solution in the calorimeter was at 18*610°.
Its water equivalent was likewise 450 grams.
The water equivalent of the instrument must be added ; in this case 18 grams.
After mixing the two solutions the temperature rose to 22*169°. The evolution of heat was, therefore,
q = 450(22*169 - 18*222) + (450 + 18) (22*169 -18*610)
= 1776 + 1648 = 8424 small calories.
We have now to multiply this number by 4, since only the fourth part of the molecular weights in grams were used.
Therefore, (NaOH aq, HC1 aq) = 18*7 0. (large calories).
The example cited is the simplest case which can be met with. The temperature equilibrium (after the reaction) is reached after a very short time, and the thermometer readings can be used directly.
But of tener this equilibrium is only established alter several minutes, and then we are not able to observe on
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