Page:Outlines of Physical Chemistry - 1899.djvu/171

 HEAT OF FORMATION (( U,\|'\T£§^ \

8. It is necessary that the thermal influen\5$£^; all energy external to the system be as far as possible el nated.

When the combination of two gases under a pressure of 760 millimetres of mercury takes place with contraction, for each molecular volume which disappears there is an evolution of (0*54 + 0*002£) Cal., due to this contraction. We have seen that one gram-molecule of any gas at normal temperature and pressure occupies a volume of 22,880 cubic centimetres. A contraction of this size corre- sponds to a pressure of 1,088 grams, exercised by the weight of the atmosphere along a path of 22,880 centimetres, centimetre-grams, which is equivalent to 0*54 Gal. This quantity of heat, due to an exterior cause, must be deducted from the total thermal effect produced by the reaction.

For instance, in determining the heat of combustion of hydrogen the result differs considerably according as the operation is carried out under constant volume or under constant pressure, that is, in a closed apparatus, or in one the interior of which is in communication with the atmosphere. The heat evolved in the first case is less than that evolved in the second, and the difference between the two is one and a half times 0*54 Cal. (when the calculation is made for the temperature 0°). In the reaction

H 2 + = H 2 (liquid)

there is a contraction of one and a half gaseous molecular volumes.

Theory here has been experimentally confirmed by the work of Than and Thomsen.

4. The reaction H 2 + Cl 2 = 2HC1 takes place without any change of volume in the gaseous mass. The calori- metric effect would appear then to be due only to the double decomposition. This is, however, not the case, because the molecular specific heat of hydrochloric acid is lower, by at least 12 per cent., than the mean caloric

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