168 OUTLINES OF PHYSICAL CHEMISTRY
The number thus obtained can be used directly because the reaction
is not accompanied by any change of volume and produces the same thermal effect under constant volume or under constant pressure.
But it must be borne in mind that in the calorimetric bomb we work at constant volume, and it is generally necessary to introduce a correction in order to arrive at the number which would be obtained if the combustion had been carried out under constant pressure. If we denote by Q tv the heat evolved in the bomb by one gram-molecule of substance, by n the number of molecular volumes occupied by the gaseous matter before the combustion, and by n' the number of molecular volumes occupied by the gaseous pro- ducts of the reaction, then the heat which would be evolved under constant pressure can be found from the equation,
Q ip = Qtv + (n - n') (0-54 + 0-002*).
t denotes the mean temperature of the calorimetric
The combustion of naphthalene takes place according to the equation C 10 H 8 + 120 2 = 10CO 2 + 4H 2 0; it is accompanied by a condensation of two molecular volumes and at 15° requires as correction an addition of 1*14 Cal.
The correction may be negative, as is the case with formic acid ; CH 2 2 + = C0 2 + H 2 0.
Here (n — n') = — ^, and at 15° it is necessary to subtract 0-28 Cal.
��1 The volume occupied by solids or liquids is negligible, and does not enter into the calculation. To understand the equation the reader should refer to the explanations on page 153.
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