heat when it is decomposed, absorbing it when it is produced.
Unfortunately there is still some doubt about the heat of formation of
calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as
+3.9 calories. De Forcrand's figure means, as before explained, that 64
grammes of carbide should absorb 0.65 large calorie when they are
produced by the combination of 40 grammes of calcium with 24 grammes of
carbon; the minus sign calling attention to the belief that calcium
carbide is endothermic, heat being liberated when it suffers
decomposition. On the contrary, Gin's figure expresses the idea that
calcium carbide is exothermic, liberating 3.9 calories when it is
produced, and absorbing them when it is decomposed. In the absence of
corroborative evidence one way or the other, Gin's determination will be
accepted for the ensuing calculation. In equation (2), therefore, calcium
carbide is decomposed and absorbs heat; water is decomposed and absorbs
heat; acetylene is produced and absorbs heat; and calcium hydroxide is
produced liberating heat. On consulting the tables of thermo-chemical
data given in the various text-books on physical chemistry, all the other
constants needed for the present purpose will be found; and it will
appear that the heat of formation of water is +69 calories, that of
acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of
calcium, 1 atom of oxygen, and 1 molecule of water unite together, is
+160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen,
and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat
of formation of the latter is 229.1 (cf. _infra_). This value is
simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.]
Collecting the results into the form of a balance-sheet, the effect of
decomposing calcium carbide with water is this:

_Heat liberated._ | _Heat absorbed._
|