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Heats of reaction always refer to the substances taking part in the reaction in the actual states in which they are used. The heat of combustion of carbon in the form of diamond in gaseous oxygen to form carbon dioxide is somewhat different from the heat of combustion of graphite, since these two forms of solid carbon contain different amounts of energy whereas the same final product is obtained in each case. The heat of combustion of methane, CH4, is not equal to the sum of the_ heat of combustion of solid carbon and two molecules of gaseous hydrogen, since in the formation of methane from these substances the carbon has been converted into a gas and the hydrogen molecules have been resolved into atoms of hydrogen. If we knew the heat of evaporation of solid carbon, the heat of dissociation of molecular hydrogen into atoms of hydrogen, and the heat of formation of methane from solid carbon and hydrogen gas, we could calculate the heat of formation of gaseous methane from gaseous carbon atoms and gaseous hydrogen atoms. In a similar way, it would be possible to calculate heats of formation of gaseous compounds from the gaseous elements in the atomic state if data for heats of evaporation of the ordinary solid substances, and of dissociation of molecules into atoms, were known.
It is possible to calculate, mainly from spectroscopic data, several such heats of dissociation into normal atoms, which are given in the following table in k, cal. (absorbed):
| H2 | 100.5 | NO = N + O | 121.4 | HF | 140 | | Cl2 | 57 | S2 | 102 | | HCl101.6 | | Br2 | 45.2 | HD | 103.5 | HBr | 86 | | I2 | 35.4 | D2 | 104.5 | HI | 66 | | O2 | 116.4 | CO2 = CO + O | 125.8 | H2) = H + OH | 118 | | N2 | 170 | N2O = NO + N | 88.5 | | |
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