The heats of reaction also add in the same way. In general, if we can add or subtract two reactions to obtain a third, then the heat of this third reaction is obtained by adding or subtracting the heats of the first two. For those who particularly enjoy filing things away by name, this is Hess' law of heat summation. It actually is a natural consequence of the first law of thermodynamics.

(In the example above, the reaction of hydrogen and oxygen gases to produce water vapor, followed by condensation of the vapor in a second step, must lead to exactly the same enthalpy change as the reaction of hydrogen and oxygen gases to produce liquid water directly. Hence the heats of the first two reactions must add to yield the heat of the direct process.)

We must be careful that the two reactions added really do yield the third, and that the heats used are for the reactions as they are written. For example, we could multiply the hydrogen combustion reaction by two to eliminate the fractional coefficient before the oxygen, but then the heat of reaction also would have to be doubled:

2H₂(g) + 0₂(g) � 2H₂0(g) DH=-116 kcal