The combination of reaction enthalpies

From the enthalpy of vaporization (AvapH = 44 kj mol at 298 K), molar mass (M = 18 g mol ), and mass density (p = 1.0 g cm , corresponding to 1.0 X 10 g dm ) of water, the energy removed as heat through evaporation per liter (cubic decimeter) of water perspired is 

When vigorous exercise promotes sweating (through the influence of heat selectors on the hypothalamus), 1 to 2 dm of perspired water can be produced per hour, corresponding to a loss of energy of approximately 2.4 to 4.8 MJ h . 

To make progress in our study of bioenergetics, we need to deveiop methods for predicting the reaction enthaipies of compiex biochemicai reactions. 

It is often the case that a reaction enthalpy is needed but is not available in tables of data. Now the fact that enthalpy is a state function comes in handy, because it implies that we can construct the required reaction enthalpy from the reaction enthalpies of known reactions. We have already seen a primitive example when we calculated the enthalpy of sublimation from the sum of the enthalpies of fusion and vaporization. The only difference is that we now apply the technique to a sequence of chemical reactions. The procedure is summarized by Hess s law, which in its modern form is  

The standard enthalpy of a reaction is the sum of the standard enthalpies of the reactions into which the overall reaction maybe divided. 

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