Hess law and thermochemical calculations

You probably know that two or more chemical equations can be combined algebraically to give a new equation. Even before the science of thermodynamics developed in the late nineteenth century, it was observed by Germaine Hess (1802-1850) that the heats associated with chemical reactions can be combined in the same way to yield the heat of another reaction. For example, the standard enthalpy changes for the oxidation of graphite and diamond can be combined to obtain AH° for the transformation between these two forms of solid carbon, a reaction that cannot be studied experimentally. 

Subtraction of the second reaction from the first (i.e., writing the second equation in reverse and adding it to the first one) yields 

This principle, known as Hess law of independent heat summation is a direct consequence of the enthalpy being a state function. Hess law is one of the most powerful tools of chemistry, for it allows the change in the enthalpy (and in other thermodynamic functions) of huge numbers of chemical reactions to be predicted from a relatively small base of experimental data. 

Because most substances cannot be prepared directly from their elements, heats of formation of compounds are seldom determined by direct measurement. Instead, Hess law is employed to calculate enthalpies of formation from more accessible data. The most important of these are the standard enthalpies of combustion. Most elements and compounds combine with oxygen, and many of these oxidations are highly exothermic, making the measurement of their heats relatively easy. 

For example, by combining the heats of combustion of carbon, hydrogen, and methane, we obtain the standard enthalpy of formation of methane, which as we noted above, cannot be determined directly  

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