Determination of the heat of formation of CO2 and CO and Hess's law

Principle

Molar standard enthalpies of formation are important thermodynamic tabular values used to calculate the standard enthalpies of reaction for reactions of all kinds. They are defined as the heat of reaction during the direct formation of one mol of the substance concerned from the stable pure elements at constant pressure. For reactions which proceed spontaneously, such as the formation of carbon dioxide from carbon, the enthalpies of formation can be determined directly by caloric measurements. If enthalpies of formation are not directly accessible in this way, as in the case of carbon monoxide, for example, they can be determined by means of the Hess's law.
To do so in the example shown here, the enthalpies of reaction for the combustion of two substances:

1. Carbon
2. Carbon monoxide

to form carbon dioxide are determined experimentally. These experimental values can be used to calculate the enthalpy of formation of carbon monoxide. 

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