Hess's law states that the total enthalpy change for a reaction is the sum of all changes, regardless of the number of steps or stages of the reaction. This allows scientists to calculate the enthalpy of various reactions using measured enthalpies of reaction. Various databases listing various enthalpies exist. Enthalpies can be calculated for both chemical and physical changes. For example, in combustion reactions a substance reacts with oxygen. During vaporization a substance changes phase from liquid to gas form. During fusion a substance changes phase from a solid to liquid.
Enthalpy of formation () is the enthalpy of the reaction in which a compound is formed from its elements. Enthalpies of formation are often shown with all reactants and products in standard state, a reference point used to calculate a substance's properties. In this case, the enthalpy of formation is called the standard enthalpy of formation and is shown as .
Standard Enthalpies of Formation for Common Substances
|Substance||Formula and Standard State||Standard Enthalpy of Formation () at 25°C (kJ/mol)|
In determining the enthalpies of formation, keep in mind that some elements in a compound can appear in more than one form. For example, oxygen is stable as a diatomic molecule (O2) or ozone (O3). Single oxygen atoms can also exist for short periods of time. When enthalpy of formation is determined, diatomic oxygen (O2) is used in the calculation because it is more stable than ozone at 1 atm pressure and 25ºC. Similarly, carbon can occur as graphite or diamond under these conditions. Graphite is more common and is used in the calculation.
It is nearly impossible to measure absolute enthalpies. All enthalpy values are changes in enthalpy. This means enthalpies of formation are calculated in terms of the enthalpies of formation of the elements, such as diatomic oxygen (O2). The enthalpy of formation of the most stable form of an element at 1 atm pressure and 25°C is reported as 0. Diatomic oxygen (O2), hydrogen (H2), nitrogen (N2), and graphite all have an enthalpy of formation of 0. Other forms of these elements do have enthalpies of formation. For example, ozone has an of 142.7 kJ/mol. Diamond has an enthalpy of formation of 1.88 kJ/mol.
Enthalpies of formation can be used to calculate the enthalpies of more complex reactions.