Each known element or compound has an associated enthalpy of formation value, designated as ฮHf. This is the enthalpy change for the reaction that forms exactly one mole of the compound from its constituent elements in their natural states.
For example, the enthalpy of formation for solid magnesium carbonate is the enthalpy of the reaction:
Notice that the reactants are all elements and they are in the states they exist naturally — oxygen is O2 molecules, not O atoms. Also notice that it is acceptable to use non-integer stoichiometric coefficients.
The enthalpy change for any reaction can be calculated using ฮHf values by adding the values for the products and subtracting that of the reactants:
Three levels on recognizing and calculating enthalpies of formation. Pick a level to begin.