The standard enthalpy of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of that substance from its component elements, at their standard states (the most stable form of the element at 25 degrees Celsius and 100 kilopascals). Its symbol is ΔHfθ.
All elements in their standard states (oxygen gas, graphite, etc.) have 0 standard enthalpy of formation, as there is no change involved.
The standard enthalpy of formation is equivalent to the sum of many separate processes included in the Born-Haber cycle of synthesis reactions. For example, to calculate the standard enthalpy of formation of sodium chloride, we use the following reaction:
- Na(s) + (1/2)Cl2(g) → NaCl(s)
This process is made of many separate sub-processes, each with their own enthalpies. Therefore, we must take into account:
- The standard enthalpy of atomization of solid sodium
- The first ionization energy of gaseous sodium
- The standard enthalpy of atomization of chlorine gas
- The electron affinity of chlorine atoms
- The lattice enthalpy of sodium chloride
The sum of all these values will give the standard enthalpy of formation of sodium chloride. Additionally, Hess's Law states that all reactions from A to B will have an identical standard enthalpy of formation, disregarding the number of steps or intermediate reactions involved.