The standard enthalpy change, ΔHo, of a reaction is defined as the enthalpy change when all reactants and products are in their standard states.
The standard enthalpy of formation of a compound, ΔHof, is the change in enthalpy for the reaction that forms one mole of the compound from its elements, with all substances in their standard states. This information is found in charts like this.
By definition, the standard enthalpy of formation of the most stable form of any element is zero, because there is no formation reaction needed when the element is already in its standard state.
Any reaction can be broken down into formation reactions:
ΔHorxn = ∑ n ΔHof (products) − ∑ m ΔHof (reactants)
The equation above states:
Note: (n and m are the coefficients from the balanced chemical equation)
- The standard enthalpy change of a reaction (ΔHorxn)
- is the (=)
- sum of (∑)
- the standard enthalpies of formation of the products (n ΔHof (products))
- minus (−)
- the sum of (∑)
- the standard enthalpies of formation of the reactants (m ΔHof (reactants)).