Hess's law states that the enthalpy change for a chemical reaction is independent of the route taken. The enthalpy change for a reaction can be calculated using Hess's law, given appropriate data.
From previous work you should know and understand the following:
ΔH = Hproducts - Hreactants
Carbon can burn to produce carbon dioxide directly by route X or it can form the intermediate CO by route Y, then the CO can burn to produce CO2 by route Z.
Previously we have used experimental data to work out ΔH combustion.
To do this we have carried out the reaction.
However sometimes we want to work out the enthalpy change for a reaction which cannot be measured directly because it does not usually occur e.g.
C(s) + 2H2(g) → CH4(g)
We can work out the enthalpy change for this reaction using Hess's law from the enthalpy of combustion of C(s), H2(g) and CH4(g)
Because the reaction is for forming one mole of compound from its elements in their standard state this is called enthalpy of formation.