CHEM 101 - Using Hess's Law to Calculate Enthalpy Change 

In this example, we use Hess’s law to calculate the change in enthalpy for a reaction.
Find ΔHrxn for the reaction in kJ:
3 C(s) + 4 H2(g) → C3H8(g)
Given:
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) ΔH1 = -2043 kJ
C(s) + O2(g) → CO2(g) ΔH2 = -393.5 kJ
2 H2(g) + O2(g) → 2 H2O(g) ΔH3 = -483.6 kJ