AUT's intrepid chemist, Prof Al, talks about enthalpy, a really important thermodynamic function, and how Hess's Law can be used to calculate an enthalpy of reaction. The worked problem: Determine the enthalpy change for the reaction 3C(s) + 4H₂(g) → C₃H₈(g) Given the following thermochemical equations C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l) ΔᵣH = -2220 kJ mol⁻¹ H₂(g) + 1/2O₂(g) → H₂O(l) ΔᵣH = -285.8 kJ mol⁻¹ CO₂(g) → C(s) + O₂(g) ΔᵣH = 393.5 kJ mol⁻¹