We can use Hess’s law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethylene:

2CH₄(g)→C₂H₄(g) + H₂(g)

Calculate the ΔH° for this reaction using the following thermochemical data:

CH₄(g) + 2O₂(g)→CO₂(g) + 2H₂O(l) ΔH° = -890.3 kJ

C₂H₄(g) + H₂(g)→C₂H₆(g) ΔH° = -136.3 kJ

2H₂(g) + O₂(g)→2H₂O(g) ΔH° = -571.6 kJ

2C₂H₆(g) + 7O₂(g)→4CO₂(g) + 6 H2O1l2 ΔH° = -3120.8 kJ