Calculate ΔH in kJ/mol for the reaction N2O5(g) → N2O4(g) + 1/2 O2(g) using these reactions:

2 N2O5(g) → 2 N2(g) + 5 O2(g) ΔH = -38.7 kJ/mol
1/2 N2(g) + O2(g) → NO2(g) ΔH = 32.7 kJ/mol
N2O4(g) → 2 NO2(g) ΔH = 51.8 kJ/mol