Which one of the following reactions is not a standard enthalpy of formation
reaction?
1) CaO(s) + CO₂(g) = CaCO3(s)
O2) S(s) + O₂(g) = SO₂(g)
3) S(s) + 3/2 O₂(g) = SO3(g)
4) C(s) + O₂(g) = CO(g)
2