Answer: CO2→CO + 12O2 is not a balanced reaction, I think you meant to post " CO2→CO + 1/2O2" The simple answer is that the enthalphy for reaction 1 reversed is the same enthalpy as for the reaction as written but with the opposite sign. If A + B = AB with 30 kJ released then AB = A + B with 30 kJ absorbed. We would write it as A + B = AB heat of reaction = -30 kJ AB = A + B heat of reaction = +30 kJ