danam04
contestada

The enthalpy change for the reaction, 2CO (g) + O2 (g) 2 CO2 (g), can be calculated from ΔHf values for each reactant and product. Which substance or substances in this equation have a ΔHf value equal to 0.0 kilojoules?
O2 (g)
CO (g)
O2 (g) and CO (g)
CO2 (g)