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)

Oxygen (O₂) will have ΔHf equal to 0.0 kJ.

Explanation:

The enthalpy of formation of elements in their native state is considered to be zero. Oxygen in nature occurs as O₂, so its enthalpy of formation will be zero.

the other given species are not elements, they are compounds.

CO and CO₂ both are compounds so they cannot have enthalpy of formation equal to zero.