Identifying reaction enthalpies from standard definitions

Steps:

• Examine first reaction: H₂(g) + ½O₂(g) → H₂O(g) forms water from elements in standard states.
• Examine second reaction: CO(g) + ½O₂(g) → CO₂(g) burns one mole of CO completely in oxygen.
• Match first to standard enthalpy of formation (ΔH_f°), energy change for forming one mole of compound from elements.
• Match second to standard enthalpy of combustion (ΔH_c°), energy released when one mole of substance combusts fully.

Why B is correct:

• B aligns with definitions: first reaction is exactly ΔH_f°(H₂O,g); second is ΔH_c°(CO,g) per Hess's law for standard states.

Why the others are wrong:

• A: Atomisation enthalpies convert elements to gaseous atoms (e.g., H₂ → 2H), not forming molecular compounds.
• C: Bond energies are averages for breaking specific bonds in isolation, unrelated to these overall reaction enthalpies.
• D: Identical to C, so equally mismatched.

Final answer: B