Reaction Represents Formation of Na₂O from Elements

Steps:

• Reactants are Na(s) and O₂(g), the standard states of sodium and oxygen.
• Product is 1 mole of Na₂O(s).
• Standard enthalpy of formation (ΔH_f°) is the ΔH when 1 mole of a compound forms from elements in standard states.
• This equation matches the definition of ΔH_f° for Na₂O.

Why D is correct:

• ΔH_f° is defined as the enthalpy change for forming 1 mole of a compound from its elements in standard states at 298 K and 1 atm, exactly as shown.

Why the others are wrong:

• A: ΔH accounts for breaking O=O bond and forming ionic lattice in Na₂O, not just one Na-O bond energy.
• B: ΔH involves multiple energy terms (O=O breaking, lattice formation), not simply twice a single bond energy.
• C: ΔH_c° for sodium is per mole of Na (Na + ½O₂ → ½Na₂O), not for 1 mole of Na₂O formation.

Final answer: D