Reaction Matches Standard Enthalpy of Formation

Steps:

• Define standard enthalpy of formation (ΔH_f°): enthalpy change when 1 mol of compound forms from elements in standard states under standard conditions.
• Examine equation: 2Na(s) + O₂(g) → Na₂O(s) shows Na(s) and O₂(g) as standard states of elements producing 1 mol Na₂O(s).
• Confirm stoichiometry aligns with forming 1 mol Na₂O, representing ΔH_f° for Na₂O.
• Eliminate other options by checking definitions of bond energy and combustion enthalpy.

Why D is correct:

• By definition, ΔH_f° is the enthalpy change for the reaction forming 1 mol of compound from elements in standard states, exactly as shown.

Why the others are wrong:

• A: ΔH includes energies for all bonds broken and formed (Na-Na, O=O, two Na-O), not just one Na-O bond.
• B: ΔH accounts for multiple bonds and reactants, not simply twice one Na-O bond energy.
• C: Standard enthalpy of combustion applies to burning 1 mol of substance in oxygen to stable products; this equation defines formation, not combustion of sodium.

Final answer: D