Standard Enthalpy of Formation Definition

Steps:

• Define standard enthalpy of formation (ΔH_f°) as the enthalpy change for forming 1 mol of a compound from its elements in standard states at 298 K and 1 atm.
• Identify water's formula as H₂O(l) in standard state (liquid).
• Confirm elements: H₂(g) and O₂(g).
• Select equation producing exactly 1 mol H₂O(l) from these elements.

Why D is correct:

• It forms 1 mol H₂O(l) from H₂(g) and ½O₂(g), matching the ΔH_f° definition.

Why the others are wrong:

• A: Forms 2 mol H₂O(l), so ΔH = 2ΔH_f°.
• B: Forms H₂O(l) from ions, not elements; this is neutralization enthalpy.
• C: Forms 2 mol H₂O(g), not liquid standard state.

Final answer: D