Standard Enthalpy of Neutralisation Defined by Water Formation

Steps:

• Recall that neutralisation involves H⁺(aq) + OH⁻(aq) → H₂O(l), the core reaction.
• Identify the standard condition: ΔH° measures energy change under standard states (298 K, 1 atm).
• Note the defining quantity: it's the enthalpy for forming exactly one mole of water.
• Compare options to this: only the one specifying one mole of water matches the precise definition.

Why D is correct:

• The definition states ΔH° as the enthalpy change when an acid and alkali react to form one mole of water, as this captures the ionic reaction's energy release (typically -57 kJ/mol for strong acid-base pairs).

Why the others are wrong:

• A: Omits specifying one mole of water; focuses only on acid without quantifying product.
• B: Similar to A, but swaps acid-alkali order; still lacks water mole specification.
• C: Specifies one mole each of acid and alkali, but not the one mole of water produced.

Final answer: D