Why is the heat of neutralisation for strong acid–strong base reactions nearly constant at about 57 kJ per kg-mole of water formed?

Introduction / Context:
Calorimetry of acid–base reactions reveals a near-constant heat release when a strong acid neutralises a strong base. Understanding the ionic basis of this constancy aids in thermochemistry calculations and in interpreting departures from the ideal value for weak acids/bases.

Given Data / Assumptions:

• Strong acids and bases are fully dissociated in aqueous solution.
• Observed standard heat of neutralisation ≈ −57 kJ per kg-mole of water formed at dilute conditions.
• Solution behaves ideally; temperature changes are modest.

Concept / Approach:
The net ionic reaction for strong acid–strong base neutralisation is simply H+ (aq) + OH− (aq) → H2O (l). Since the reacting species are the same regardless of the specific acid/base identities, the enthalpy change is essentially the enthalpy of water formation from these ions, giving a nearly constant value.

Step-by-Step Solution:

Verification / Alternative check:
For weak acids/bases, the measured heat deviates because additional enthalpy is consumed or released in ionisation steps, confirming that the constancy hinges on full dissociation and the H+/OH− reaction only.

Why Other Options Are Wrong:

• (a) Completion is true but not the fundamental reason for identical ΔH.
• (b) Salt hydrolysis can be negligible, but this is not the root cause.
• (d) Aqueous medium is common, yet the key is which ions actually react.
• (e) Ionic strength varies with concentration; it does not fix ΔH at one value.

Common Pitfalls:
Confusing extent of reaction with identity of reacting species, or attributing constancy to specific salts rather than to the universal H+/OH− neutralisation step.

Final Answer:
Because in essence only H+ and OH− ions react to form water