For the neutralization reaction between a strong acid and a strong base, the enthalpy value is -57.1 kJ.
The enthalpy of neutralization is the heat energy released or absorbed when one mole of water is formed from the reaction of an acid and a base. It is a specific type of reaction enthalpy.
Constant Enthalpy for Strong Acid-Strong Base Neutralization
A key point regarding neutralization is that for the reaction between any strong acid (like HCl, HNO₃, H₂SO₄) and any strong base (like NaOH, KOH, Ca(OH)₂) in dilute aqueous solutions, the enthalpy of neutralization is remarkably consistent. This is because strong acids and strong bases are completely dissociated into ions in water.
The essential reaction occurring is the combination of hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base to form water (H₂O):
H⁺(aq) + OH⁻(aq) → H₂O(l)
Since the spectator ions (like Na⁺, Cl⁻, K⁺, SO₄²⁻) do not participate in this core reaction and the enthalpy change is primarily associated with the formation of the water molecule, the value remains nearly constant regardless of which strong acid and strong base are used.
As stated in the reference: "The enthalpy of neutralization of all strong acids and strong bases is always constant i.e -57.1 kJ."
This negative value indicates that the reaction is exothermic, meaning heat is released into the surroundings during the neutralization process.
Variations for Weak Acids or Weak Bases
It is important to note that this constant value of -57.1 kJ applies specifically to the neutralization of strong acids and strong bases.
When a weak acid or a weak base is involved in the neutralization reaction, the enthalpy of neutralization will be different. This is because energy is required to ionize the weak acid or base (break their molecules into ions), and this ionization process is often endothermic or less exothermic than the formation of water itself, affecting the overall enthalpy change. Therefore, the enthalpy of neutralization involving weak acids or bases will typically be less negative (or even positive in some cases) than -57.1 kJ.
Understanding this specific value helps in predicting the heat changes involved in strong acid-strong base reactions, which is fundamental in fields like thermochemistry and quantitative analysis.
