Question

Why is ΔH from the neutralization of HCl with NaOH is greater than that evolved for the neutralization of CH3COOH with NaOH?

Answer 1

NaOH (aq) + HCl(aq) NaCl (aq) + H₂O (l) ; ΔH = -57.1 kJ/mole

When strong acid & strong base completly neutralise releases 57.1 kJ of heat Where as if any of acid or base is weak like neutralization of CH₃COOH with NaOH the heat released is less than 57.1 kJ , this is because strong acids ,strong bases and salt that they form are all completely ionized in dilute aqueous solutions.

There can dissociates as
Na⁺ (aq) + OH^- (aq) + H⁺ (aq) + Cl^-(aq) Na⁺ (aq) + Cl^- (aq) + H₂O (l)
If common ions will cancel then we get
H⁺ (aq) + OH^- (aq) H₂O (l)

Thus neutralization is simply a reaction between H⁺ ions given by acids and OH^- ions given by base to form one mole of H₂O.Since strong acid and strong base completely ionize in aqueous solution number of H⁺ and OH^- produced by 1 gram equivalent of strong acid and strong base is always the same so the enthalpy of neutralization is same.

If either the acid or base or both are weak the enthalpy of neutralization is less than 57.1 kj The reason for this behavious can be explained by considering the neutralization between a strong base like NaOH and weak acid like acetic acid.
now acetic acid ionizes to a small extent whereas NaOH ionizes completely as :
NaOH (aq) Na⁺(aq) + OH^- (aq) ( Completly ionize)
CH₃COOH (aq)CH₃COO-(aq) + H⁺(aq)         (an equilibrium)
when H⁺ given by acid combine with OH^- given by base the equilibrium shifts to right so more of acetic acid dissociates some part of heat produced during combination of H⁺ and OH^- ions is used up for complete dissociation of acetic acid (is called enthalpy of dissociation) hence net heat evolved in above reaction is less than 57.1which less than that evolved in strong base and strong acid reaction.