R3.1.8 Neutralisation & Salt Hydrolysis - IB Chemistry

📘 IB Understanding

Neutralisation is an exothermic reaction between an acid and a base producing a salt and water. The pH of the resulting salt solution depends on the strength of the parent acid and base.

Neutralisation Reactions

The general equation for neutralisation:

Acid + Base → Salt + Water

The net ionic equation (for strong acid + strong base):

H⁺(aq) + OH^-(aq) → H₂O(l)

Common Salt Examples

HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
HNO₃ + NH₃ → NH₄NO₃
2CH₃COOH + MgO → (CH₃COO)₂Mg + H₂O

Salt Hydrolysis

When a salt dissolves, its ions may react with water (hydrolyse). The pH of the solution depends on the parent acid and base:

Salt Type	Made From	Solution pH	Example
Neutral	SA + SB	= 7	NaCl, KNO₃
Basic	WA + SB	> 7	CH₃COONa, Na₂CO₃
Acidic	SA + WB	< 7	NH₄Cl, FeCl₃

Why Do Some Salt Solutions Have a Non-Neutral pH?

Acidic Hydrolysis Example

NH₄Cl dissolves to give NH₄⁺ and Cl^-. The NH₄⁺ acts as a weak acid:

NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)

This produces H₃O⁺, making the solution acidic.

Basic Hydrolysis Example

CH₃COONa dissolves to give CH₃COO^- and Na⁺. The ethanoate ion acts as a weak base:

CH₃COO^-(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH^-(aq)

This produces OH^-, making the solution basic.

Worked Example

Q: Predict whether Na₂CO₃(aq) is acidic, basic, or neutral.

A: Na₂CO₃ dissociates into Na⁺ and CO₃^2-. Na⁺ comes from a strong base (NaOH), so it does not hydrolyse. CO₃^2- is the conjugate base of the weak acid HCO₃^-, so it hydrolyses water:

CO₃^2-(aq) + H₂O(l) ⇌ HCO₃^-(aq) + OH^-(aq)

The solution is basic (pH > 7).

⚠️ Exam Tip

When naming salts: the cation comes from the base, the anion from the acid. HCl forms chlorides, H₂SO₄ forms sulfates, HNO₃ forms nitrates.

Neutralisation & Salt Hydrolysis