Salts can behave as acids or bases in water

A salt is an ionic compound formed when an acid reacts with a base. When dissolved in water, the ions from the salt can interact with water molecules in a process called hydrolysis.

In a hydrolysis reaction, the salt ion reacts with water, forming the conjugate acid or base of the ion. The direction and extent of hydrolysis depends on the strengths of the parent acid and base that formed the salt.

Types of salt solution

Salt solutions are classified into four main types based on the possible combinations of strong and weak acids and bases:

1. Salts of strong acids and strong bases - Examples include NaCl and KNO_3_.
2. Salts of strong acids and weak bases - Examples include NH_4_Cl and NH_4_NO_3_.
3. Salts of weak acids and strong bases - Examples include CH_3_COONa and NaHCO_3_.
4. Salts of weak acids and weak bases - Examples include CH_3_COONH_4_ and (NH_4_)_2_CO_3_.

The type of salt formed affects whether hydrolysis will occur and consequently influences the pH of the resulting solution. Strong acids and bases do not hydrolyse, while weak acids and bases do.

Strong acid-strong base salts - no hydrolysis

Salts formed from strong acids and strong bases, like sodium chloride, dissociate completely in water:

NaCl_(aq) ➔ Na⁺_(aq) + Cl^-_(aq)

Neither the sodium cation (Na⁺) nor the chloride anion (Cl^-) react with water. Their hypothetical reactions would produce the strong base sodium hydroxide (NaOH) and the strong acid hydrochloric acid (HCl), respectively, which do not exist in aqueous solutions in undissociated forms.

Na⁺_(aq) + H₂O_(l) ➔ (no reaction)

Cl^-_(aq) + H₂O_(l) ➔ (no reaction)

The only equilibrium in these solutions is the self-ionisation of water:

H₂O_(l) ⇌ H⁺_(aq) + OH^-_(aq)

Equal amounts of H⁺ and OH^- are produced, so the concentration of H⁺ equals the concentration of OH^- and the solution remains neutral (pH = 7).

Strong acid-weak base salts - cation hydrolysis

In salts like ammonium chloride (NH₄Cl), only the cation undergoes hydrolysis:

NH₄Cl_(aq) ➔ NH₄⁺_(aq) + Cl^-_(aq)

The ammonium ion (NH₄⁺) is a weak acid, as its conjugate base ammonia (NH₃) is weak. It reacts with water in a reversible process:

NH₄⁺_(aq) + H₂O_(l) ⇌ NH_3(aq) + H₃O⁺_(aq)

or

NH₄⁺_(aq) ⇌ NH_3(aq) + H⁺_(aq)

This hydrolysis produces excess hydronium ions (H₃O⁺) or hydrogen ions (H⁺), making [H⁺] > [OH^-]. The solution becomes acidic (pH < 7).

Weak acid-strong base salts - anion hydrolysis

In salts of weak acids and strong bases, like sodium ethanoate, only the anions undergo hydrolysis:

CH₃COONa_(aq) ➔ CH₃COO^-_(aq) + Na⁺_(aq)

The ethanoate ion has a weak conjugate acid (ethanoic acid) so acts as a weak base, accepting protons from water:

CH₃COO^-_(aq) + H₂O_(l) ⇌ CH₃COOH_(aq) + OH^-_(aq)

This generates an excess of hydroxide ions, so [OH^-] > [H⁺] and the solution becomes alkaline with a pH greater than 7.

For salts containing anions of polyprotic weak acids, like carbonate (CO₃^2-), the anion undergoes stepwise hydrolysis, accepting one proton at a time:

CO₃^2-_(aq) + H₂O_(l) ⇌ HCO₃^-_(aq) + OH^-_(aq)

HCO₃^-_(aq) + H₂O_(l) ⇌ H₂CO_3(aq) + OH^-_(aq)

Weak acid-weak base salts - both ions hydrolyse

Salts derived from weak acids and weak bases, such as ammonium ethanoate, have both cations and anions that can hydrolyse:

CH₃COONH_4(aq) ➔ CH₃COO^-_(aq) + NH₄⁺_(aq)

The ethanoate ion acts as a weak base while the ammonium ion acts as a weak acid:

CH₃COO^-_(aq) + H₂O_(l) ⇌ CH₃COOH_(aq) + OH^-_(aq)

NH₄⁺_(aq) + H₂O_(l) ⇌ NH_3(aq) + H₃O⁺_(aq)

or

NH₄⁺_(aq) ⇌ NH_3(aq) + H⁺_(aq)

Both H⁺ and OH^- ions are produced. The solution's pH depends on the relative strengths of the parent acid and base:

• If the anion's conjugate acid is slightly stronger than the cation's conjugate base, the solution will be mildly acidic (pH < 7).
• If the cation's conjugate base is slightly stronger than the anion's conjugate acid, the solution will be mildly basic (pH > 7).

For ammonium ethanoate, the pK_a of ethanoic acid (4.76) and the pK_b of ammonia (4.75) are nearly equal, so the solution pH will be about 7.

More generally, the relationship between the acid dissociation constant (K_a) and the base dissociation constant (K_b) determines the solution pH:

• If K_a > K_b: [H⁺] > [OH⁻] and pH < 7
• If K_a < K_b: [H⁺] < [OH⁻] and pH > 7
• If K_a ≈ K_b: [H⁺] ≈ [OH⁻] and pH ≈ 7

Summary of salt hydrolysis

The key points regarding the acid-base properties of different salts are summarised in this table:

Worked example 1 - Predicting pH of salt solution

Explain, using equations, whether an aqueous solution of methylammonium chloride (CH_3_NH_3_Cl) will be neutral, acidic or basic.

Step 1: Dissociation in water

Methylammonium chloride dissociates in water:

CH₃NH₃Cl_(aq) ➔ CH₃NH₃⁺_(aq) + Cl^-_(aq)

Step 2: Hydrolysis of the methylammonium ion

The methylammonium ion (CH_3_NH_3_⁺) acts as a weak acid in water:

CH₃NH₃⁺_(aq) + H₂O_(l) ⇌ CH₃NH_2(aq) + H₃O⁺_(aq)

This reaction increases the concentration of hydronium ions (H₃O⁺) in solution, making it acidic.

Worked example 2 - Predicting pH of salt solution

Explain whether an aqueous solution of potassium iodide (KI) will be neutral, acidic or basic.

Step 1: Dissociation in water

KI dissociates completely in water:

KI_(aq) ➔ K⁺_(aq) + I^-_(aq)

Step 2: Hydrolysis analysis

Neither the potassium ion (K⁺) nor the iodide ion (I^-) undergo significant hydrolysis in water.

Since there are no significant contributions to H⁺ or OH^- ions from hydrolysis, the pH of the solution remains close to 7.

Worked example 3 - Predicting pH of salt solution

Explain, using equations, whether an aqueous solution of sodium methanoate (HCOONa) will be neutral, acidic or basic.

Step 1: Dissociation in water

HCOONa dissociates in water:

HCOONa_(aq) ➔ Na⁺_(aq) + HCOO^-_(aq)

Step 2: Hydrolysis of the methanoate ion

The methanoate ion (HCOO^-) acts as a weak base in water:

HCOO^-_(aq) + H₂O_(l) ⇌ HCOOH_(aq) + OH^-_(aq)

This reaction increases the concentration of hydroxide ions (OH^-) in solution, making it basic.