R3.1.12 The pH of a salt solution (Higher Level only)

Hydrolysis of Ions in a Salt

When a salt such as ammonium chloride (NH $_{4}$ Cl) dissolves in water, it dissociates into ions—NH $_{4}^{+}$ and Cl $^{-}$ .
While it may seem that these ions simply exist in solution without further interaction, the ammonium ion (NH $_{4}^{+}$ ) actually reacts with water, releasing hydrogen ions (H $^{+}$ ) and slightly lowering the pH.

This process, called hydrolysis, explains why some salt solutions are acidic, basic, or neutral.

Understanding Hydrolysis: The Basics

Definition

Hydrolysis of a salt

When a salt dissolves in water, its ions may undergo hydrolysis, reacting with water to form either H $^{+}$ (acidic) or OH $^{-}$ (basic) ions.

The pH of the resulting solution depends on the nature of the ions involved:

Cations (positive ions): Certain cations, like NH $_{4}^{+}$ , act as weak acids by donating protons to water.
Anions (negative ions): Certain anions, like CH $_{3}$ COO $^{-}$ , act as weak bases by accepting protons from water.

Key Rule:

Only ions from weak acids or weak bases undergo hydrolysis. Ions derived from strong acids (e.g., Cl $^{-}$ ) or strong bases (e.g., Na $^{+}$ ) are too weak to react with water and do not hydrolyze.

Analogy

Think of strong acids and bases as "stable parents" whose ions are too unreactive to interact with water, while weak acids and bases are "reactive parents," producing ions that actively engage in hydrolysis.

Predicting the pH of Salt Solutions

To determine the pH of a salt solution, consider the parent acid and base from which the salt is derived.
These combinations dictate whether the solution is acidic, basic, or neutral:

1.Salts of Strong Acids and Strong Bases

Example: NaCl (sodium chloride)
Parent acid: HCl (strong acid)
Parent base: NaOH (strong base)
Result: Neither ion hydrolyzes, so the solution is neutral (pH = 7).

2.Salts of Strong Acids and Weak Bases

Example: NH $_{4}$ Cl (ammonium chloride)
Parent acid: HCl (strong acid)
Parent base: NH $_{3}$ (weak base)
Result: The NH $_{4}^{+}$ ion hydrolyzes:
${NH}_{4}^{+} + H_{2} O ⇋ {NH}_{3} + H_{3} O^{+}$
The solution becomes acidic (pH < 7).

3.Salts of Weak Acids and Strong Bases

Example: CH $_{3}$ COONa (sodium ethanoate)
Parent acid: CH $_{3}$ COOH (weak acid)
Parent base: NaOH (strong base)
Result: The CH $_{3}$ COO $^{-}$ ion hydrolyzes:
${CH}_{3} {COO}^{-} + H_{2} O ⇋ {CH}_{3} COOH + {OH}^{-}$
The solution becomes basic (pH > 7).

4.Salts of Weak Acids and Weak Bases

Example: NH $_{4}$ CH $_{3}$ COO (ammonium ethanoate)
Parent acid: CH $_{3}$ COOH (weak acid)
Parent base: NH $_{3}$ (weak base)
Result: Both ions hydrolyze:
${CH}_{3} {COO}^{-} + H_{2} O ⇋ {CH}_{3} COOH + {OH}^{-}$
${NH}_{4}^{+} + H_{2} O ⇋ {NH}_{3} + H_{3} O^{+}$
The pH depends on the relative strengths of the parent acid and base, determined by their dissociation constants ( $K_{a}$ and $K_{b}$ ).

Tip

To predict the pH of a salt solution, always identify the parent acid and base, and assess whether the resulting ions will hydrolyze.

Example

Examples of Hydrolysis Reactions

Here are some specific examples illustrating how hydrolysis affects pH:

1.Ammonium Ion (NH $_{4}^{+}$ )

Acts as a weak acid:
${NH}_{4}^{+} + H_{2} O ⇋ {NH}_{3} + H_{3} O^{+}$
The release of H $^{+}$ ions makes the solution slightly acidic.

2.Carboxylate Ion (RCOO $^{-}$ )

Acts as a weak base:
${RCOO}^{-} + H_{2} O ⇋ RCOOH + {OH}^{-}$
The formation of OH $^{-}$ ions makes the solution slightly basic.

3.Carbonate Ion (CO $_{3}^{2 -}$ )

Contributes to alkalinity through stepwise hydrolysis:
${CO}_{3}^{2 -} + H_{2} O ⇋ {HCO}_{3}^{-} + {OH}^{-}$
${HCO}_{3}^{-} + H_{2} O ⇋ H_{2} {CO}_{3} + {OH}^{-}$

4.Hydrogen Carbonate Ion (HCO $_{3}^{-}$ )

Functions as a buffer:
${HCO}_{3}^{-} + H_{2} O ⇋ H_{2} {CO}_{3} + {OH}^{-}$
${HCO}_{3}^{-} + H_{2} O ⇋ {CO}_{3}^{2 -} + H_{3} O^{+}$
It can act as either an acid or a base, depending on the conditions.

Example question

Predicting pH

Which of the following salts will produce a basic solution when dissolved in water?

NaCl
NH $_{4}$ Cl
CH $_{3}$ COONa

Solution

NaCl: Neutral (neither ion hydrolyzes).
NH $_{4}$ Cl: Acidic (NH $_{4}^{+}$ hydrolyzes to produce H $^{+}$ ).
CH $_{3}$ COONa: Basic (CH $_{3}$ COO $^{-}$ hydrolyzes to produce OH $^{-}$ ).

Answer:bCH $_{3}$ COONa produces a basic solution.

Common Mistake

Do not confuse the strength of the parent acid or base with the concentration of the salt. A strong acid or base does not hydrolyze, while a weak acid or base does.

Reflection

Self review

How does the hydrolysis of salts influence the pH of natural water bodies like rivers and lakes?
Why might salts like ammonium nitrate (NH $_{4}$ NO $_{3}$ ) or sodium carbonate (Na $_{2}$ CO $_{3}$ ) impact aquatic ecosystems differently?

Theory of Knowledge

How does the classification of acids and bases (e.g., strong vs. weak) shape our understanding of chemical behavior?
Are there limitations to these classifications when applied to real-world systems?

You've read 2/2 free chapters this week.

Upgrade to PLUS or PRO to unlock all notes, for every subject.