Bronsted-Lowry acids and bases and pHEdexcel A-Level Chemistry: Revision notes

A Brønsted–Lowry acid is a proton donor. A Brønsted–Lowry base is a proton acceptor. A proton is an H⁺ ion.

An acid–base reaction involves the transfer of a proton from the acid to the base. In an aqueous solution of an acid, water acts as the base:

HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)

In this reaction HCl is the acid (it donates H⁺) and H₂O is the base (it accepts H⁺). H₃O⁺ is usually written as H⁺(aq).

When an acid donates a proton, what remains is its conjugate base. When a base accepts a proton, it becomes its conjugate acid. The two species in a pair differ by one H⁺.

CH₃COOH(aq) + NH₃(aq) ⇌ CH₃COO⁻(aq) + NH₄⁺(aq)

• Pair 1: CH₃COOH (acid) and CH₃COO⁻ (base)
• Pair 2: NH₄⁺ (acid) and NH₃ (base)

Every acid–base equilibrium contains two conjugate pairs. To identify them, find the species that has lost H⁺ and the one that has gained it.

pH is a measure of acidity defined by

pH = −log₁₀[H⁺]

where [H⁺] is the hydrogen ion concentration in mol dm⁻³. Rearranging gives [H⁺] = $10^{-\mathrm{pH}}$.

• pH of 0.0100 mol dm⁻³ H⁺: −log₁₀(0.0100) = 2.00
• [H⁺] for pH 3.40: $10^{-3.40}$ = 4.0 × 10⁻⁴ mol dm⁻³

Each pH unit is a tenfold change in [H⁺]. A lower pH means a higher [H⁺]. Give pH to 2 decimal places.

A strong acid is fully dissociated in aqueous solution. A weak acid is partially dissociated, so an equilibrium is set up.

• Strong: HCl(aq) → H⁺(aq) + Cl⁻(aq)
• Weak: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

The degree of dissociation is the fraction of acid molecules that have dissociated. Strong and weak describe the degree of dissociation, not concentration.

For a strong monoprotic acid, [H⁺] equals the acid concentration, so pH = −log₁₀(concentration). For 0.0500 mol dm⁻³ HNO₃: pH = −log₁₀(0.0500) = 1.30.

Equimolar acids (0.100 mol dm⁻³): HCl pH 1.00 gives [H⁺] = 0.100 (fully dissociated, strong); ethanoic acid pH 2.88 gives [H⁺] ≈ 1.3 × 10⁻³ (about 1% dissociated, weak).

Equimolar bases: NaOH (pH 13.00) is higher than NH₃ (pH 11.12), so ammonia produces fewer OH⁻ ions: a weak base.

Salts: NaCl, from a strong acid and a strong base, is neutral (pH 7). NH₄Cl, from a strong acid and a weak base, is acidic. CH₃COONa, from a weak acid and a strong base, is alkaline.

Dilution: diluting a solution by 10, 100 and 1000 times raises the pH of a strong acid by exactly 1, 2 and 3 units, because [H⁺] falls in proportion. A weak acid pH rises by less than this, because the equilibrium shifts right and a greater proportion of the acid dissociates.