Bronsted-Lowry acids and bases

Pass the proton

Arrhenius could not explain why Na₂CO₃, K₂CO₃ and NH₃ behave as bases even though they have no OH group, and his idea only works in water. Bronsted and Lowry fixed both problems with a simpler idea.

A Bronsted-Lowry acid donates a proton (H⁺). A Bronsted-Lowry base accepts a proton. An acid-base reaction is simply a proton moving from donor to acceptor, and it can happen in a gas or in any solvent, not only in water.

When HCl gas meets water, HCl gives its proton to water: HCl is the acid and water is the base. When ammonia dissolves in water, water gives a proton to NH₃, making NH₄⁺ and OH⁻: now water is the acid and ammonia is the base. Because water can do both, it is called amphoteric.

In the reverse reaction, NH₄⁺ gives a proton back to OH⁻. So NH₄⁺ is called the conjugate acid and OH⁻ the conjugate base. A conjugate pair differs by just one proton.

Follow the H⁺: whoever gives it is the acid, whoever takes it is the base.

Key terms

Bronsted-Lowry acid

Any solvent, or none.

Bronsted-Lowry base

Explains NH₃ as a base.

Amphoteric

Describing water.

Conjugate acid

Reverse reaction.

Conjugate base

Reverse reaction.

Short questions with model answers

1. Q1. Label the acid, base, conjugate acid and conjugate base in HCN + H₂O ⇌ H₃O⁺ + CN⁻. (p. 100)

   • Who gives H⁺?
   • Who takes H⁺?

   Acid HCN, base H₂O, conjugate acid H₃O⁺, conjugate base CN⁻.

2. Q2. Give two examples of Bronsted-Lowry bases which are not bases by the Arrhenius definition. (Exercise, p. 100)

   • Accepts a proton
   • Gives no OH⁻ of its own

   NH₃ and CN⁻ (Cl⁻ also works).

Common mistakes

• ✗ Water is always a base.

  ✓ Water is amphoteric: a base with HCl, an acid with NH₃ (p. 99).

• ✗ Every Bronsted-Lowry base is also an Arrhenius base.

  ✓ Only OH⁻ is both; NH₃, CN⁻ and Cl⁻ are not Arrhenius bases (p. 99).

• ✗ The conjugate base of HCN is H₃O⁺.

  ✓ A conjugate base is the acid minus one H⁺: CN⁻.

MCQs

1. 1. A Bronsted-Lowry acid is a:

   1. (a) Proton donor
   2. (b) Proton acceptor
   3. (c) OH⁻ donor
   4. (d) Electron donor
   Show answer

   (a) p. 99.

2. 2. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, water acts as:

   1. (a) A base
   2. (b) An acid
   3. (c) A salt
   4. (d) A spectator
   Show answer

   (b) Water gives its proton to NH₃ (p. 99).

3. 3. In CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺, the conjugate base is: (Exercise ix, p. 103)

   1. (a) CH₃COOH
   2. (b) H₂O
   3. (c) CH₃COO⁻
   4. (d) H₃O⁺
   Show answer

   (c) Acetic acid minus one H⁺.

4. 4. A substance that can act both as an acid and as a base is called:

   1. (a) Neutral
   2. (b) Amphoteric
   3. (c) Diprotic
   4. (d) Spectator
   Show answer

   (b) p. 100.

5. 5. Which is a Bronsted-Lowry base but NOT an Arrhenius base?

   1. (a) NaOH
   2. (b) KOH
   3. (c) NH₃
   4. (d) Ba(OH)₂
   Show answer

   (c) p. 99.

Quick revision

• Bronsted-Lowry: acids donate protons, bases accept them; water is not needed.
• Every Arrhenius acid and base fits, but NH₃, CN⁻ and Cl⁻ are bases only under Bronsted-Lowry.
• Water is amphoteric.
• A conjugate pair differs by one proton.