Acids, Bases, Conjugates, Proticity, and Amphiprotic Substances

• HA: Acid (proton donor)

• B: Base (proton acceptor)

• HB⁺: Conjugate acid

• A⁻: Conjugate base

Examples

1. Reaction: NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)

   • NH₃: Base

   • H₂O: Acid

   • NH₄⁺: Conjugate acid

   • OH⁻: Conjugate base

2. Reaction: HClO₂ (aq) + H₂O (l) ⇌ ClO₂⁻ (aq) + H₃O⁺ (aq)

   • HClO₂: Acid

   • H₂O: Base

   • ClO₂⁻: Conjugate base

   • H₃O⁺: Conjugate acid

3. Reaction: HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

   • HCl: Acid

   • NaOH: Base

   • NaCl: Neutral salt (not a conjugate)

   • H₂O: Neutral molecule

Conjugate Bases:

• HNO₃ → NO₃⁻

• CH₃COOH → CH₃COO⁻

• H₂SO₄ → HSO₄⁻

Conjugate Acids:

• OH⁻ → H₂O

• NH₃ → NH₄⁺

• CO₃²⁻ → HCO₃⁻

3. Proticity

Proticity refers to the number of protons an acid can donate in solution.

Classes of Acids:

1. Monoprotic Acids (1 proton):
   Examples: HCl, HNO₃, CH₃COOH
   Dissociation: HCl → H⁺ + Cl⁻

2. Diprotic Acids (2 protons):
   Examples: H₂SO₄, H₂CO₃
   Stepwise Ionization:

   • H₂SO₄ → H⁺ + HSO₄⁻ (strong ionization)

   • HSO₄⁻ ⇌ H⁺ + SO₄²⁻ (weak ionization)

3. Triprotic Acids (3 protons):
   Examples: H₃PO₄, Citric acid (C₆H₈O₇)
   Stepwise Ionization:

   • H₃PO₄ → H⁺ + H₂PO₄⁻

   • H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻

   • HPO₄²⁻ ⇌ H⁺ + PO₄³⁻

4. Amphiprotic Substances

Amphiprotic substances can act as both an acid and a base.

Sodium Hydrogen Carbonate (NaHCO₃):

1. Acting as an acid:
   HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

2. Acting as a base:
   HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O

Potassium Dihydrogen Phosphate (KH₂PO₄):

1. Acting as an acid:
   H₂PO₄⁻ + OH⁻ → HPO₄²⁻ + H₂O

2. Acting as a base:
   H₂PO₄⁻ + H₃O⁺ → H₃PO₄ + H₂O

Water (H₂O):

1. Acting as an acid:
   H₂O + NH₃ → NH₄⁺ + OH⁻

2. Acting as a base:
   H₂O + HCl → H₃O⁺ + Cl⁻

5. Acid Strength

The strength of an acid depends on its degree of ionization. Strong acids fully ionize, while weak acids only partially ionize.

Strong Acids (Complete Ionization):

1. HCl → H⁺ + Cl⁻

2. HNO₃ → H⁺ + NO₃⁻

3. H₂SO₄ → H⁺ + HSO₄⁻

Weak Acids (Partial Ionization):

1. CH₃COOH ⇌ H⁺ + CH₃COO⁻

2. H₂CO₃ ⇌ H⁺ + HCO₃⁻

Degree of Ionization Formula:

% Ionization = ([H₃O⁺] produced / [HA] initial) × 100%

6. Strength of Conjugates

The strength of conjugate acid/base pairs follows these trends:

1. A strong acid has an extremely weak conjugate base.
   Example: HCl (strong acid) → Cl⁻ (extremely weak base).

2. A weak acid has a weak conjugate base.
   Example: CH₃COOH (weak acid) → CH₃COO⁻ (weak base).