Arrhenius acids and bases

What happens in water

Sour taste and litmus describe acids, but chemists needed an explanation at the level of molecules. Svante Arrhenius, a Swedish chemist, defined acids and bases by what they do in water.

An Arrhenius acid dissociates in water to give hydrogen ions (H⁺), which join water to make hydroxonium ions (H₃O⁺). Examples: HCl, HNO₃, H₂SO₄ and HCN. An Arrhenius base dissociates in water to give hydroxide ions (OH⁻). Examples: NaOH, KOH and Ba(OH)₂.

Why H₃O⁺? A proton is tiny and its charge is packed very tightly, so it bonds strongly to a lone pair on a water molecule. The book notes that up to four water molecules may crowd around one proton (Figure 7.1).

Arrhenius also explained neutralization. HCl and NaOH both split completely into ions in water. The H₃O⁺ and OH⁻ ions then join to form water and give out heat. Na⁺ and Cl⁻ take no part; they stay in solution as hydrated spectator ions.

Arrhenius needs water: acids make H₃O⁺ in it, bases make OH⁻ in it.

Key terms

Arrhenius acid

The water-based definition.

Arrhenius base

The water-based definition.

Hydroxonium ion

How H⁺ really exists in water.

Dissociate

What acids and bases do.

Spectator ion

Na⁺ and Cl⁻ in neutralization.

Short questions with model answers

1. Q1. How do chloride ions exist in water? (Exercise, p. 96)

   • HCl dissociates
   • Water surrounds ions

   As separate hydrated Cl⁻ ions, surrounded by water molecules.

2. Q2. Choose the Arrhenius acids: HF, NH₄⁺, H₂SO₃, SO₃, H₂S. (Short question i, p. 104)

   • Look for an H to give away
   • Rule out the rest

   HF, H₂SO₃ and H₂S.

Common mistakes

• ✗ H⁺ floats around on its own in water.

  ✓ It bonds to water as H₃O⁺ (p. 96).

• ✗ Na⁺ and Cl⁻ react to make solid salt during neutralization.

  ✓ They stay dissolved as spectator ions (p. 97).

• ✗ NH₃ is an Arrhenius base.

  ✓ It has no OH group of its own; this is a limitation of Arrhenius (p. 98).

MCQs

1. 1. According to Arrhenius, an acid in water gives:

   1. (a) OH⁻ ions
   2. (b) H⁺ (H₃O⁺) ions
   3. (c) Electrons
   4. (d) Na⁺ ions
   Show answer

   (b) p. 95.

2. 2. Which is an Arrhenius base?

   1. (a) HCN
   2. (b) Ba(OH)₂
   3. (c) NH₃
   4. (d) Na₂CO₃
   Show answer

   (b) p. 96: NaOH, KOH, Ba(OH)₂.

3. 3. How many hydroxide ions does one Ca(OH)₂ release in water? (Exercise iv, p. 103)

   1. (a) 1
   2. (b) 2
   3. (c) 0
   4. (d) 3
   Show answer

   (b) The formula has two OH groups.

4. 4. In HCl + NaOH neutralization, which ions are spectators?

   1. (a) H₃O⁺ and OH⁻
   2. (b) Na⁺ and Cl⁻
   3. (c) H₃O⁺ and Cl⁻
   4. (d) None
   Show answer

   (b) p. 97.

5. 5. A limitation of the Arrhenius idea is that it:

   1. (a) Needs water as the solvent
   2. (b) Ignores HCl
   3. (c) Has no bases
   4. (d) Uses litmus only
   Show answer

   (a) p. 98.

Quick revision

• Arrhenius acids give H⁺ (as H₃O⁺) in water; Arrhenius bases give OH⁻ in water.
• A proton bonds to a water molecule because it is tiny and highly charged.
• Neutralization: H₃O⁺ and OH⁻ form water and heat; Na⁺ and Cl⁻ are spectators.
• Arrhenius cannot explain NH₃ or Na₂CO₃ as bases, and needs water.