9.2 Base and Alkali

Base:

A base is a metal oxide or metal hydroxide that reacts with acids to form salt and water. Some examples of bases are calcium oxide (CaO), sodium hydroxide (NaOH), and ammonium hydroxide (NH₄OH). The general reaction between a base and an acid is an acid-base neutralization, where acid neutralizes the base, and the base neutralizes the acid.

Examples of reactions with bases:

• CaO + 2HCl → CaCl₂ + H₂O

• NaOH + H₂SO₄ → Na₂SO₄ + H₂O

• NH₄OH + HCl → NH₄Cl + H₂O

Common bases include sodium oxide (Na₂O), copper oxide (CuO), sodium hydroxide (NaOH), and ferrous hydroxide (Fe(OH)₂).

Alkali:

An alkali is a base that is soluble in water and contains a hydroxide ion (OH⁻). Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH). Not all bases are alkalis. For example, calcium oxide (CaO) is a base but not an alkali because it does not contain an OH⁻ ion.

9.2.1 Properties of Dilute Bases:

• Litmus Test: A dilute base turns red litmus paper blue. It has no effect on blue litmus paper.

• Feel: Bases feel slippery to the touch but can harm the skin.

9.2.2 Dilute Base in Reaction with Metallic Salts:

When a dilute base reacts with metallic salts, a metallic hydroxide and a salt are formed.

Examples:

1. Reaction between dilute NaOH and Al(NO₃)₃:

Al(NO₃)₃ + 3NaOH → Al(OH)₃ (white precipitate) + 3NaNO₃

2. Reaction between dilute NaOH and Fe(NO₃)₂:

Fe(NO₃)₂ + 2NaOH → Fe(OH)₂ (green precipitate) + 2NaNO₃

3. Reaction between dilute NaOH and Fe(NO₃)₃:

Fe(NO₃)₃ + 3NaOH → Fe(OH)₃ (reddish-brown precipitate) + 3NaNO₃

4. Reaction between dilute NaOH and Cu(NO₃)₂:

Cu(NO₃)₂ + 2NaOH → Cu(OH)₂ (light blue precipitate) + 2NaNO₃

5. Reaction between dilute NaOH and Zn(NO₃)₂:

Zn(NO₃)₂ + 2NaOH → Zn(OH)₂ (white precipitate) + 2NaNO₃

9.2.3 Reaction Between Alkali and Ammonium Salts:

When an alkali reacts with ammonium salts, ammonia gas is released.

Examples:

• NH₄Cl + NaOH → NH₃ + NaCl + H₂O

• NH₄Cl + KOH → NH₃ + KCl + H₂O

• 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O

9.2.4 Role of Water in the Chemical Properties of Alkali:

• Alkali Dissolution: When alkalis like NaOH or KOH dissolve in water, they fully ionize to produce hydroxide ions (OH⁻).

• NaOH(s) + Water → Na⁺(aq) + OH⁻(aq)

• KOH(s) + Water → K⁺(aq) + OH⁻(aq)

• Ammonia Dissolution: Ammonia gas dissolves in water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻) but only partially ionizes.

• NH₃(g) + H₂O → NH₄⁺(aq) + OH⁻(aq)

Weak alkalis only partially ionize in water, while strong alkalis fully ionize and produce more hydroxide ions.

Ionic Equations:

1. Addition of iron to dilute sulfuric acid solution:

Fe + H₂SO₄ → FeSO₄ + H₂

2. Addition of solid sodium carbonate to dilute hydrochloric acid solution:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

3. Addition of ammonia solution to copper sulfate solution:

CuSO₄ + 2NH₃ + 2H₂O → Cu(OH)₂ + (NH₄)₂SO₄