Bases

A base is a metallic oxide or hydroxide which neutralizes an acid to form salt and water only.

The term was originally used to describe substances that turned red litmus blue.

(i) ZnO_(g) + 2HCl_(aq) → ZnCl_2(s) + H₂O

(ii) 2NaOH_(aq) + H₂SO_4(aq) → Na₂SO_4(s) + 2H₂O_(l)

Definitions

1. Arrhenius definition of a Base:

A base is a substance that produces hydroxyl ion (OH^–) when dissolved in water or in aqueous solution e.g NaOH, KOH.

NaOH_(s) + H₂O → Na⁺_(aq)  + OH^–_(aq)

or

NaOH → Na⁺_(aq) + OH^–_(aq)

2. Brønsted–Lowry Concept of Bases:

A base is a proton acceptor. Example,

(i) NH₃ + HCl → NH₄⁺ + Cl^–

(ii) H₂O + HCl → H₃O⁺ + Cl^–

In examples (i) and (ii), NH₃ and H₂O molecules are bases, they accept proton from the acids.

3. Lewis Concept of Base:

Lewis base is an electron-pair donor. They are species that possess a Lone-pair of electrons that can be shared with an empty valence orbital.

(i) NH₃ + H₃O⁺ → NH₄⁺ + H₂O

(ii) Cl^– + AlCl₃ → AlCl₄^–

In (i), NH₃ donates the Lone-pair of electrons on the nitrogen atom to the proton in H₃O⁺, and in (ii) Cl^– donates a Lone-pair electron to the empty valence orbital on the aluminum atom in AlCl₃.