Science with Tara

Alkalis are substances that produce hydroxide ions (OH⁻) in aqueous solution. In this lesson, we will learn the typical chemical reactions of alkalis.

Common alkalis include Group (I) metal hydroxides, calcium hydroxide and barium hydroxide.

Alkalis have pH values more than 7. The higher the pH value, the more alkali the solution. Strongest alkali has pH of 14.

Reaction with Acids (Neutralisation)

Alkalis react with Acids to form salt and water. This reaction is called neutralisation reaction.

Example:
Reaction between hydrochloric acid and sodium hydroxide

Chemical equation:
HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Let’s work out the ionic equation of this reaction. Substances that ionises in aqueous solution are represented with (aq) state symbols.
Hydrochloric acid, HCl, will ionise into hydrogen ions and chlorides ions.
Sodium hydroxide, NaOH, will ionise into sodium ions and hydroxides ions.
Sodium chloride, NaCl, will ionise into sodium and chloride ions.
Water will not ionise, hence it is represented with (l) state symbols.

Ionic equation:
H⁺(aq) + Cl^–(aq) + Na⁺(aq) + OH^–(aq) → Na⁺(aq) + Cl^–(aq) + H₂O(l)

Ionic equation of neutralisation reaction:
H⁺ (aq) + OH^– (aq) → H₂O (l)

Any ions that appear at the left and right of the equation are considered as spectator ions. They do not take part in the reaction and can be cancelled in the equation. We will cancel Cl^–  (aq) and Na⁺ (aq), leaving us with H⁺ (aq) and OH^– (aq).

Reaction with Ammonium Salts

Alkalis react with ammonium salts to form salt, water and ammonia gas.

Example:
Reaction between ammonium chloride and sodium hydroxide

Chemical equation:
NaOH(aq) + NH₄Cl(s) → NaCl(aq) + H₂O(l) + NH₃(g)

Ammonia gas can be tested with the use of damp red litmus paper. As ammonia gas is very soluble in water, reaction mixture is heated to drive out the gas for testing. Ammonia gas, a pungent gas, turns damp red litmus paper blue.

Reaction with Metal Salt Solutions

Alkalis react with solution of metal salt to form another metal salt and an insoluble metal hydroxide.

Example:
Reaction between sodium hydroxide and copper(II) sulfate

Chemical equation:
2NaOH(aq) + CuSO₄(aq) → Na₂SO₄(aq) + Cu(OH)₂(s)

This reaction may be confusing at first glance. Working out the ionic equation will make things clearer.

Substances that ionises in aqueous solution are represented with (aq) state symbols.
Sodium hydroxide, NaOH, will ionise into sodium ions and hydroxides ions.
Copper(II) sulfate will ionise into copper(II) ions and sulfate ions.
In the reaction mixture, there are four ions present, that is, sodium ions, hydroxides ions, copper(II) ions and sulfate ions. They are free to form a new compound.
Copper(II) ions and hydroxide ions come together to form a solid, leaving sodium ions and sulfate ions in the aqueous solution.

As to why copper(II) and hydroxide ions form solid, recall that all bases (metal hydroxides and oxides) are insoluble except alkali, which is soluble hydroxide.

Soluble hydroxides include group(I) metal hydroxides, calcium hyroxide and barium hydroxide. All the other hydroxides are insoluble, with state symbol (s).

Ionic equation:
Cu²⁺(aq) + SO₄^2-(aq) + Na⁺(aq) + OH^–(aq) → Cu(OH)₂(s) + Na⁺(aq) + SO₄^2-(aq)

Cu²⁺ (aq) + OH^– (aq) → Cu(OH)₂ (s)

Any ions that appear at the left and right of the equation are considered as spectator ions. They do not take part in the reaction and can be cancelled in the equation. We will cancel SO₄^2- (aq) and Na⁺ (aq), leaving us with Cu(OH)₂ (s).