Neutralisation

Definition of Neutralisation

Neutralisation is a chemical reaction where an acid reacts with a base to produce a salt and water. During this reaction, the pH of the solution moves closer to neutral (pH 7) because the acid and base cancel each other out.

The general word equation for neutralisation is:

acid + base 1212 salt + water

Acids and Alkalis

Acids release hydrogen ions (H⁺) when dissolved in water. Alkalis are bases that dissolve in water and release hydroxide ions (OH^21).

When an acid reacts with an alkali, the H⁺ ions from the acid combine with the OH^21 ions from the alkali to form water:

$\mathrm{H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)}$

This formation of water is the key feature of neutralisation.

For example, hydrochloric acid (HCl) releases H⁺ ions, and sodium hydroxide (NaOH) releases OH^21 ions:

$\mathrm{HCl \rightarrow H^+ + Cl^-}$

$\mathrm{NaOH \rightarrow Na^+ + OH^-}$

When these react:

$\mathrm{H^+ + OH^- \rightarrow H_2O}$

For instance, when 1 mole of hydrochloric acid reacts with 1 mole of sodium hydroxide, 1 mole of sodium chloride and water are formed.

Neutralisation Reactions

Neutralisation involves acids reacting with different types of bases to form salts and water. The main types of neutralisation reactions are:

• Acid + base 1212 salt + water
• Acid + metal oxide 1212 salt + water
• Acid + alkali 1212 salt + water

Here, the base can be a metal oxide or an alkali (a soluble base).

For example, when sulfuric acid reacts with copper oxide (a metal oxide), the reaction is:

$\mathrm{H_2SO_4 (aq) + CuO (s) \rightarrow CuSO_4 (aq) + H_2O (l)}$

Copper sulfate is the salt formed, and water is produced.

Similarly, when hydrochloric acid reacts with sodium hydroxide (an alkali), the reaction is:

$\mathrm{HCl (aq) + NaOH (aq) \rightarrow NaCl (aq) + H_2O (l)}$

Sodium chloride (table salt) and water are formed.

Learning example: Calculate the products when nitric acid reacts with magnesium oxide.

Nitric acid is $\mathrm{HNO_3}$ and magnesium oxide is $\mathrm{MgO}$.

The reaction is:

$\mathrm{2HNO_3 + MgO \rightarrow Mg(NO_3)_2 + H_2O}$

Magnesium nitrate and water are produced.

Applications of Neutralisation

Neutralisation reactions have many practical uses:

• Making salts: Neutralisation is used to produce salts by reacting acids with bases, metal oxides, or alkalis (see separate notes on Making Salts for details).
• Antacid tablets: These contain bases such as magnesium hydroxide or calcium carbonate that neutralise excess stomach acid, relieving indigestion and heartburn.
• Agricultural lime: Farmers add lime (calcium oxide or calcium hydroxide) to acidic soils to neutralise the acid and improve soil quality for crops.