Properties of Period 3 Elements & their Oxides (A Level only) (AQA A-Level Chemistry): Revision Notes

6.1.4 Oxides Reacting with Water

Reactions of Oxides with Water and Resulting pH

The oxides of Period 3 elements from sodium to sulphur exhibit different reactions with water based on their bonding types and structures. The resulting solutions vary in pH, reflecting whether the oxides are basic, amphoteric, or acidic.

Sodium Oxide ($\text{Na}_2\text{O}$)

Reaction with Water: Dissolves and reacts with water to form sodium hydroxide, a strong alkali.

Equation:

$$\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH (aq)}$$

pH: The resulting solution is strongly alkaline, with a pH around 14.

Magnesium Oxide ($\text{MgO}$)

Reaction with Water: Reacts sparingly with water to form magnesium hydroxide, a weak alkali due to its low solubility.

Equation:

$$\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \text{ (aq)}$$

pH: The solution is weakly alkaline, with a pH around 9.

Aluminium Oxide ($\text{Al}_2\text{O}_3$)

Reaction with Water: Insoluble in water and does not react due to strong ionic bonds.

pH: No effect on pH, but aluminium oxide is amphoteric and can react with both acids and bases in other conditions.

Silicon Dioxide ($\text{SiO}_2$)

Reaction with Water: Insoluble in water and unreactive due to strong covalent bonds in its giant lattice structure.

pH: No effect on pH. However, silicon dioxide can react with bases as it behaves as an acidic oxide.

Phosphorus(V) Oxide ($\text{P}_4\text{O}_{10}$)

Reaction with Water: Reacts vigorously with water to produce phosphoric acid.

Equation:

$$\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4$$

pH: The solution is strongly acidic, with a pH around 0.

Sulfur Dioxide ($\text{SO}_2$)

Reaction with Water: Dissolves to form sulphurous acid, a weak acid because the reaction is reversible.

Equation:

$$\text{SO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{SO}_3$$

pH: The resulting solution is weakly acidic, with a pH around 3.

Sulfur Trioxide ($\text{SO}_3$)

Reaction with Water: Reacts vigorously to produce sulfuric acid.

Equation:

$$\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$$

pH: The solution is strongly acidic, with a pH around 0.

Structures and Anions Formed by Acidic Oxides

Phosphorus(V) Oxide ($\text{P}_4\text{O}_{10}$)

Forms phosphoric acid ($\text{H}_3\text{PO}_4$), dissociating into phosphate ions ($\text{PO}_4^{3-}$)

Sulfur Dioxide ($\text{SO}_2$)

Forms sulphurous acid ($\text{H}_2\text{SO}_3$), dissociating into sulfite ions ($\text{SO}_3^{2-}$)

Sulfur Trioxide ($\text{SO}_3$)

Forms sulfuric acid ($\text{H}_2\text{SO}_4$), dissociating into sulphate ions ($\text{SO}_4^{2-}$)

Reactions of Oxides with Acids and Bases

Basic Oxides (e.g., $\text{Na}_2\text{O}$, $\text{MgO}$)

React with acids to form salt and water.

$$\text{Na}_2\text{O} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O}$$ $$\text{MgO} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2\text{O}$$

Acidic Oxides (e.g., $\text{SiO}_2$, $\text{P}_4\text{O}_{10}$, $\text{SO}_2$, $\text{SO}_3$)

React with bases to form salts.

$$\text{SiO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SiO}_3 + \text{H}_2\text{O}$$ $$\text{SO}_3 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}$$

Amphoteric Oxides (e.g., $\text{Al}_2\text{O}_3$)

React with both acids and bases.

$$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$ $$\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$$