Oxides Reacting with Water (AQA A Level Chemistry): Revision Note

Oxides Reacting with Water

• Going across Period 3, electronegativity increases from 0.9 (Na) to 2.5 (S), causing the shift from ionic to covalent bonding in the oxides

Structure and bonding of the Period 3 oxides table

Behaviour of the Period 3 Oxides with Water

Metal oxides (to the left of the periodic table):

• Sodium oxide, Na₂O, and magnesium oxide, MgO, are made up of ions

• They contain an oxide ion, O^2-, which is a strong base and will readily produce hydroxide ions through reaction with water

O^2- (aq) + H₂O (l) → 2OH^- (aq)

• This is why the solutions formed are strongly alkaline

• Sodium oxide forms a more alkaline solution than magnesium oxide because it is far more soluble in water

Oxides in the middle of the periodic table

• Although ionic, aluminium oxide does not react with water because the oxide ions are held too strongly in the ionic lattice

• This means the ions cannot be separated

• Silicon dioxide is a giant covalent molecule - it is the main component of sand

• It has millions of strong covalent bonds, so it does not react with water

Non-metal oxides (to the right of the periodic table):

• Oxides of phosphorus and sulfur are simple covalent molecules that react with water to produce acidic solutions

• They form an acid which donates H⁺ ions to water

Reaction of Period 3 oxides with water table

Structures of Acids and Anions Formed

• When P₄O₁₀, SO₂ and SO₃ react with water, they form acids

  • These acids can then dissociate to form anions.

Phosphoric acid, H₃PO₄

• Phosphoric acid is formed by:

P₄O₁₀ (s) + 6H₂O (l) → 4H₃PO₄ (aq)

• H₃PO₄ has a central P atom double-bonded to one O and single-bonded to three OH groups

• Successive dissociation produces three anions:

  • H₂PO₄^-

  • HPO₄^2-

  • PO₄^3-

Sulfurous acid, H₂SO₃

• Sulfurous acid is formed by:

SO₂ (g) + H₂O (l) → H₂SO₃ (aq)

• H₂SO₃ has a central S atom bonded to two OH groups and one =O

• Dissociation produces two anions:

  • HSO₃^-

  • SO₃^2-

Sulfuric acid, H₂SO₄

• Sulfuric acid is formed by:

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

• H₂SO₄ has a central S atom double-bonded to two O atoms and single-bonded to two OH groups

• H₂SO₄ is a strong acid

  • The first dissociation uses a one-way arrow:

H₂SO₄ (aq) → H⁺ (aq) + HSO₄^- (aq)

• The HSO₄⁻ formed is a weak acid

  • So, the second dissociation uses an equilibrium arrow:

HSO₄^- (aq) ⇌ H⁺ (aq) + SO₄^2- (aq)

• The SO₄^2- anion has S double-bonded to two O and single-bonded to two O^-

Acid-Base Reactions of the Oxides

Acid/base Nature of the Period 3 Oxides

• Aluminium oxide is amphoteric which means that it can act both as a base (and react with an acid such as HCl) and an acid (and react with a base such as NaOH)

Reactions of the Period 3 oxides with acid/base table