6.1.4 Oxides Reacting with Water

Structure, bonding & electronegativity of the Period 3 elements table

• The oxides of Na and Mg which show purely ionic bonding produce alkaline solutions with water as their oxide ions (O^2-) become hydroxide ions (OH^-):

O^2- (aq) + H₂O (l) → 2OH^- (aq)

• The oxides of P and S which show purely covalent bonding produce acidic solutions with water because when these oxides react with water, they form an acid which donates H⁺ ions to water
  • Eg. SO₃ reacts with water as follows:

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

• • The H₂SO₄ is an acid which will donate a H⁺ to water:

H₂SO₄ (aq) + H₂O (l) → H₃O⁺ (aq) + HSO₄^- (aq)

• Al and Si are insoluble and when they react with hot, concentrated alkaline solution they act as a base and form a salt
  • This behaviour is very typical of a covalently bonded oxide

• Al can also react with acidic solutions to form a salt and water
  • This behaviour is very typical of an ionic bonded metal oxide

• This behaviour of Al proves that the chemical bonding in aluminium oxide is not purely ionic nor covalent: therefore it exhibits amphoteric character 

Reaction of Period 3 oxides with water 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
  • 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
  • They will react with water to produce acidic solutions

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