2.1.4 Period 3 Oxides & Hydroxides: Acid/Base Behaviour

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)

Acidic & basic nature of the Period 3 oxides

Reactions of the Period 3 oxides with acid/base table

• The acidic and basic nature of the Period 3 elements can be explained by looking at their structure, bonding and the Period 3 elements’ electronegativity

Structure, bonding & electronegativity of the Period 3 elements table

• The difference in electronegativity between oxygen and Na, Mg and Al is the largest
• Electrons will therefore be transferred to oxygen when forming oxides giving the oxide an ionic bonding
• The oxides of Si, P and S will share the electrons with the oxygen to form covalently bonded oxides
• The giant ionic and giant covalent structured oxides will have high melting points as it is difficult to break the structures apart

The diagram shows the structure of some Period 3 oxides

• 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: it is amphoteric

Period 3 hydroxide

• NaOH is a strong base and will react with acids to form a salt and water:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

• Mg(OH)₂ is also a basic compound which is often used in indigestion remedies by neutralising the excess acid in the stomach to relieve pain:

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

• Al(OH)₃ is amphoteric and can acts both as an acid and base:

Al(OH)₃(s) + 3HCl(aq) → AlCl₃(s) + 3H₂O(l)

Al(OH)₃(s) + NaOH(aq) → NaAl(OH)₄(aq)