Acid/Base Behaviour of the Period 3 Oxides & Hydroxides (Cambridge International A-Level Chemistry)

Acid / Base Behaviour of Period 3 Oxides & Hydroxides

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 and basic nature of Period 3 oxides table

Reaction of 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 of the Period 3 oxides table

Electronegativity of Period 3 elements table

• Oxygen has an electronegativity value of 3.5
• Therefore, the difference in electronegativity between oxygen and Na, Mg and Al is the largest
• So, electrons will be transferred to oxygen when forming oxides giving the oxide an ionic binding
• 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 structure of some Period 3 oxides

Magnesium oxide is giant ionic, silicon dioxide is giant covalent and sulfur dioxide is simple molecular

• 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 an acid 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 hydroxides

• 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 act 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)

Exam Hint

• Electronegativity is the power of an element to draw the electrons towards itself in a covalent bond.
• For example, in Na₂O the oxygen will draw the electrons more strongly towards itself than sodium does.