Acid/Base Behaviour of the Period 3 Oxides & Hydroxides (CIE AS Chemistry)

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

Period 3 oxide	Na2O	MgO	Al2O3	SiO2	P4O10	SO2 SO3
Acid / base nature	Basic	Basic	Amphoteric	Acidic	Acidic	Acidic

Reaction of Period 3 oxides with acid / base table

Oxide	Chemical equation	Comment
Na2O	Na2O (s) + 2HCl (aq) → 2NaCl (aq) + H2O (l)
MgO	MgO (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l)	Used in indigestion remedies by neutralising excess acid in the stomach
Al2O3	Al2O3 (s) + 3H2SO4 (aq) → Al2(SO4)3 (aq) + 3H2O (l) Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2NaAl(OH)4 (aq)	Reacts with acid to form salt and water Reacts with hot, concentrated alkali to form salt
SiO2	SiO2 (s) + 2NaOH (aq)  → Na2SiO3 (aq) + H2O (l)
P4O10	P4O10 (s) + 12NaOH (aq)  → 4Na3PO4 (aq) + 6H2O (l)
SO2  SO3	SO2 (g) + 2NaOH (aq)  → Na2SO3 (aq) + H2O (l) SO3 (g) + 2NaOH (aq)  → Na2SO4 (aq) + H2O (l)

• 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

Period 3 oxide	Na2O	MgO	Al2O3	SiO2	P4O10	SO2 SO3
Relative melting point	High	High	Very high	Very high	Low	Low
Chemical bonding	Ionic	Ionic	Ionic (with some covalent character)	Covalent	Covalent	Covalent
Structure	Giant ionic	Giant ionic	Giant ionic	Giant covalent	Simple molecular	Simple molecular

 

Electronegativity of Period 3 elements table

Period 3 element	Na	Mg	Al	Si	P	S	Cl
Electronegativity	0.9	1.2	1.5	1.8	2.1	2.5	3.0

• 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 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 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)