CIE AS Chemistry (9701) exams from 2022

Revision Notes

2.1.4 Period 3 Oxides & Hydroxides: Acid/Base Behaviour

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 & basic nature of the Period 3 oxides

The Periodic Table - Table 1_Acid - Base Behaviour of Period 3 Oxides & Hydroxides, downloadable AS & A Level Chemistry revision notes

Reactions of the Period 3 oxides with acid/base table

The Periodic Table - Table 2_Acid - Base Behaviour of Period 3 Oxides & Hydroxides, downloadable AS & A Level Chemistry revision notes

  • 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 Periodic Table - Table 3_Acid - Base Behaviour of Period 3 Oxides & Hydroxides, downloadable AS & A Level Chemistry revision notes

  • 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 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 Periodic Table - Oxide Structure, downloadable AS & A Level Chemistry revision notes

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 (O2-) become hydroxide ions (OH):

O2-(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)

Exam Tip

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.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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