Classifying Oxides

• Physics
• Secondary Education
• Age 12,13

07 April 2023

Classifying Oxides

Content objectives

• I can classify oxides as acidic, including SO₂ and CO₂, or basic, including CuO and CaO, related to metallic and non-metallic character.
• I can describe amphoteric oxides as oxides that react with acids and with bases to produce a salt and water.
• I can classify Al₂O3 and ZnO as amphoteric oxides.

Proton Transfer

What are oxides?

• Oxides are compounds made from one or more atoms of oxygen combined with one other element.
• Examples of oxides include: MgO, ZnO, K2O, CO2, SO2, H2O
• Oxides can be classified based on their acid-base characteristics.

Acid and basic oxides

• have different properties and values of pH
• the difference in their pH = whether they are bonded to a metal or to a non-metal element
• The metallic character of the element influences the acidic or basic behaviour of the molecule.

Acidic Oxides

• formed when a non-metal element combines with oxygen
• react with bases to form a salt and water
• When dissolved in water, they produce an acidic solution with a low pH.
• Common examples include CO2, SO2, NO2 and SiO2

Basic Oxides

• formed when a metal element combines with oxygen
• react with acids to form a salt and water
• When dissolved in water they produce a basic solution with a high pH.
• Common examples include CuO and CaO

07 April 2023

Classifying Oxides

Neutral Oxides

• Some oxides do not react with either acids or bases and thus are said to be neutral.
• Examples include N2O, NO, and CO.

Amphoteric oxides

• a group of oxides that can behave as both acidic and basic, depending on whether the other reactant is an acid or a base
• In both cases, salt and water are formed.
• most common amphoteric oxides: zinc oxide, ZnO and aluminum oxide, Al2O3
• The hydroxides of both of these elements also behave amphoterically.

Example

• Example of aluminium oxide behaving as a base:

Al2O3 + 6HCl → 2AlCl3 + 3H2O

• Example of aluminium oxide behaving as an acid:

Al2O3 + 2NaOH → 2NaAlO2 + H2O

Exemption to the Proton transfer concept

• This acidic and basic behaviour of amphoteric oxides is not easily explained by donating or accepting protons. A separate theory called the Lewis acid-base theory can identify acids or bases in these situations.