Oxides- CIE iGCSE Chemistry Notes - New Syllabus
Oxides for iGCSE Chemistry Notes
Core Syllabus
- Classify oxides as acidic, including SO₂ and CO₂, or basic, including CuO and CaO, related to metallic and non-metallic character
Supplement Syllabus
- Describe amphoteric oxides as oxides that react with acids and with bases to produce a salt and water
- Classify Al₂O₃ and ZnO as amphoteric oxides
Classification of Oxides: Acidic and Basic
Classification of Oxides: Acidic and Basic
Oxides are chemical compounds that contain oxygen combined with another element. They can be classified based on their chemical behaviour into:
Acidic oxides:
- Formed from non-metals.
- Do not contain hydroxide ions, but react with water to form acids.
- They can react with bases (alkalis) to produce a salt and water.
- Do not react with acids, because they are themselves acidic.
- Examples:
- \( \text{CO}_2 \) – Carbon dioxide
- \( \text{SO}_2 \) – Sulfur dioxide
- Reactions:
\( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \) (carbonic acid)
\( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \) (sulfurous acid)
Basic oxides:
- Formed from metals.
- React with acids to form a salt and water.
- Some basic oxides are insoluble in water (e.g. \( \text{CuO} \)), while others dissolve to form alkaline solutions (e.g. \( \text{CaO} \)).
- Do not react with bases, because they are themselves basic.
- Examples:
- \( \text{CuO} \) – Copper(II) oxide
- \( \text{CaO} \) – Calcium oxide
- Reactions:
\( \text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} \)
\( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \) (forms limewater)
Example
Identify whether each of the following oxides is acidic or basic: \( \text{CO}_2 \), \( \text{SO}_2 \), \( \text{CuO} \), and \( \text{CaO} \). Justify your classification with reasoning.
▶️Answer/Explanation
\( \text{CO}_2 \) – This is a non-metal oxide. It dissolves in water to form carbonic acid: \( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \). Therefore, it is an acidic oxide.
\( \text{SO}_2 \) – Another non-metal oxide that forms sulfurous acid in water: \( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \). Hence, it is also an acidic oxide.
\( \text{CuO} \) – Copper(II) oxide is a metal oxide. It reacts with acids but not with bases, forming salt and water. For example: \( \text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} \). It is a basic oxide.
\( \text{CaO} \) – Calcium oxide is a reactive metal oxide that forms an alkali in water: \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \). It is a basic oxide.
Amphoteric Oxides
Amphoteric Oxides
An amphoteric oxide is a special type of oxide that can react with both acids and bases to form a salt and water. These oxides show both acidic and basic behaviour depending on what they are reacting with.
- They do not behave like neutral oxides (which are unreactive), but instead react with both acids and alkalis.
- Usually formed by metals with both ionic and covalent character in their oxides.
- Examples include:
- \( \text{Al}_2\text{O}_3 \) – Aluminium oxide
- \( \text{ZnO} \) – Zinc oxide
Reactions of Amphoteric Oxides:
With acids:
\( \text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O} \)
With alkalis (e.g. sodium hydroxide):
\( \text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] \)
This forms a complex ion called sodium zincate, showing that the oxide is behaving as an acid in this case.
Example
Explain why \( \text{ZnO} \) is classified as an amphoteric oxide, and provide balanced equations for its reactions with both acids and alkalis.
▶️Answer/Explanation
Reason: Zinc oxide reacts with both acids and alkalis. This dual behaviour classifies it as amphoteric.
Reaction with hydrochloric acid (acts as a base):
\( \text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O} \)
Reaction with sodium hydroxide (acts as an acid):
\( \text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] \)
Since it reacts with both an acid and a base to form salt and water (or complex salt), it is an amphoteric oxide.
Two important amphoteric oxides at IGCSE level are:
- Aluminium oxide (\( \text{Al}_2\text{O}_3 \))
- Zinc oxide (\( \text{ZnO} \))
Why are they amphoteric?
- They behave as bases when reacting with acids → forming salt and water.
- They behave as acids when reacting with alkalis → forming complex salts in solution.
Examples of their reactions:
1. Reactions of \( \text{Al}_2\text{O}_3 \):
With hydrochloric acid (as a base):
\( \text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} \)
With sodium hydroxide (as an acid):
\( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na[Al(OH)}_4] \)
2. Reactions of \( \text{ZnO} \):
With sulfuric acid (as a base):
\( \text{ZnO} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\text{O} \)
With sodium hydroxide (as an acid):
\( \text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] \)
Example
Explain why both \( \text{Al}_2\text{O}_3 \) and \( \text{ZnO} \) are considered amphoteric oxides. Write balanced equations to support your answer.
▶️Answer/Explanation
Amphoteric behaviour means the oxide reacts with both acids and alkalis.
For \( \text{Al}_2\text{O}_3 \):
With acid: \( \text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} \)
With base: \( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na[Al(OH)}_4] \)
For \( \text{ZnO} \):
With acid: \( \text{ZnO} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\text{O} \)
With base: \( \text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] \)
Thus, both oxides exhibit amphoteric behaviour and are classified accordingly.