IGCSE Chemistry 0620

Topic: Acids, Bases and Salts - Oxides

Objective: Classify $Al_2O_3$ and $ZnO$ as amphoteric oxides

This section focuses on understanding oxides and their classification, specifically highlighting amphoteric oxides like $Al_2O_3$ and $ZnO$. We will explore the properties of oxides and how they react with both acids and bases.

What are Oxides?

Oxides are chemical compounds formed from two or more elements, one of which is oxygen. They can be classified as acidic, basic, or neutral depending on their reactivity with acids and bases.

Classification of Oxides

Oxides are broadly classified into three categories:

• Acidic Oxides: These oxides react with water to form acidic solutions. Examples include $SO_3$, $CO_2$, and $P_2O_5$.
• Basic Oxides: These oxides react with water to form basic (alkaline) solutions. Examples include $Na_2O$ and $CaO$.
• Neutral Oxides: These oxides do not react with water to form acidic or basic solutions. Examples include $N_2O$ and $ZnO$.

Amphoteric Oxides

Amphoteric oxides are oxides that can react with both acids and bases. This means they exhibit both acidic and basic properties. The most common examples are $Al_2O_3$ and $ZnO$.

Reaction of Amphoteric Oxides with Acids

$Al_2O_3$ reacts with acids to form a soluble salt and water.

$ZnO$ reacts with acids to form a soluble salt and water.

Reaction of Amphoteric Oxides with Bases

$Al_2O_3$ reacts with bases to form a soluble aluminate salt and water.

$ZnO$ reacts with bases to form a soluble zincate salt and water.

Chemical Equations

Reaction	Reactants	Products
Reaction with Acid	$Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)$	Soluble salt (e.g., $AlCl_3$), Water
Reaction with Acid	$ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$	Soluble salt (e.g., $ZnCl_2$), Water
Reaction with Base	$Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2Na[Al(OH)_4](aq)$	Soluble salt (e.g., $Na[Al(OH)_4]$), Water
Reaction with Base	$ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$	Soluble salt (e.g., $Na_2ZnO_2$), Water

Why are $Al_2O_3$ and $ZnO$ Amphoteric?

The amphoteric nature of $Al_2O_3$ and $ZnO$ arises from the presence of both acidic (oxygen atoms) and basic (metal ions) components in their structure. The oxygen atoms can donate electrons to a proton (acid), while the metal ions can accept electrons from a hydroxide ion (base). This dual behavior is characteristic of amphoteric substances.

Conclusion

$Al_2O_3$ and $ZnO$ are important examples of amphoteric oxides, demonstrating their ability to react with both acids and bases. This property is crucial in various chemical applications.