Elements react with oxygen (aka. burn in oxygen) in the atmosphere to form oxides.
Oxides are classified as:
- Acidic
- Basic
- Amphoteric
- Neutral
Acidic Oxides
Acidic oxides react with water to form acids.
- Recall: Solutions of these acids in water have a pH value of less than 7.
Acidic oxides are oxides of non-metals, which is also known as non-metallic oxides. Examples:
- $\text{Carbon} + \text{oxygen } \rightarrow \text{ carbon dioxide}$ $$\text{C} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow \text{CO}_{2} (\text{g})$$
- $\text{Sulphur} + \text{oxygen } \rightarrow \text{ sulphur dioxide}$ $$\text{S} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow \text{SO}_{2} (\text{g})$$
Examples of acidic oxides are shown in the table below:
Acidic Oxides | Formula | Acid produced with water |
---|---|---|
Sulphur Dioxide | $\text{SO}_{2}$ | Sulphurous Acid $\text{H}_{2}\text{SO}_{3}$ |
Sulphur Trioxide | $\text{SO}_{3}$ | Sulphuric Acid $\text{H}_{2}\text{SO}_{4}$ |
Carbon Dioxide | $\text{CO}_{2}$ | Carbonic Acid $\text{H}_{2}\text{CO}_{3}$ |
Phosphorus(V) Oxide | $\text{P}_{4}\text{O}_{10}$ | Phosphoric Acid $\text{H}_{3}\text{PO}_{4}$ |
Basic Oxides
Basic oxides react with acid to produce salt and water only.
- Notice that this is a neutralization reaction.
- Hence, basic oxides neutralize acids.
Basic oxides that are soluble in water are called alkalis.
- Recall: Solutions of these alkalis in water have a pH value of more than 7.
Basic oxides are oxides of metals, which is also known as metallic oxides. Examples:
- $\text{Sodium} + \text{oxygen } \rightarrow \text{ sodium oxide}$ $$\text{Na} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow 2\text{Na}_{2}\text{O} (\text{s})$$
- $\text{Copper} + \text{oxygen } \rightarrow \text{ copper(II) oxide}$ $$\text{Cu} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow 2\text{CuO} (\text{s})$$
Examples of basic oxides are shown in the table below:
Basic Oxides | Formula |
---|---|
Sodium Oxide | $\text{Na}_{2}\text{O}$ |
Magnesium Oxide | $\text{MgO}$ |
Calcium Oxide | $\text{CaO}$ |
Copper Oxide | $\text{CuO}$ |
Iron (III) Oxide | $\text{Fe}_{2}\text{O}_{3}$ |
Amphoteric Oxides
Amphoteric oxides behave as an acidic oxide or as a basic oxide.
- Amphoteric oxides show both acidic and basic properties.
- Amphoteric oxides can react with both acids and alkalis to form salts and water. (Recall: Neutralization reaction)
Examples of amphoteric oxides are shown in the table below:
Amphoteric Oxides | Formula |
---|---|
Aluminium Oxide | $\text{Al}_{2}\text{O}_{3}$ |
Zinc Oxide | $\text{ZnO}$ |
Lead(II) Oxide | $\text{PbO}$ |
Reactions of amphoteric oxides with acids:
- Aluminium oxide with hydrochloric acid $$\text{Al}_{2}\text{O}_{3} + 6 \text{HCl} \rightarrow 2\text{AlCl}_{3} + 3\text{H}_{2}\text{O}$$
- Zinc oxide with sulphuric acid $$\text{ZnO} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{ZnSO}_{4} + \text{H}_{2}\text{O}$$
- Lead(II) oxide with nitric acid $$\text{PbO} + 2 \text{HNO}_{3} \rightarrow \text{Pb}\left( \text{NO}_{3} \right)_{2} + \text{H}_{2}\text{O}$$
Reactions of amphoteric oxides with bases:
- Aluminium oxide with sodium hydroxide to form sodium aluminate $$\text{Al}_{2}\text{O}_{3} + 2 \text{NaOH} \rightarrow 2\text{NaAlO}_{2} + 3\text{H}_{2}\text{O}$$
- Zinc oxide with sodium hydroxide to form sodium zincate $$\text{ZnO} + 2 \text{NaOH} \rightarrow \text{Na}_{2}\text{ZnO}_{2} + \text{H}_{2}\text{O}$$
- Lead(II) oxide with sodium hydroxide to form sodium plumbate(II) $$\text{PbO} + 2 \text{NaOH} \rightarrow \text{Na}_{2}\text{PbO}_{2} + \text{H}_{2}\text{O}$$
Neutral Oxide
Neutral oxide has no acidic or basic properties. Examples:
- Water
- Carbon monoxide