Oxides
Non-metallic elements react with oxygen to form acidic oxides. These oxides do not dissolve in water to form alkalis; instead, they react with bases or alkalis to form salts and water. Some acidic oxides (like CO_2 and SO_2) dissolve in water to form weak acids.
Key Examples:
- Carbon dioxide (CO_2): Formed from carbon (a non-metal).
- Sulfur dioxide (SO_2): Formed from sulfur (a non-metal).
- Phosphorus(V) oxide (P_2O_5): Formed from phosphorus (a non-metal).
Why is this important?
This relates to the concept of neutralization. Just as an acid reacts with a base, an acidic oxide acts as an acid in chemical reactions.
2. Basic Oxides (from Metals)
Metallic elements react with oxygen to form basic oxides. These oxides are often insoluble in water but react with acids to form salts and water. They neutralize acidic solutions.
Key Examples:
- Calcium oxide (CaO): Formed from calcium (a metal).
- Copper(II) oxide (CuO): Formed from copper (a metal). Note: CuO is a classic example of a basic oxide used in exams to test if you recognize that transition metals also form basic oxides.
- Magnesium oxide (MgO): Formed from magnesium (a metal).
Some metal oxides exhibit properties of both acids and bases. These are called amphoteric oxides. They react with both acids and bases to produce a salt and water.
Key Examples:
- Aluminium oxide (Al_2O_3): Aluminium is a metal, but its oxide is amphoteric.
- Zinc oxide (ZnO): Zinc is a metal, but its oxide is amphoteric.
Building on the previous concept, while most metal oxides are basic, Al_2O_3 and ZnO are the primary exceptions required for this syllabus.
| Amphoteric Oxides |
|---|
| Metal (specifically Al, Zn) |
| Reacts to form salt + water |
| Reacts to form salt + water |
| Al_2O_3, ZnO |
This definition is critical because it distinguishes them from basic oxides (which only react with acids) and acidic oxides (which only react with bases). The term 'amphoteric' comes from the Greek word for 'both', reflecting this dual reactivity.
1. Reaction with an Acid (acting as a base):
When ZnO reacts with dilute sulfuric acid (H_2SO_4), it behaves like a typical basic oxide.
ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)
Here, zinc sulfate and water are produced. The oxide accepts protons (H^+) from the acid.
2. Reaction with a Base (acting as an acid):
When ZnO reacts with aqueous sodium hydroxide (NaOH), it behaves like an acidic oxide. It dissolves to form a complex ion.
ZnO(s) + 2NaOH(aq) + H_2O(l) \rightarrow Na_2<a href="aq">Zn(OH)_4</a>
Here, sodium tetrahydroxozincate(II) is formed. The oxide donates protons or accepts hydroxide ions.
Similarly, Aluminium oxide (Al_2O_3) reacts with NaOH to form sodium tetrahydroxoaluminate(III).
The Error: Students often assume that because Copper (Cu) is a transition metal or less reactive than Calcium, its oxide might be neutral or acidic. Others mistakenly think all metal oxides are basic and forget the amphoteric exceptions.
The Correct Understanding:
- Copper(II) oxide (CuO) is definitely a basic oxide. The rule is based on whether the element is a metal or non-metal, not its reactivity series position. Copper is a metal, so CuO is basic.
- Amphoteric oxides are specific exceptions. Only certain metals (primarily Aluminium and Zinc in this syllabus) form amphoteric oxides. Do not generalize this to all transition metals.
Why examiners accept this: Examiners look for the application of periodic trends. The boundary between metals and non-metals runs diagonally from Boron to Astatine. Elements to the left are metals (forming basic oxides), and elements to the right are non-metals (forming acidic oxides).
Correct Usage Example:
Question: 'Classify the oxide of element X, which is in Group 17.'
Answer: 'Element X is a halogen (non-metal). Therefore, its oxide is acidic.'
Do not just say 'it's acidic'. Explicitly link it to the non-metallic character of the parent element.