Chapter 3 · Question 3

How do metals react with oxygen? Taking aluminium as an example, explain the concept of amphoteric oxides. Write balanced chemical equations for the reaction of aluminium oxide with hydrochloric acid and with sodium hydroxide.

Answer Revealed

Direct Answer:

Almost all metals combine with oxygen on heating to form metal oxides: Metal + Oxygen → Metal oxide. Most metal oxides are basic in nature, but some metal oxides such as aluminium oxide and zinc oxide react with both acids and bases to produce salt and water — these are called amphoteric oxides. Aluminium burns in air to form aluminium oxide:

4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3

. Its amphoteric nature is shown by: (i) With HCl:

\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}

. (ii) With NaOH:

\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}

(sodium aluminate).

Simple Explanation

Metals burn in air to form metal oxides. Most metal oxides are basic — they neutralise acids. But aluminium oxide and zinc oxide are special: they are 'amphoteric,' meaning they can neutralise BOTH acids and bases. Aluminium oxide plus HCl gives aluminium chloride and water. Aluminium oxide plus NaOH gives sodium aluminate and water. It is like a chemical diplomat — friendly with both sides.

Exam-Ready Structure

The reaction of metals with oxygen is the first chemical property explored in NCERT Chapter 3, demonstrating reactivity trends and the concept of amphoteric oxides. 1. General reaction: Most metals react with oxygen to form metal oxides. It is a combination reaction: Metal + Oxygen → Metal oxide. 2. Reactivity trends: (a) Sodium and potassium react so vigorously that they catch fire if kept in air — they are stored in kerosene. (b) Magnesium burns with a dazzling white flame to form MgO. Aluminium, zinc, and lead form a thin protective oxide layer at room temperature that prevents further oxidation. (c) Iron does not burn on heating, but iron filings burn vigorously when sprinkled into a flame. (d) Copper does not burn but on prolonged heating forms a black coating of CuO. (e) Silver and gold do not react with oxygen even at high temperatures. 3. Aluminium oxide as an amphoteric oxide:

4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3

. While most metal oxides are basic, Al₂O₃ and ZnO exhibit amphoteric behaviour — they react with both acids and bases. 4. Reaction with acid (HCl):

\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}

(aluminium chloride and water). 5. Reaction with base (NaOH):

\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}

(sodium aluminate and water). 6. Zinc oxide (ZnO) also shows amphoteric behaviour. 7. Most metal oxides are insoluble in water, but sodium oxide and potassium oxide dissolve to form alkalis:

\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}

and

\text{K}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{KOH}

Key Points

Metals + O₂ → Metal oxides (combination reaction); most are basic oxides
Amphoteric oxides react with BOTH acids and bases to give salt and water
Examples of amphoteric oxides: Al₂O₃ (aluminium oxide) and ZnO (zinc oxide)
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
NaO and K₂O are soluble metal oxides that form alkalis in water (NaOH, KOH)
Ag and Au do not react with oxygen; Na and K are stored in kerosene to prevent reaction

How do metals react with oxygen? Taking aluminium as an example, explain the concept of amphoteric oxides. Write balanced chemical equations for the reaction of aluminium oxide with hydrochloric acid and with sodium hydroxide.