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Question

Which among the following boron family element form A2O3 type oxide on heating in air?

A
B
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B
Al
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C
Neither a nor b
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D
Both (a) and (b) above.
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Solution

The correct option is D Both (a) and (b) above.
Amorphous boron and aluminium metal on heating in air form B2O3 and Al2O3.
4B(s)+3O2(g)Δ2B2O3(s)
4Al(s)+3O2(g)Δ2Al2O3(s)

THEORY
1. Reactivity towards air:
Boron is unreactive in crystalline form; Al forms a very thin oxide layer on the surface.

Amorphous boron and aluminium metal on heating in air form B2O3 and Al2O3
4E(s)+3O2(g)2E2O3(s)
(Where E = B, Al)
The nature of the oxides varies down the group.
Boron trioxide is acidic.
Aluminium and gallium oxides are amphoteric.
Indium and thallium Oxides are basic.

2. They react with dinitrogen at high temperature to form nitrides.
2E(s)+N2(g)2EN(s)
(Where E = B, Al)

3. Reactivity towards Acids and Alkalies:
Boron does not react with acids and alkalies even at moderate temperature.
Aluminium dissolves in mineral acids and aqueous alkalies and thus shows its amphoteric nature.
2Al(s)+6HCl(aq)2Al3+(aq)+6Cl(aq)+3H2(g)
Aluminium also reacts with aqueous alkali and liberates dihydrogen.
2Al(s)+2NaOH(aq)+6H2O(l)2Na+[Al(OH)4](aq)+3H2(g)Sodium tetrahydroxoaluminate (III)
Concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.

4. Reactivity towards Halogens
These elements react with halogens to form trihalides (except TlI3)
2E(s)+3X2(g)2EX3(s)
(X = F, Cl, Br, I)

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