Question

Which among the following boron family element form $A_2{O}_3$ type oxide on heating in air?

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

Answer 1

The correct option is D Both (a) and (b) above.

Amorphous boron and aluminium metal on heating in air form $B_2{O}_3$ and $A{l}_2{O}_3.$
$4B\left(s\right)+3O_2\left(g\right)\stackrel{\Delta }{\to }2B_2{O}_3\left(s\right)$
$4Al\left(s\right)+3O_2\left(g\right)\stackrel{\Delta }{\to }2A{l}_2{O}_3\left(s\right)$

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 $B_2{O}_3$ and $A{l}_2{O}_3$
$4E\left(s\right)+3O_2\left(g\right)\stackrel{△}{\to }2E_2{O}_3\left(s\right)$
(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.
$2E\left(s\right)+N_2\left(g\right)\stackrel{△}{\to }2EN\left(s\right)$
(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.
$2Al\left(s\right)+6HCl\left(aq\right)\to 2A{l}^{3+}\left(aq\right)+6C{l}^{-}\left(aq\right)+3H_2\left(g\right)$
Aluminium also reacts with aqueous alkali and liberates dihydrogen.
$2Al\left(s\right)+2NaOH\left(aq\right)+6H_{2}O\left(l\right)\to \underset{\text{Sodium tetrahydroxoaluminate (III)}}{2N{a}^{+}\left[Al\right(OH{)}_4{]}^{-}\left(aq\right)+3H_2\left(g\right)}$
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 $Tl{I}_3$)
$2E\left(s\right)+3X_2\left(g\right)\to 2E{X}_3\left(s\right)$
(X = F, Cl, Br, I)