4.0 Compounds of Aluminium

  p Block Elements

1. Aluminium Oxide: Alumina $(Al_2O_3)$

Preparation:

1. Using calcination: $2Al{(OH)_3}\mathop \to \limits^{calcination} A{l_2}{O_3} + 3{H_2}O$

Properties:

1. Alumina is also known as corundum used as gemstone.

2. $Al_2O_3$ is amphoteric oxide.

3. Alumina exists mostly in two crystalline forms called $\alpha$- $Al_2O_3$ and $\gamma $-$Al_2O_3$. $$2Al{(OH)_3}\mathop \to \limits^{450^\circ C} \gamma A{l_2}{O_3}\mathop \to \limits^{1000^\circ C} \alpha A{l_2}{O_3}$$

2. Alums:

They are obtained by mixing equimolar quantities of their constituent sulphates.

General formula: ${\left[ {M\left( I \right)} \right]_2}S{O_4}M''\left( {III} \right)S{O_4}.x{H_2}O$

where ${\left[ {M{{\left( {{H_2}O} \right)}_6}} \right]^ + }$, ${\left[ {M''{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$ and $S{O_4}^{2 - }$ are in ratio $1:1:2$.

Alums are:

• Potash alum ${K_2}S{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$

• Ammonium alum ${\left( {N{H_4}} \right)_2}S{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$

• Sodium alum $N{a_2}S{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$

• Ferric alum ${\left( {N{H_4}} \right)_2}S{O_4}.F{e_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$

Properties:

They are more soluble in hot water and less in cold water and also loose water of crystallisation when heated. When strongly heated give burnt alum.