General characteristic of the compounds of the alkali metals

All the alkali metals, their oxides, peroxides and superoxides readily dissolve in water to produce corresponding hydroxides which are strong alkalies eg

2Na + 2H₂O → 2NaOH + H₂                   

Na₂O + 2H₂O 2NaOH               

Na2O₂ + 2H₂O → 2NaOH + H₂O₂               

2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂               

Thus peroxides and superoxides also act as oxidizing agents since they react withH₂O forming H₂O₂ and O₂ respectively. The hydroxides of all the alkali metals are white crystalline solids. They are strongest of all base and readily dissolve in water with the evolution of much heat.

The basic strength of these hydroxides increases as we move    down the group Li to Cs. The hydroxides of alkali metals behave as strong bases due to their low ionization energies which decrease down the group. The decrease in ionization energies leads to weakening of the bond between metal and hydroxide ion and M – O bond in M – O – H can easily break giving M⁺ and OH^- This results in the increased concentration of hydroxyl ions in  the solution i.e increased basic characters.

(b) Solubility and stability

All these hydroxides are highly soluble in water and thermally    stable except lithium hydroxide.

(c)  Formation of salts with acids:

Alkali metals hydroxides being strongly basic react with all acids forming salts.

NaOH + HCI → NacI + H₂O               

 2NaOH + H₂ SO₄ → Na₂SO₄ + 2H₂O              

Name the alkali metal which forms superoxides when heated in excess of air and why?

Potassium forms superoxides when heated in excess of air. This is due to the stabilization of large size cation by large size anion.

Why lithium forms only lithium oxide and not peroxide or super oxides?

Due to the small size of lithium, it has a strong positive field around it. On combination with the oxide anion (O^2–), the positive field of lithium ion restricts the spread of negative charge towards another oxygen atom and thus prevents the formation of higher oxides.

Account for the fact that the basicity of the alkali metal hydroxides increases down the group.