General characteristic of the compounds of the alkali metals
(i) Oxides and Hydroxides
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.
(a) Basic Strength
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.
2LiOH → Li2O + H2O
(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.
K + O2 → KO2
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 (O2–), the positive field of lithium ion restricts the spread of negative charge towards another oxygen atom and thus prevents the formation of higher oxides.
What happens when potassium superoxide is dissolved in water?
Account for the fact that the basicity of the alkali metal hydroxides increases down the group.