Class 11 Chemistry The S Block Elements Chemical Properties of alkali metals

Chemical Properties of alkali metals

(i) Reactivity towards air

Alkali metals get tarnished in the presence of dry air due to the formation of their respective oxides.

The metallic oxide so formed further reacts with moisture to form hydroxides.

The alkali metals burn vigorously in oxygen and form their respective oxides.

4Li + O2 --> 2Li2O (Oxide)

 

 (ii) Reactivity towards water:

The alkali metals react with water and forms hydroxide and dihydrogen.

Lithium due to its small size and very high hydration energy does not react with water vigorously even though it has most negative E0 value.

But on the other hand sodium do reacts with water vigorously even though it has least negative E0 value.

(iii) Reactivity towards dihydrogen

The alkali metals react with dihydrogen at about 673K (lithium at 1073K) and form hydrides.

2M + H2 --> 2M+H-

(iv) Reactivity towards halogens

The alkali metals react vigorously with halogens and form ionic halides, M+X

(v) Reducing nature

The alkali metals are strong reducing agents.

Lithium is the most powerful and sodium is the least powerful.

(vi) Solutions in liquid ammonia

The alkali metals dissolve in liquid ammonia and impart a deep blue colour to the solutions. The colour in the solution nature is due to the ammoniated electron which absorbs the energy in the visible region of light and imparts the blue colour to the solution.

The solutions are paramagnetic and liberate hydrogen on standing which in turn results in the formation of amide.

The blue colour in concentrated solution changes to bronze colour and becomes diamagnetic.

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