Question

When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reason for this type of colour change.

Answer 1

Hint: Alkali metal dissolves in liquid ammonia and gives a certain coloured solution. The colour changes as the concentration of the metal increases. When we add liquid ammonia to alkali metals, we get a solution where there are free ammoniated electrons and ammoniated metal ions.

Complete step by step solution:
We know that the elements of group 1 are called the alkali metals. There are 6 alkali metals- lithium, sodium, potassium, rubidium, caesium and francium.
They are called alkali metals because they form alkali on the reaction with water. Alkali is a strong base which can neutralise an acid.
These alkali metals react with liquid ammonia. They dissolve reversibly in liquid ammonia. The reaction of alkali metals with liquid ammonia gives us hydrogen gas and the metal salt of the conjugate base of the solvent. It forms an ammoniated solution by the formation of an amide ion. We can write the reaction as-
     \[M(s)+N{{H}_{3}}(l)\to \dfrac{1}{2}{{H}_{2}}(g)+{{M}^{+}}+N{{H}_{2}}^{-}\]
The solution is ammoniated here and it forms an ammoniated metal ion and ammoniated electrons. This ammoniated solution of alkali metal is intensely coloured and a good conductor of electricity due to the presence of solvated ammonia electrons. The solvated electron is loosely attached to the ammonia solvent and is stabilised by hydrogen bonding.
The colour of the solution changes as the concentration of the metal changes.
With an increase in the concentration of metal, the solution of the colour changes, a 3M solution is intense blue coloured and changes to metallic copper bronze. The solution also gets a metallic lustre due to the formation of metal clusters.
When the solution is blue coloured, it is paramagnetic and slowly becomes diamagnetic when the solution becomes golden.

Therefore, we can say that this colouration of the solution is due to the presence of ammoniated free electrons in the solution.

Note: The ammoniated solution of alkali metals conducts electricity due to the presence of free electrons in the solution. The high mobility of the solvated electron results in higher conductivity. The conductivity of the deep blue solution is about 10 times higher than that of an aqueous sodium chloride solution and as the concentration of metal increases, the conductivity also increases and is comparable to that of pure liquid metals.