Question

When a metal of group 1 was dissolved in liquid ammonia, the following observations were obtained:

(i) Blue solution was obtained initially.

(ii) On concentrating the solution, blue colour changed to bronze colour.

How do you account for the blue colour of the solution? Give the name of the product formed on keeping the solution for some time.

Answer 1

(i) The alkali metals dissolve in liquid ammonia and give a blue solution, which is conductive in nature. A solution of sodium in liquid ammonia at – 30°C conducts electricity. The ammoniated electrons are responsible for the blue color of the solution as they absorb energy in the visible region of light and impart blue color to the solution. Both the ammoniated cations and ammoniated electrons are responsible for the electrical conductivity of the solution.

\[\ce{Na + (x + y)NH3 -> [Na(NH3)x]^{+} + [e(NH3)y]-}\]

(ii) The blue color changes to bronze color in concentrated solution due to the formation of a cluster of metal ions. The standing blue solution liberates hydrogen gas with the formation of amide.

\[\ce{M^{+} + e- + NH3 -> MNH2 + 1/2 H2}\]