11th Class Chemistry The s-Block Elements

  • question_answer 15) When an alkali metal dissolves in liquid ammonia, the solution can acquire different colours. Explain the reasons for this type of colour change.

    Answer:

    The alkali metal atoms on dissolution in liquid ammonia ionise according to following equation: \[M+(x+y)N{{H}_{3}}\to {{M}^{+}}{{(N{{H}_{3}})}_{x}}+{{e}^{-}}{{(N{{H}_{3}})}_{y}}\] The solutions thus contain ammoniated cations and ammoniated electrons. When light falls on this solution, the ammoniated electrons are excited by absorbing energy corresponding to red light and the light transmitted is blue. This imparts blue colour to the solution. This colour is observed in dilute solutions. However, when concentration increases above 3M the colour changes to bronze and solution acquires metallic lustre due to the formation of metal ion clusters.    

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