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QUESTION

# When sodium metal is dissolved in liquid ammonia a blue solution is formed which is highly conducting because electrons are solvated. Dissolving more sodium in solution results in a reduction in conductivity. Why is this?

The redox chemistry of sodium in liquid ammonia is fairly complex. Solvated electrons form (to give the blue colour). As more metal is added the electrons COUPLE to form bronzes, reducing conductivity.

Ammonia is a water like , that allows much more basic chemistry. When sodium is added to liquid ammonia, a beautiful blue colour forms, which is attributed to a solvated electron, which has a fairly long lifetime. On further reduction (i.e. addition of further sodium), the electrons are conceived to couple (i.e. 2 electrons are solvated by a cluster of ammonia molecules) to give the so called sodium bronze (and a reduction in electrical conductivity). These are very beautiful reactions to do, though admittedly smelly and time consuming.

The acid base chemistry of any solvent is limited to the properties of the solvent. Liquid ammonia solvents allow much more basic chemistry than is possible in water. The blue colour is the solvated electron, an electron that is stabilized by several ammonia molecules. Solvated electrons are also possible in water, but in this solvent the electrons have a vanishingly short lifetime. Note that this reaction should be demonstrated in 2nd year inorganic chemistry: you need a cylinder of ammonia gas, and a condenser cooled by dry ice and acetone to condense the ammonia.

As regards ammonia, the balanced equation with sodium is:

Na(s) + NH_3(l) rarr 1/2H_2(g)uarr + NaNH_2(s)darr.

Sodium amide is a white solid. This is very similar to the reaction of sodium metal with water.