Taylor12SolvatedElectron p.1
Solvated electrons
Source: Chemistry At Work by J.R.Taylor 1992 (Publisher John Murray) Exercise 1.2
Activation energy, electrolysis, Group I, solvation
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Liquid ammonia, which may be formed by cooling the gas to below -33C, is an
example of a non-aqueous solvent which will dissolve ionic species. Many analogies
may be drawn between reactions which occur in ammonia and those in water. Both
solvents are slightly self-ionising. In water we define an alkali as a substance which
dissolves giving OH- ions; so in liquid ammonia we may define an alkali as a
substance which dissolves to give NH2- ions. Thus NaNH2 is an alkali in ammonia
(equivalent in water: NaOH), and so is K2NH (equivalent to K2O). Ammonium salts
may be regarded as acids in liquid ammonia, and a typical neutralization reaction
might be:
NH4Cl + KNH2  KCl + 2 NH3
One of the most remarkable properties of the alkali metals is their ability to
dissolve in liquid ammonia, forming solutions which in some ways resemble liquid
metals. On evaporation of the solvent, the solid metal is reformed (except in the case
of lithium). It is known that these solutions contain alkali metal cations and
electrons, e.g.:
Na(s) + am  Na+(am) + e-(am)
[am = ammonia]
The removal of an electron from an alkali metal in the gas phase is highly
endothermic, but in solution both the metal cation and the electron are heavily
solvated, i.e., surrounded by ammonia molecules which are tightly held by ion-dipole
forces, so the overall reaction is exothermic. The properties of the solutions are
determined largely by the properties of the solvated electron, which we can think of as
the simplest odd-electron species. The electron is highly mobile, and so the solution
has a high conductivity, higher than solutions of most alkali metal salts. Alkali metal
ions are colourless, but the solvated electron absorbs in the red and infra-red region of
the electromagnetic spectrum. Thus the solutions of alkali metals in ammonia all
have the same blue colour.
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At first sight it might seem that there are few similarities between the actions of
water and ammonia on the alkali metals. Ammonia produces a solution containing
electrons, whereas water reacts readily evolving hydrogen. The reason for the
difference is the variation in the stability of the solvated electron in the two solvents.
It has been shown that the lifetime of an electron in water is less than one thousandth
Taylor12SolvatedElectron p.2
of a second.
It reacts as follows:
e (aq) + H2O(l)  OH-(aq) + ½ H2(g)
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We may therefore imagine the reaction of sodium with water as involving the
formation of Na+ ions and electrons, followed by reaction of the electrons with the
solvent. In contrast, the electron is stable almost indefinitely in ammonia if the
solvent is pure, and so a metal solution is formed. If a suitable catalyst such as
iron(III) oxide is added, however, then decomposition is rapid:
e-(am) + NH3(l)  NH2-(am) + ½ H2(g)
which is precisely analogous to the reaction in water. The reaction of ammonia with
the electron is energetically favourable, but in the absence of a suitable catalyst the
activation energy is too high for the reaction to occur at a suitable rate. The catalyst
reduces the activation energy by providing an alternative pathway, and decomposition
then occurs rapidly.
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Adapted from Odd Electron Species, P.R. Scott
Cambridge University Press (1981)
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Questions
1. Write equations for the self-ionization of water and ammonia.
2. Write an equation for the reaction of K2NH with ammonia. How is this
analogous to the corresponding aqueous reaction?
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4.
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What name is given to the removal of an electron in the gas phase? Give a
precise definition, illustrating your answer with an equation for sodium.
Explain the meaning of the terms
a) odd-electron species;
b) catalyst;
c) activation energy
Draw diagrams to illustrate the way in which a sodium cation and an electron
become solvated, using four solvent molecules for each, clearly indicating the
method of bonding.
Draw a suitable energy level diagram for the processes which occur during the
dissolution of sodium in liquid ammonia.
What reactions, if any, will occur at the anode and cathode during the passage of
an electric current through a solution of sodium in liquid ammonia? Is the
author justified in referring to ‘conduction’ rather than electrolysis?
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