Self-ionization liquid ammonia

It was found that this heterogeneous reaction can be stopped by the addition of small amounts of lithium. It is likely that lithium with its higher electrode potential is able to prevent the self-ionization of liquid ammonia, and the heterogeneous reaction between ammonium in solution and liquid ammonia is inhibited. When this heterogeneous reaction is prevented, the decomposition follows a perfectly satisfactory second-order reaction, and the temperature must be raised up to the range from zero to 20° in order to obtain a measurable reaction rate. The reaction is satisfactorily explained on the assumption that ammonium dissociates in mercury giving ammonium ions and free electrons NH4—>NH4+ -be . The older view that the ammonium exists as a free radical, NH4, seems less likely. Such a radical would be unstable and it would not be expected to have such a long life. Ammonium ions, however, are more stable. 

These results emphasize the fact that ions produced by self-ionization of the solvent, e.g., HsO and 0H in water, ROHt and R0 in alcohols, and NHi" and NHJ in liquid ammonia, do not of necessity possess abnormal conductance, although they frequently do so. It is seen from Table XIX that the conductance of the hydrogen ion in liquid ammonia, i.e., NHi", is normal the same is true for the NHi" ion. The anilinium and pyridinium ions also have normal conductances in the corresponding solvents. The conductance of the HSOr ion in sulfuric acid as solvent is, however, abnormally high it is probable that a Grotthuss type of... 

If the solvent is water, these two ions are always H+(aqj and OH (aqj, but in the case of liquid ammonia, which is also a good solvent, the corresponding ions would be NH4 and NH2. That the solvent does play some special role is implied by the self-ionization reactions... 

Liquid ammonia is associated through hydrogen bonds, the bond energies of which are somewhat smaller than those in water. The small conductivity of the purified solvent has been attributed to self-ionization of the liquid and this has been confirmed by potentiometric measurements 25-... 

The ionic product of the self-ionization of liquid ammonia has been determined from potentiometric measurements using the cellos... 

In liquid ammonia solution, an acid may be defined as a substance that provides NH4 ions, while a base produces amide ions, NH2 . More generally in a self-ionizing solvent, an acid is a substance that produces the cation characteristic of the solvent, and a base is a substance that produces the anion characteristic of the solvent. 

Self-ionization is exhibited by many pure liquids for example, in liquid ammonia. 

The commercial preparation of ammonia is accomplished in huge quantities by the Haber process [Equation (16.12)], discussed in Section 16.4 under Nitrogen Fixation. In the laboratory the most common preparation is the treatment of ammonium salts with strong bases, as represented in Equation (16.13).The self-ionization of liquid ammonia (analogous to that of liquid water) is shown in Equation (16.14). NH3(/) is often used as a nonaqueous solvent. As shown in Equation (16.15), NH3 acts as a weak base in water and serves as a prototype for a number of nitrogen-containing bases such as methylamine, pyridine, and aniline, which you may recall from studying acid-base equilibria in earlier courses. 

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