Metal-ammonia solutions explanation

After having listed the observed properties of metal-ammonia solutions in Section I and describing the various models for them in Section II, the important job now remains of discussing the explanation of the observed properties by the proposed models. 

The explanation of the optical band is not easy using the cavity model. Jortner showed that excited states higher than 2p are stable for the polarons in metal-ammonia solutions and deduced the following approximate relation for the energy values of higher hydrogen-like states with n > 2,... 

Explanation of Thermoelectric Data. The nature of the thermoelectric data obtained by Dewald and Lepoutre in saturated metal-ammonia solutions justifies their assumed metallic structure. ... 

In Part I- F the magnetic properties of metal-ammonia solutions were listed. As we have seen, the obseiwed magnetic properties consisted of results of total susceptibility measurements, spin susceptibility measurements using electron spin resonance techniques, dynamic features of electron spin resonance involving measurements on the relaxation times, and nuclear resonance studies. We shall first take up the explanation of the susceptibility data using the cavity, cluster, and unified models and subsequently consider the interpretation of the results of resonance studies. 

The qualitative explanation of the observed relaxation time data in dilute metal-ammonia solutions in terms of the cavity model is thus quite satisfactory. One does not have to consider the monomers in solutions with concentration equal to and less than O.OIM because the monomer concentration is lower by about a factor of... 

Careful electron-resonance work in other alkali metal-ammonia solutions besides sodium and potassium and over a wide range of frequencies is necessary to test the correctness of Poliak s explanation of the relaxation process in these solutions. 

Solutions of different acids having the same concentration might not have the same pH. For instance, the pH of 0.10 M CH3COOH(aq) is close to 3 but that of 0.10 M HCl(aq) is close to 1. We have to conclude that the concentration of H,() ions in 0.10 M CH3COOH(aq) is lower than that in 0.10 M HCl(aq). Similarly, we find that the concentration of OH ions is lower in 0.10 M NH,(aq) than it is in 0.10 M NaOH(aq). The explanation must be that in water CH.COOH is not fully deprotonated and NH3 is not fully protonated. That is, acetic acid and ammonia are, respectively, a weak acid and a weak base. The incomplete deprotonation of CH3COOH explains why solutions of HC1 and CH3COOH with the same molarity react with a metal at different rates (Fig. 10.14). 

Experimental evidence in support of this explanation is the fact that lithium added to a solution of lithium iodide in ethylenediamine dissolves without imparting a blue color to the solution—i.e., reacts immediately to give the amide. By contrast, lithium added to a solution of lithium chloride in ethylenediamine dissolves and imparts a deep blue color to the solution. The catalytic effect of iodide anion may be related to the effect of iodide anion on the electron spin resonance (ESR) absorption of solutions of alkali metals in liquid ammonia. Catterall and Symons (2) observed a drastic change in the presence of alkali iodides but very little change in the presence of alkali bromides or chlorides. They attributed this change to interaction of the solvated electron with the 6 p level of the iodide anion. 

It is observed in the experiment that the iron nail immediately creates a copper deposit in a blue colored copper sulfate solution (see E8.1), whereby this does not happen in the violet colored ammine complex solution. A trace of copper deposit can only be observed after it has been dipped into the complex solution for a while (see E9.6). It is possible to verify this hypothesis with the help of a second reaction, the metal hydroxide precipitation (see E9.6) a greenish blue deposit is commonly observed in the blue solution of hexaaquacopper ions, but not in the solution of tetraamminecopper ions. Apparently, copper ions and water molecules are not very tightly bonded in aqua complexes, but copper ions and ammonia molecules in ammine complexes are there is a weak stability of aquacopper ions, but a great stability of tetraamminecopper complexes. The stability constants can be taken and interpreted if one wants a quantitative explanation of these phenomena. 

The reactions between some metallic salts, ammonium salts, and ammonia will be taken up briefly. Many of the bivalent metals such as nickel, magnesium, etc., form hydroxides insoluble in water but soluble in solutions of ammonium salts. The generally accepted explanation for the solubility in solutions of ammonium salts or for the non-precipitation by ammonia, if ammonium salts are present, is that the ammonium ion of the ammonium salts drives back or represses the electrolytic dissociation of the ammonium hydroxide so that the hydroxide ion is not present in sufficient concentration to exceed with the metal ion the solubility product of the metal hydroxide. The new explanation depends upon hydrolytic reactions and equilibria as outlined. 

---