Solvation of Alkali metal ions, Chemistry tutorial

INTRODUCTION

In this chapter we will be studying the behaviour of the alkali metals in solutions. We will as well study the complexation behaviour of the alkali metals. In our study of the chemical properties of the alkali metals, we must contain noticed that lithium behaved differently from the rest of the alkali metals. In this chapter we will be studying the anomalous behaviour of lithium.

Solvation of the alkali metal:

Whenever a metal is surrounded via solvent molecules, the phenomenon is termed solvation of the metal ion. When the solvent is water the phenomenon is now termed hydration. Hydration is thus solvation with water as the solvent. The alkali metal ions are highly hydrated. The smaller the size of the ion, the greater its degree of hydration. This is since the smaller the size, the more will be its charge density and the more will be its attraction for the polar solvent molecules. Therefore Li+ ion, that is the smallest gets more hydrated than Na+ and so on. The degree of hydration reduces on moving down the group. Consequently of differences in their degree of hydration, the hydrated ionic radii of the alkali metal ions decrease as we go down the group from lithium - caesium. Lithium has the largest hydrated radius while Cs + has the smallest hydrated radius in the 1st group. We will agree that the smaller the size of the ion or the lighter it is, the more will be its mobility and therefore conductance. In this regard we should expect the highest conductance of the alkali metals, but it is not so. Since the hydrated Li+ is the largest of all the alkali metal ions. In explanation its mobility is less and so Li+ ion is the least conducting in solution. The ionic conductance in solution in fact decreases in the order C s+> Rb+ > K+> Na+ > Li+.

Solutions of Alkali Metals in Liquid Ammonia All the alkali metals are extremely soluble in liquid ammonia giving a deep blue colour. The solubilities of the metals for 100g of ammonia are Li, 10g; Na, 25g and K, 49g. The dissolution of the alkali metal is accompanied by its dissociation into the metal ions and the e-s. The metal ion and the electrons then obtain associated with ammonia solvent molecules. Electrons associated with the solvent are familiar as

Solvated electrons:

Na(s)   +NH3 ( L) → Na+(NH3)x+ + e (NH3)y  

The alkali metal solutions in liquid ammonia are extremely conducting and act approximately as metals. Their explicit conductivities are about similar, since the anion for example solvated electron, in all the cases is similar. The little difference in the conductivity is due to the value of the metal itself. The solution of alkali metals in liquid ammonia is blue in colour due to the presence of solvated electrons, and consequently the solutions are as well paramagnetic. With increasing concentration there is a decrease in Para magnetizing suggesting that the electron can get associated to form diamagnetic electron pairs. Even though there might be other equilibrium as well.

Na(s) (dispersed) ↔ Na (in solution) ↔Na+e-

                       2e ↔ e2

On increasing the concentration over 3M, the colour of the solution varies to copper bronze having metallic luster, since the metal ions form clusters. Separately from lithium another alkali metals can be recovered unchanged from solution. Lithium, in ammonia solution form a difficult of the type [Li (NH3)4] +.

The blue solutions of alkali metals are moderately stable at temperatures wherever ammonia is still a liquid. The reaction that results in the formation of an amide.

Na + NH3(c) ↔ NaNH2 + 1/2H2  

Can take place photochemically and is catalysed through transition metal salts. The alkali metal solutions in liquid ammonia are powerful reducing agents or are utilized for this purpose in inorganic or organic reactions.

Cmplexation Behaviour of Alkali Metals:

A complex compound can be described as a compound with a central atom or ion enclosed through a group of ions of molecules termed "ligands". These ligands are generally bond to the metal through a "coordinate bond" example a bond formed via the donation of a lone pair of electrons from one atom (of the liquid) to the other (metal/ion). Even though both metal and the legend are generally capable of independent existence as stable chemical species, up till now whenever the complex is formed, it usually retains its identity in solution. For instance, in solution, Fe++ and CN- can exist independently but once the complex [Fe (CN) 6] -4 is formed it exists in solution as such. It does not dissociate into Fe+ and CN- , consequently it will not give any positive consequence whenever tested in Fe+ and CN- . It is therefore complex specie. The most stable complexes would be formed through the lightly polarizing cations. This is because they contain a strong tendency of interacting with electron clouds of another anionic and neutral electron rich species (ligands).

According to the model above, a dreadfully weak coordinating ability is expected in the group one metals since of their large size or low charge of the cations M+. According to this examination, stability of the complexes of the alkali metals should reduce in the order Li > Na > K>Rb > Cs and this is the observed trend. Alkali metals form few complexes mostly chelates with the ligands like B- ducetones, nitrophenils, nitroso naphthols etc as given in fig. have low stability.

1426_Source complex of alkali metal ions.jpg

Fig: Source complex of alkali metal ions

Lithium, being the most polarizing cation of all the alkali metals forms tetrahedral complexes with ligands like NH3, L5H5N etc. With ammonia it forms the complex of the kind [Li (NH5)4]I, while with pyridine a complex of the kind [LiCl(C5H5N)2 (H2 0)] formed.

Anomalous Natures of Lithium:

On descending any group of S or P block elements of the periodic table, one notices that, there is a general decrease in electronegativity or amplify in electronegativity. The difference in electronegativety between the 1st and 2nd elements of each and every group is much greater than that between any two successive elements. This is reflected in the properties of the elements. Therefore not only is the element more electronegative than the other element of the group, but it is much more electronegative than expected through easy extra polations.

This trend of bigger than expected differences in properties between the 1st member of a group and the rest of the elements is shown in group one whereas Li elements markedly different from the rest of the members of the group. Let us assume a look at such summaries again as they apply in group one. Due to the very small size of lithium, the metallic bonding between the atoms in the metallic lattice is very strong giving increase to burly cohesive forces. This is shown in its relatively higher melting point, or boiling point, hardness and homonuclear bond energy. The relatively higher attraction of lithium for its outer electron consequences in its relatively higher centre electronegativity ionization energy, hydration energy electron affinity and of course smaller atomic radii relative to the other homologues. Similar anomalies are as well found in the chemical properties, but the different appear greater as we shall see in the given accounts.

(i ) Lithium salts of large polarisable anions are thermally less stable than those of other alkali metals for example Lithium carbonate decomposes at 950K, while no decomposition of sodium carbonate occur below 1050K.

(ii) Lithium salts of anions of high charge density are less soluble than those of other alkali metals. The halides of lithium are more soluble in organic solvents.

 (iii) Lithium does not form solid bicarbonate trioxide or superoxides, since such are unstable at room temperature. On another hand those of other alkali metals entail a higher temperature to effect their decomposition.

 (iv) Lithium forms stable salts through anions of high charge density owing to their high lattice energy. For instances, in air lithium forms the normal oxide, while the others form higher oxides lithium reacts with nitrogen to form nitride, Li3N the others do not react. Lithium hydride is more constant than the other hydrides or lithium carbide is formed more simply with acetylene.

(v) With water Lithium reacts slowly.

(vi) Lithium forms more stable covalent bonds than other alkali metals as well as, thus, forms more stable complex compounds as previous seen. For instance, lithium cannot be recovered unchanged from its liquid ammonia solution. Owing to the formation of [Li (NH3)4] +.

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