Halides of Alkali metals:-

The alkali metals combine directly with halogens under appropriate conditions forming halides of general formula MX. These halides can also be prepared by the action of aqueous halogen acids (HX) on metals oxides, hydroxides or carbonate.

M₂O + 2HX → 2MX + H₂O                

MOH + HX → MX + H₂O               

M₂CO₃ + 2HX → 2MX + CO₂ + H₂O (M = Li, Na, K, Rb or Cs)

(X = F, Cl, Br or I)

All these halides are colourless, high melting crystalline solids having high negative enthalpies of formation.

Table – II

[Standard enthalpies of formation in (kJ/mol^-1)]

The value decreases in the order:

Thus fluorides are the most stable while iodides are the least stable.

The trends in melting points, boiling points and solubility of alkali metals halides can be understood in terms of polarization effects, lattice energy and hydration of ions.

Comparison of ionic and covalent character of alkali metal halides.

 

When a cation approaches an anion, the electron cloud of the anion is attracted towards the cation and hence gets distorted. This effect is called polarization. The power of the cation to polarize the anion is called its polarizing power and the tendency of the anion to get polarized is called its polarizability. The greater the polarization produced more is the concentration of the electrons between the two atoms thereby decreasing the ionic character or increasing the covalent character. The covalent character of any compound in general depends upon the following factors.

(i) Size of the cations

Smaller the cation greater is its polarizing power and hence larger is the covalent character. The covalent character decreases as size of cation increases.

LiCl > NaCl > KCl > RbCl > CsCl

(ii) Size of the anion

Larger the anion, greater is its polarizability. This explains the covalent character of lithium halides is in order

Greater the charge on the cation greater is its polarizing power and hence larger is the covalent character. The covalent character of some halides increase in the order

Na⁺CI^- < Mg⁺²CI₂ < AI⁺³ CI₃               

Similarly greater the charge on the anion, more easily it gets polarized thereby imparting more covalent character to the compound formed eg covalent character increase in the order

NaCI < Na₂SO₄ < Na₃PO₄               

Thus the covalent character decreases as the charge of the anion decrease.   

 If two cations have the same charge and size, the one with pseudo noble gas configuration i.e. having 18 electrons in the outermost shell has greater polarizing power than a cation with noble gas configuration i.e having 8 electrons. For example CuCl is more covalent than NaCl.

Lattice energy is defined as the amount of energy required to separate one mole of solid ionic compound into its gaseous ions. Evidently greater the lattice energy, higher is the melting point of the alkali metals halide and lower is its solubility in water

Table – III

It is the amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.

M⁺ (g) + aq → M⁺ (aq) + hydration energy

X^- (g) + aq → X^- (aq) + hydration energy              

Higher the hydration energy of the ions greater is the solubility of the compound in water.

Further the extent of hydration depends upon the size of the ions. Smaller the size of the ion, more highly it is hydrated and hence greater is its hydrated ionic radius and less is its ionic mobility (Conductance).

From above arguments, the melting point and solubility in water or organic solvent of alkali metal halides can be explained

(i)  A delicate balance between lattice enthalpy and hydration enthalpy determines the ultimate solubility of a compound in water. For eg. Low solubility of LiF (0.27 g/100 g H₂O ) is due to its high lattice energy ( - 1005KJmol^-1) whereas the low solubility of CsI (44g/100g H₂O ) is due to smaller hydration energy of the two ions (-670 KJ/mol)

(ii)  The solubility of the most of alkali metal halides except those of fluorides decreases on descending the group since the decrease in hydration energy is more than the corresponding decrease in the lattice energy.

(iii)  Due to small size and high electronegativity, lithium halides except LiF are predominatantly covalent and hence are soluble in covalent solvents such as alcohol, acetone, ethyl acetate, LiCl is also soluble in pyridine. In contrast NaCl being ionic is insoluble in organic solvents.

(iv) Due to high hydration energy of Li⁺ ion, Lithium halides are soluble in water except LiF which is sparingly soluble due to its high lattice energy.

(v)    For the same alkali metal the melting point decreases in the order

 fluoride > chloride > bromide > iodide

because for the same alkali metal ion, the lattice energies decreases as the size of the halide ion increases.

(vi) for the same halide ion, the melting point of lithium halides are lower than those of the corresponding sodium halides and thereafter they decrease as we move down the group from Na to Cs.

The low melting point of LiCl (887 K) as compared to NaCl is probably because LiCl is covalent in nature and NaCl is ionic.

Alkali metal halides are soluble in water due to their high ionic character and low lattice energy.