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)]

Element
MF
MCl
MBr
MI
Li
-612
-398
-350
-271
Na
-569
-400
-360
-288
K
-563
-428
-392
-328
Rb
-549
-423
-389
-329
Cs
-531
-424
-395
-337
 

The value decreases in the order:

Fluoride > Chloride > bromides > Iodide
 

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.

 
(a) Polarization effects

Comparison of ionic and covalent character of alkali metal halides.

 2284_Polarization effects.JPG

 

Alt text: polarizing effects of alkali metals

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

Thus LiCl is more covalent than KCl.
 

(ii) Size of the anion

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

LiI > LiBr > LiCl > LiF
 

(iii) Charge of the ion

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.   

 
 
(iv)   Electronic configuration of the cation

 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.

 
(b)    Lattice Energies

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

Compound
Lattice energy

Hydration* energy

Solubility
Melting point
LiCl
-845
-876
63.7
887
NaCl
-770
-776
35.7
1084
KCl
-703
-700
34.7
1039
RbCl
-674
-680
77.0
988
CsCl
-644
-646
162
925
NaF
-893
-919
4.22
1261
NaCl
-770
-776
35.7
1028
NaBr
-730
-745
116
1084
NaI
-685
-685
184
944
LiF
-1005
-1019
0.27
1115
CsI
-582
-670
44.0
1115
 
 
(c)    Hydration Energy

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.

 
Illustration:

Why are alkali metal halides soluble in water?

Solution:

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

 

Exercise:

Why is LiF less soluble in water?

 

Exercise:

Why is CsI less soluble in water?

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