General
characteristics of the compounds of alkali metals
All the common compounds of the alkali metals are
generally ionic in nature. General characteristics of some of their compounds
are discussed here .
On combustion in excess of air, alkali metals forms normal
oxides with formula M2O. They react with water to form corresponding
hydroxides which are basic in nature.
M2O + H2O → 2 MOH
Alkali metals apart from lithium form peroxides in
addition to normal oxides upon combustion with excess air. These peroxides
produce hydroxides and H2O2 upon reacting with water.
M2O2+2 H2O → 2MOH+H2O2
(M = Na, K, Rb, Cs)
Except lithium and sodium, all the other alkali metals
form superoxides also. These superoxides also gives basic hydroxides upon
treatment with water.
2MO2 + 2 H2O → 2 MOH + H2O2 + O2
(M = K, Rb, Cs)
Under appropriate conditions pure compounds M2O,
M2O2 or MO2 may be prepared.
The oxides and the peroxides are colourless when pure, but
the superoxides are yellow or orange in colour. The peroxides are diamagnetic
while the superoxides are paramagnetic. Sodium peroxide is widely used as an
oxidising agent. The hydroxides which are obtained by the reaction of the
oxides with water are all white crystalline solids. The alkali metal hydroxides
are strong bases. They dissolve in water with evolution of heat on account of
intense hydration.
The alkali metal halides, MX, (X=F, Cl, Br, I) are
colourless crystalline solids with high melting points. They can be prepared by
the reaction of the appropriate oxide, hydroxide or carbonate with aqueous
hydrohalic acid (HX). As the electropositive character of alkali metal
increases from Li to Cs, the ease with which the metals form halides increases
from Li to Cs. All halides are ionic in nature except LiBr and LiI. Except LiF,
all other halides are soluble in water. The low solubility of LiF in water is
due to its high lattice enthalpy (small size of Li+ and F-).
Due to the presence of covalent nature both LiBr and LiI are soluble in organic
solvents.
Alkali metals form salts with all the oxo-acids. Most of
these salts are soluble in water and are thermally stable. As the
electropositive character increases down the group, the stability of the
carbonates and bicarbonates increases. This is due to the decrease in
polarising power of alkali metal cations. The carbonates (M2CO3)
of alkali metals are remarkably stable up to 1273 K, above which they first
melt and then eventually decompose to form oxides. However, Li2CO3
is considerably less stable and decomposes readily.
Li2CO3 - -∆→Li2O + CO2
This is presumably due to large size difference between Li+
and CO2-3 which makes the crystal lattice unstable. Being
strongly basic, alkali metals except lithium form solid bicarbonates. No other
metal forms solid bicarbonates.
M2CO3 + CO2 + H2O
→ 2 MHCO3
(M = Na, K, Rb, Cs)
All the carbonates and bicarbonates are soluble in water
and their solubilities increase rapidly on descending the group. This is due to
the reason that lattice energies decrease more rapidly than their hydration
energies on moving down the group.
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