Chapter 4
B.sc 1st year Book
(Page 3)

Compounds of Alkali metals

  1. Oxides :

    Alkali metals are highly reactive. After exposure to air or O2, they get tarnished quickly due to the formation of a film of oxides on their surface. In order to protect them from O2 or air, the metals are put into kerosene oil or paraffin oil. Oxides are also formed when they are burnt in air or oxygen, In this respect reactions of lithium and sodium are different. Li forms monoxide whereas Na forms peroxide.

In this respect reactions of lithium and sodium are different. Li forms monoxide whereas Na forms peroxide
Peroxide contains O2power2- as derived from the H2O2 molecule by removing both hydrogen atoms. Except for Li all alkali metals form peroxide of the formula M2O2 or M−O−O−M. These peroxides are formed by the action of O2 or air on metals when dissolved in NH3.K, Rb and Cs also form superoxides of the formula MO2 i.e. M++O2 – The structure of monoxide, peroxide, and superoxide ions are
Peroxide contains O2power2-O2power2 ion has 18 electrons in its valence shell and each oxygen atom has one σ bond pair and 3 lone pairs and is surrounded by a total of 4 electron pairs hence they are in an sp3 hybridization state.
Figure 4.02; Structure of peroxide ion, O2power2-ion.
Super oxides are also obtained by burning metals in excess of U2,
All the superoxides are paramagnetic in nature due to the presence of one unpaired electron in the OR-ion.

2. Hydrides :

All the alkali metals form hydrides of the formula MH when they are heated in presence of H2. Lithium forms LiH at700∘−800∘. whereas other alkali metals form their hydrides at a temperature between 350∘−400∘. Thus, lithium is less reactive toward H2.
2M+H2400∘C750−00∘C2MH:M=Na,K,Rb and Cs2 L+H2⟶7Li
The hydrides are also formed by heating alkali metal nitride in a current of hydrogen e.g.
2M3 N+3H2⟶6MH+N2M=Li,Na,K,Rb,Cs

The metal hydrides of heavier metals are formed by reducing their carbonates in a current of H2 gas.
e.g. Rb2CO3+2H2⟶620CSOC2RbH+CO2+H2O
Properties:

All the hydrides are crystalline ionic solid. With the increase in the electropositive character of metals the ionic character of hydrides increases in the order of:
LiH < NaH < KH < AbH <CsH
The reactivity of hydrides of alkali metals also follows the same trend,
Lithium hydride (LH) is stable in dry air up to red heat and sodium hydride (NaH) is inflammable in the presence of O2 at 277∘. Whereas potassium hydride (KH) is inflammable at room temperature. All the hydrides are decomposable except LiH. On exposure to light, they turn blue without decomposition.
All the hydrides are decomposable except LiH. On exposure to light, they turn blue without decomposition.
All the hydrides are ionic M+H−. They form non-directional bonds between M+ and H− hence they have no geometry. All have face-centered cubic lattices.
Structure :
All the hydrides are ionic M+H−. They form non-directional bonds between M+and H−hence they have no geometry. All have face-centered cubic lattices.Fig 4.03: A face centre structure of M+X−

3. Halides :

Since alkali metals have low ionization energy hence they form halides by the direct combination of metals with halogens.
2M+X2⟶2MX;M=L,Na,K,Pb,Cs and X=FFCl,Br,1
The ease of formation of halides increases from Li to Cs. These halides are also prepared
MOH+HX⟶MX+H2O;M=Li,Na,K,Pb,Cs and X=F,Cl,Br,1

Properties :

Alkali metal halides are crystalline solid, highly stable, and have high melting and boiling points, They are soluble in water except for lithium halide which has a covalent character. In this respect, Lithium halides have the least ionic character whereas cesium halides have the maximum ionic character hence lithium halides are soluble in organic solvents like alcohol and pyridine.
All the halides are volatile without decomposition at high temperatures. In a fused state, they are a good conductor of heat and electricity. The thermal stabilities of alkali metal halides increase with the increase of their atomic number as
LlCl < NaCl < KCl < RbCl < CsCl
Also, there is a decrease in melting point and boiling point with the increase in the atomic number of halogens.
 Halides : LiF M.P. (2C):870LiCl613→
Hydrolysis: Lithium halides undergo hydrolysis to some extent in hot water but other alkali metal halides do not undergo hydrolysis.

Formation of hydrates

LiCl forms hydrate of the formula LiCl.2H2O but other alkali metal halides do not form such hydrates.

Formation of Polyhalides :

Most of the alkali metals form poly halldes like Kl3, Pbl3Csl3, RbBrCl2, KBrF4, CsFICl2etc. The structure of alkali metal halides depends upon radius ratio (r+/r−) and lattice energy as discussed in an earlier chapter.
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