Properties of Alkali Metals

Introduction to Properties of Alkali Metals

Alkali metals have large affinity to non-metals like oxygen, halogens. Hence alkalimetals reduce oxides and halides to the metal.

MgO + 2Na '+ Mg + Na2O


Alkali metals are strong reducing agents. Due to low value of ionization energy alkalimetals easily give electrons for reduction. According to this lithium having highest ionization potential should be the weakest reducing agent. But it is found to be the strongest. This anomaly is explained as follows:

The function of a metal as a reducing agent is controlled by three factors. They are,
1. Enthalpy of sublimation: M(s) '> M(g) ; H2 = x
2. Ionization of the metal atom: M(g) '> M+(g) + e-; 'Hi = y
3. Hydration of the ion: M+(g) + aq ' M+(aq) ; z = z

Out of these three processes the hydration process is exothermic. Due to the small size of Li+ ion, it has high charge density and hence under goes hydration to the maximum extent. The energy released compensates high value ionization energy for Li. Hence it becomes the strongest reducing agent.

Alkali metals reaction with air

When an alkali metal is exposed to air, the surface tarnishes due to the formation of oxide, hydroxide and finally the carbonate. Hence all the alkali metals are preserved under kerosene. On burning lithium forms only the monoxide, sodium forms the normal oxide and the peroxide and others can form even the super oxides.

4L1 + O2'> 2Li2O 2Na + O2'> Na2102 M + O2'+ MO2 where M is K, Rb, Cs

Alkali metals reaction with water

All the alkali metals react with water. The activity increases from Li to Cs. The hydroxide of the metal is formed. Hydrogen is evolved.

Example:

2M + 2H20 '+ 2MOH + H2


2Na + 2H20 '+ 2NaOH + H2

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