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Chemical Properties of Group – II elements
Reaction with water – (Formation of hydroxides)
The electrode potential of Be (Be2+/Be = -1.97 V) is least negative amongst all the alkaline earth metals. This means that Be is much less electropositive than other alkaline earth metals and hence does not react with water or steam even at red heat.
The electrode potential of Mg (Mg+2/Mg = -2.37 V), although more negative than that of Be yet is still less negative than those of alkali metals and hence it does not react with cold water but reacts with boiling water or steam.
Mg + H2O → MgO + H2
or, Mg + 2H2O → Mg (OH)2 + H2
Mg, infact, forms a protective layer of oxide on its surface, therefore, despite its favourable electrode potential it does not react readily with water unless the oxide layer is removed by amalgamating it with mercury. In the formation of oxide film, Mg resembles Al.
Ca, Sr and Ba have more negative electrode potentials similar to those of the corresponding group I alkali metals and hence react with even with cold water, liberating H2 and forming the corresponding metal hydroxides.
Ca + 2H2O → Ca(OH)2 + H2
Reactivity of alkaline earth metals increases as we move down the group. However, the reaction of alkaline earth metals is less vigorous as compared to alkali metals.
Reaction with air (Nitrogen and Oxygen)
(a) Formation of oxides and nitrides
Be metal is relatively unreactive in the massive form and hence does not react below 873K. However, powdered Be is more reactive and burns brilliantly on ignition to give a mixture of BeO & Be3N2.
2Be + O2 (air) → 2BeO
3Be + N2 (air) → Be3N2
magnesium is more electropositive than Be and hence burns with dazzling brilliance in air to form a mixture of MgO and magnesium nitride.
Mg + air → MgO + Ng3N2
Ca, Sr and Ba being even more electropositive react with air readily to form a mixture of their respective oxides and nitrides.
The reactivity towards oxygen increases as we go down the group. Thus Ca, Ba and Sr are stored in paraffin but Be and Mg are not because they form protective oxide layer on their surface.
(b) Formation of Nitrides
All the alkaline metals burn in dinitrogen to form ionic nitrides of the formula, M3N2. This is in contrast to alkali metals where only Li forms Li3N.
3M + N2 → M3N2
Be3N2 being covalent is volatile while the nitrides of all other elements are crystalline solids.
All these nitrides decompose on heating and react with water liberating NH3.
Be3N2 → 3Be + N2
Ba3N2 + 6H2O → 3Ba (OH)2 + 2NH3
Ca3N2 + 6H2O → 3Ca (OH)2 + 2NH3