>> Study Material
>> IIT JEE Chemistry
>> Inorganic Chemistry
>> S and P-Block Elements
>> Chemical Properties of Alkaline Earth Metals
Reaction with hydrogen – (Formation of hydrides)
All the alkaline earth metals except be combine with hydrogen directly on heating to form metal hydrides of formula MH2.
The hydride of beryllium can also be obtained by the reduction of BeCl2 with LiAlH4
2BeCI2 + LiAIH4 → 2BeH2 + LiCI + AlCl3
Both BeH2 and MgH2 are covalent compounds having polymeric structures in which H – atoms between beryllium atoms are held together by three
centre – two electron (3C - 2e) bonds as shown below:
The hydrides of other elements of this group i.e. CaH2, SrH2 and BaH2 are ionic and contain the H- ions.
All the hydrides of alkaline earth metals reacts with water liberating H2 gas and thus act as reducing agents.
MH2 + 2H2O → M (OH)2 + 2H2
CaH2 is called Hydrolith and is used for production of H2 by action of water on it.
Reaction with carbon – (Formation of carbides)
When BeO is heated with carbon at 2175 – 2275 K a brick red coloured carbide of the formula Be2C is formed
2175 - 2275k
2BeO + 2C ———→ Be2C + 2CO.
It is a covalent compound and react water forming methane.
Be2C + 4H2O → 2Be (OH)2 + CH4
The rest of the alkaline earth metals (Mg, Ca, Sr & Ba) form carbides of the general formula, MC2 either when the metal is heated with carbon in an electric furnace or when their oxides are heated with carbon.
Ca + 2C ———→ CaC2
CaO + 3C ———→ CaC2 + CO
All these carbides react with water producing acetylene gas.
CaC2 + 2H2O ———→ HC ≡ CH + Ca (OH)2
Reaction with Halogens
The alkaline earth metals react with halogens at elevated temperature to form the halides of the types MX2.
Action of Acids
The alkaline earth metals readily react with acids liberating hydrogen.
M + 2HCI → MCI2 + H2 ( M = Be, Mg, Ca, Sr, Ba)
Reaction with Ammonia
Like alkali metal, the alkaline earth metals dissolve in liquid ammonia to give deep blue black solution from which ammoniates [ M (NH3)6 ]2+ can be recovered.
How does the basicity of oxides of group 2 increases down the group?
The basicity increases down the group
BeO < MgO < CaO < SrO < BaO
amphoteric strongly - basic
Why is BeCl2 covalent in nature?
Formation of Peroxides
Since larger cations stabilize larger anions. Therefore, tendency to form peroxide increases as the size of the metal ion becomes larger. Thus BaO2 is formed by passing air over heated BaO at 773K.
2BaO + O2 ———→2BaO2
2SrO + O2 ———→ 2SrO2
SrO2 is prepared in similar way but under high pressure and temperature. CaO2 is not formed this way but can be prepared as the hydrate by treating Ca (OH)2 with H2O2and then dehydrating the product.
Ca ( OH)2 + H2O2 ——→ CaO2. 2H2O
Crude MgO2 has been made using H2O2 but peroxide of beryllium is not known.
All peroxide are white crystalline ionic solids containing the peroxide ion O2-2. Treatment of peroxide with acids liberates H2O2.
BaO2 + 2HCI ———→BaCI2 + H2O2