Oxides of Magnesium and Calcium

Oxides of Mg, Ca

 
MgO (Magnesia)
 

It is made by heating magnesite (MgCO3).

MgCO3  MgO + CO2
 

It is very slightly soluble in water imparting an alkaline reaction to the solution.

MgO + H2O → Mg(OH)2
 

Calcium oxide, Quick lime CaO 

Preparation

It is prepared by heating limestone in a rotatory kiln at 1070 – 1270 K.

CaCO3 1070-1270k CaO + CO2; ΔH = + 179.9 KJ       
 

The temperature should not be raised above 1270 K. Otherwise silica present as impurity in lime will combine with calcium oxide to form infusible calcium silicate.

            above 1270k

CaO + SiO2 ———→ CaSiO3         

 
Properties
 
(i)     It is a white amorphous solid with m.p. of 2870 K.

(ii)    When exposed to atmosphere, it absorbs moisture and CO2 forming slaked lime and calcium carbonate respectively.

CaO + H2O (Moisture)  Ca(OH)2

CaO + CO2  CaCO3

(iii) On adding water, it produces a hissing sound a large amount of heat is evolved which converts water into steam. This process is called slaking of lime and the fine powder obtained is called slaked lime.

CaO + H2O  Ca(OH)2 ; ΔH = -63KJ

 

(iv)   Action of acids and acidic oxides

It is a basic oxide and hence combines with acids and acidic oxides forming salts.

CaO + 2HCl  CaCl2 + H2O

CaO + SO2  CaSO3 

(v)    Reaction with coke

When heated with coke in electric furnace at 2273 – 3273 K, it forms calcium carbide.

        2273-3273k
CaO + 3C ————→ CaC2 + Co       

 

(vi)   Reaction with ammonium salt

On heating with ammonia salts, it liberates ammonia gas.

CaO + 2NH4Cl  CaCl2 + 2NH3 + H2O
 
Uses

(i) It is used as a building materials.

(ii)It is used for drying alcohols and non acidic gases.

(iii)It is used in the preparation of ammonia and soda lime (CaO + NaOH).

(iv)It is used as a basic lining in furnaces.
 
Illustration:

Why does a piece of burning magnesium continue to burn in SO2?

Solution:

This is because the reaction of Mg with SO2 is exothermic.

2Mg + SO2 —→ 2MgO + 1/8S8 + Heat

Exercise:

Name the substance used for drying alcohol and non-acidic gases.

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