Calcium Oxide CaO

It is also called quick lime.

Preparation

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

CaCO₃(s) CaO(s) + CO₂(g);ΔH = +179.9 kJ

The necessary conditions for obtaining a good yield of quick lime are:

(i) Since the reaction is reversible, carbon dioxide should be removed as soon as it is formed in order to shift the equilibrium in the forward direction in accordance with
Le Chatelier’s Principle.

(ii) The temperature should not be allowed to rise above 1270 K otherwise silica present as impurity in lime will combine with calcium oxide to form infusible calcium silicate.

CaO + SiO₂CaSiO₃

(calcium silicate)

Properties

(i) It is a white amorphous solid with melting point of 2870 K.

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

CaO + H₂O(moisture) Ca(OH)₂

(slaked lime)

CaO + CO₂ CaCO₃ (calcium carbonate)

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

CaO + H₂O Ca(OH)₂;ΔH = −63 kJ

(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 treated with coke in an electric furnace at 2273−3273 K, it forms calcium carbide.

CaO + 3C CaC₂ + CO

(vi) Reaction with ammonium salts: On heating with ammonium salts, it liberates ammonia gas.

CaO + 2NH₄Cl CaCl2 + 2NH₃ + H₂O

Uses

(i) It is used as a building material.

(ii) In the preparation of cement, glass, calcium carbide and sodium carbonate (from caustic soda).

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

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

(v) It is used as basic lining in furnaces.