Calcium Oxide

Calcium Oxide

Preparation of Calcium Oxide

Calcium oxide is called quick lime. It is prepared by heating limestone in a rotatory kiln at 1070-1270 K.

CaCO₃ ⇔ CaO + CO₂       ΔH = 179.9 KJ

Since the reaction is reversible, therefore, the carbon dioxide is removed as soon as it is formed to enable the reaction to proceed to completion.

The temperature is not allowed to rise above 1270K otherwise ,silica present as impurity in limestone will react with CaO to form calcium silicate.

CaO + SiO₂ → CaSiO₃

Properties of Calcium Oxide

1) It is a white amorphous solid ,melting point , 2870 K.

2) When heated in oxyhydrogen flame, it emits brilliant white light called limelight .

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

CaO + H₂O → Ca(OH)₂

CaO + CO₂ → CaCO₃

4) It is obtained in form of hard lumps. The addition of limited amount of water breaks these lumps. During the process, a hissing sound and large amount of heat is evolved which convert 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 = -64.5 KJ

5) Quicklime when slaked with caustic soda gives a solid called soda-lime ( CaO + NaOH)

6) Action of acids and acidic oxides: It is a basic oxide and hence combines with acids and acidic oxides at high temperature forming salts.

CaO + 2 HCl → CaCl₂ + H₂O

CaO + SiO₂ → CaSiO₃

6 CaO + P₄O₁₀ → 2 Ca₃(PO4)₂

CaO + SO₂ → CaSO₃

7) Reaction with Coke:  When heated with coke in an electric furnace at 2273 K, it forms calcium carbide.

CaO + 3 C → CaC₂ + CO

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

CaO + 2 NH₄Cl → CaCl₂ + 2 NH₃ + H₂O

Uses of Calcium Oxide

1) It is an important primary material and is the cheapest form of alkali.

2) It is employed in the purification of sugar and in manufacture of dye stuff.

3) It is used on a large scale in building construction.

4) In the preparation of cement ,glass ,calcium carbide and sodium carbonate.

5) It is used in the preparation of ammonia and soda lime.

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

7) It is used as a basic lining in furnaces.

8) It is used as a flux in metallurgy.