Oxides of Mg, Ca
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
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.
CaO + SiO2 ———→ CaSiO3
(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
(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
(v) Reaction with coke
When heated with coke in electric furnace at 2273 – 3273 K, it forms calcium carbide.
CaO + 3C ————→ CaC2 + Co
(vi) Reaction with ammonium salt
On heating with ammonia salts, it liberates ammonia gas.
CaO + 2NH4Cl → CaCl2 + 2NH3 + H2O
(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.
Why does a piece of burning magnesium continue to burn in SO2?
This is because the reaction of Mg with SO2 is exothermic.
2Mg + SO2 —→ 2MgO + 1/8S8 + Heat
Name the substance used for drying alcohol and non-acidic gases.