Oxides and Hydroxides
Group - I & II
Sodium Oxide (Na2O) :
(i) It is obtained by burning sodium at 180°C in a limited supply of air or oxygen and distilling off the excess of sodium in vacuum.
(ii) By heating sodium peroxide, nitrate or nitrite with sodium.
(i) It is white amorphous mass.
(ii) It decomposes at 400°C into sodium peroxide and sodium.
(iii) It dissolves violently in water, yielding caustic soda.
Na2O + H2O -----> 2NaOH
Sodium Peroxides (Na2O2) :
Preparation: It is formed by heating the metal in excess of air or oxygen at 300°, which is free from moisture and CO2.
2Na + O2 -----> Na2O2
(i) It is a pale yellow solid, becoming white in air from the formation of a film of NaOH and Na2CO3.
(ii) In cold water (~ 0°C) produces H2O2 but at room temperature produces O2. In ice-cold mineral acids also produces H2O2.
(iii) It reacts with CO2, giving sodium carbonate and oxygen and hence it is used for purifying air in a confined space e.g. submarine, ill-ventilated room.
2Na2O2 + 2CO2 -----> 2Na2CO3 + O2
(iv) It is an oxidising agent and oxidises charcoal, CO, NH3, SO2.
3Na2O2 + 2C -----> 2Na2CO3 + 2Na [Deposition of metallic Na]
CO + Na2O2 ------> Na2CO3
SO2 + Na2O2 ------> Na2SO4
2NH3 + 3Na2O2 ------> 6NaOH + N2
(v) It contains peroxide ion [-O-O-]-2.
(i) For preparing H2O2, O2.
(ii) Oxygenating the air in submarines.
(iii) Oxidising agent in the laboratory.
Oxides of Potassium:
(iii) Passage of O2 through a blue solution of K in liquid NH3 yields oxides K2O2 (white), K2O3 (red) and KO2 (deep yellow) i.e;
Magesium Oxide (MgO):
It is also called magnesia and obtained by heating natural magnesite.
MgCO3--------- >. MgO + CO2
(i) It is white powder.
(ii) It's m.p. is 2850°C. Hence used in manufacture of refractory bricks for furances.
(iii) It is very slightly soluble in water imparting alkaline reaction.
Calcium Oxide (CaO) :
It is commonly called as quick lime or lime and made by decomposing lime stone at high temperature about 1000°C.
CaCO3 ---------> CaO + CO2 + 42,000 cal
1. It is white amorphous powder of m.p. 2570°C.
2. It emits intense light (lime light), when heated in oxygen - hydrogen flame.
3. It is a basic oxide and combines with some acidic oxide e.g.
CaO + SiO2 ---- >. CaSiO3
CaO + CO2 ------> CaCO3
4. It combines with water to produce slaked lime.
CaO + H2O------ >. Ca(OH)2
Magnesium Peroxide (MgO2) and Calcium Peroxide (CaO2) :
These are obtained by passing H2O2 in a suspension of Mg(OH)2 and Ca(OH)2.
Uses: MgO2 is used as an antiseptic in toothpaste and as a bleaching agent.
(i) Electrolysis of Brine:
(ii) Caustication of Na2CO3 (Gossage's method):
Since the Ksp(CaCO3) < Ksp(Ca(OH)2), the reaction shifts towards right.
(i) It is white crystalline, deliquescent, highly corrosive solid.
(ii) It is stable towards heat.
(iii) It's aqueous solution alkaline in nature and soapy in touch.
(v) Acidic and amphoteric oxides gets dissolved easily e.g.
CO2 + 2NaOH ---> Na2CO3 + H2O
Al2O3 + 2NaOH- ---> 2NaAlO2 + H2O
(vi) Aluminium and Zn metal gives H2 from NaOH.
2Al + 2NaOH + 2H2O --->3H2 + 2NaAlO2
(vii) Several non metals such as P, S, Cl etc. yield a hydride instead of hydrogen e.g.
4P + 3NaOH + 3H2O --->PH3 + 3NaH2PO2 (Disproportionation reaction)
Potassium Hydroxide :
Preparation : Electrolysis of KCl aqueous solution
Properties : Same as NaOH
**(a) It is stronger base compared to NaOH.
(b) Solubility in water is more compared to NaOH.
(c) In alcohol, NaOH is sparingly soluble but KOH is highly soluble.
(d) As a reagent KOH is less frequently used but in absorption of CO2, KOH is preferably used compared to NaOH. Because KHCO3 formed is soluble whereas NaHCO3 is insoluble and may therefore choke the tubes of apparatus used.
Magnesium Hydroxide : It occurs in nature as the mineral brucite.
Preparation : It can be prepared by adding caustic soda solution to a solution.
Mg+2 + 2NaOH ------>Na2SO4 + Mg(OH)2
1. It can be dried at temperature upto 100°C only otherwise it breaks into its oxide at higher temperature.
Mg(OH)2------> MgO + H2O
2. It is slightly soluble in water imparting alkalinity.
3. It dissolves in NH4Cl solution.
Mg(OH)2 + 2NH4Cl ------> MgCl2 + 2NH4OH
**Thus, Mg(OH)2 is not, therefore, precipitated from a solution of Mg+2 ions by NH4OH in presence of excess of NH4Cl.
Preparation : By spraying water on quicklime.
CaO + H2O ----> Ca(OH)2
(i) It is sparingly soluble in water.
(ii) It's solubility in hot water is less than that of cold water. Hence solubility decreases with increase in temperature.
(iii) It readily absorbs CO2 and used as a test for the gas.
(iv) It is used as a mortar.
[Mortar is a mixture of slaked lime (1 part) and sand (3 parts) made into a paste with water].