Notes-Part-2-Class-11-Science-Chemistry-Chapter-8- Elements of Group 1 and 2-Maharashtra Board

Topics to be Learn : Part-1 Hydrogen Alkali metals and elements of group 2 Topics to be Learn : Part-2 Some important compounds of elements of s—block

Some important compounds of elements of s-block :

Sodium Carbonate (washing soda) Na₂CO₃.10H₂O

Preparation: Sodium Carbonate is commercially prepared by Solvay process, which takes place in the following stages.

Stage 1: CO₂ is passed into a concentrated solution of NaCl which is saturated with NH₃.

Crystals of sodium bicarbonate separate as a result of the following reactions.

2NH_3(aq) + H₂O + CO_2(g) → (NH₄)₂CO_3(aq)

(NH₄)₂CO_3(aq) + H₂O + CO₂ → 2NH₄HCO_3(aq)

NH₄HCO_3(aq) + NaCl_(aq) → NH₄Cl_(aq) + NaHCO_3(s)

Sodium bicarbonate crystals precipitate out because of its low solubility.

[NaHCO₃ is formed as a result of double decomposition reaction in equation (ii)].

Stage 2 : Sodium bicarbonate crystals are heated to obtain sodium carbonate.

2NaHCO_3(s) \(\underrightarrow{Δ}\) Na₂CO_3(s) + H₂O_(g) + CO_2(g)

In this Solvay process the recovery of ammonia is done by treating the solution of NH₄Cl obtained with slaked lime, Ca(OH)₂. The byproduct of this reaction is calcium chloride.

2NH₄Cl_(aq) + Ca(OH)_2(s) → 2NH_3(g) + CaCl_2(aq) + H₂O_(l)

Properties of Sodium Carbonate :

• Sodium carborate (washing soda) is a white crystalline solid having the formula Na₂CO₃, 10H₂O.
• It is highly soluble in water.
• On heating the decahyadrate loses water molecules to form monohydrate.
• On heating above 373 K temperature monohydrate further loses water and changes into white anhydrous powder called soda-ash.

Na₂CO₃.10H₂O_(s) \(\underrightarrow{373\,\,K}\) Na₂CO₃.H₂O_(s) + 9H₂O_(g)

Na₂CO₃.H₂O_(s) \(\underrightarrow{373\,\,K}\)  Na₂CO₃._(s) + H₂O_(g)

• Aqueous solution of sodium carbonate is alkaline because of its hydrolysis by the following reaction:

Na₂CO₃ + H₂O → NaHCO₃ + NaOH

• The solution becomes alkaline as NaOH is a strong base.

Uses of sodium carbonate :

• Sodium carbonate is used as a cleaning material (as due to its alkaline nature it has emulsifying effect on grease and dirt).
• It is used to make hard water soft (as a water softener), as it precipitates out the soluble calcium and magnesium salts in hard water as carbonates.  Thus water becomes soft and gives lather with soap.

For example : Ca(HCO₃)_2(aq) + Na₂CO_3(aq) → CaCO_3(s) + 2NaHCO_3(aq)

• It is used for commercial production of soap and caustic soda.
• It is an important laboratory reagent.

Sodium hydroxide (caustic soda) NaOH

Preparation :

(1) Commercially sodium hydroxide is prepared by the electrolysis of aqueous sodium chloride solution in Castner—Kellner cell, using mercury cathode and graphite anode.

(2) The following reactions take place,

NaCl → Na⁺ + Cl⁻

(3) Reaction at cathode :

Na⁺ + e⁻ \(\underrightarrow{Hg}\) Na—Hg.

Sodium ions get reduced to metallic sodium at cathode. At cathode sodium metal combines with mercury to form amalgam, Na—Hg.

(4) Reaction at anode : Cl⁻ ions are oxidised to form chlorine gas at anode.

Cl⁻ → Cl + e⁻

Cl +Cl → Cl₂

The sodium amalgam when treated with water forms sodium hydroxide and hydrogen gas.

2Na—Hg + 2H₂O → 2NaOH + 2Hg + H₂.

Properties of sodium hydroxide (NaOH) :

• Sodium hydroxide is a white deliquescent solid with melting point 591 K.
• It is highly soluble in water and gives a strongly alkaline solution.
• The surface of the solution absorbs atmospheric CO₂ to form Na₂CO₃.

Uses of sodium hydroxide :

• Sodium hydroxide is used in the purification of bauxite.
• It is used in the manufacture of soap, paper, artificial silk and many chemical compounds.
• It is used in petroleum refining.
• It is used in textile industries (for mercerizing cotton).
• It is used as an important laboratory reagent.

Calcium Carbonate (CaCO₃)

Calcium carbonate is found in nature as chalk, limestone or marble.

Preparation :

(1) From slaked lime : When carbon dioxide is bubbled through slaked lime Ca(OH)₂, water insoluble calcium carbonate is formed.

Ca(OH)_2(aq) + CO₂O_(s) → CaCO_3(s) + H2O_(l)

Controlled addition of CO₂ is essential. Excess CO₂ transforms precipitate of CaCO₃ into water soluble calcium bicarbonate.

(2) From calcium chloride : When solution of calcium chloride is added to a solution of sodium carbonate, calcium carbonate is formed as a precipitate.

CaCl_2(aq) + Na₂CO_3(aq) → CaCO_3(s) + 2NaCl_(aq)

Physical properties of calcium carbonate :

• Calcium carbonate is soft, light, white powder.
• It is insoluble in water.

Action of heat on calcium carbonate :

On heating to 1200 K, calcium carbonate decomposes into calcium oxide and carbon dioxide.

CaCO_3(s) \(\underrightarrow{1200\,\,K}\) CaO_(s) + CO_2(g)

Action of dilute acids on calcium carbonate :

Calcium carbonate reacts with dilute acids to give corresponding calcium salt and carbon dioxide.

CaCO₃ + 2HCl → CaCl₂ + CO₂ ↑ + H₂O

CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ ↑ + H₂O

Uses of calcium carbonate:

• Calcium carbonate in the form of marble is used as building material.
• Calcium carbonate is used in the manufacture of quicklime (CaO) which is the major ingredient of cement.
• A mixture of CaCO₃ and MgCO₃ is used as flux in the extraction of metals from ores.
• It is required for the manufacture of high quality paper.
• It is an important ingredient in toothpaste, chewing gum, dietary supplements of calcium and filler in cosmetics.

Hydrogen peroxide (H₂O₂) :

Hydrogen peroxide is a low cost, clean and mild oxidizing agent. A 30% aqueous solution hydrogen peroxide is commercially available.

Preparation :

(i) From hydrated barium peroxide : When hydrated barium peroxide, BaO₂.8H₂O is acidified with dilute sulphuric acid, hydrogen peroxide is obtained along with a white precipitate of barium sulphate.

BaO₂.8H₂O_(s) + H₂SO_4(dil) \(\underrightarrow{273\,\,K}\) BaSO_4(s) + H₂O_2(aq) + 8H₂O_(l)

Insoluble barium sulphate is filtered off to get hydrogen peroxide solution.

(ii) Merk process : Small quantity of sodium peroxide is added to ice cold solution of dilute sulphuric acid, with stirring gives hydrogen peroxide.

Na₂O_2(aq) + H₂SO_4(aq) \(\underrightarrow{273\,\,K}\) H₂O_2(aq) + Na₂SO_4(aq)

(iii) From electrolytic oxidation of sulphuric acid : A 50% solution of sulphuric acid is subjected to an electrolytic oxidation to form peroxydisulphuric acid at anode.

2HSO₄ \(\underrightarrow{electrolysis}\) H₂S₂O₈ + 2e⁻

Hydrolysis of peroxydisulphuric acid yields hydrogen peroxide.

HO—SO₂—O—O—SO₂—OH + 2H₂O → 2 H₂SO₄+ H₂O₂.

[This method can be used to prepare D₂O₂ in laboratory].

(iv) industrial preparation of hydrogen peroxide :

• Industrially hydrogen peroxide is prepared by the air oxidation of 2-Ethylanthraquinol.
• Air is bubbled through a solution of 2-ethylan-thraquinol to get hydrogen peroxide and the oxidised product (2-ethylanthraquinone).
• 2-Ethylanthraquinone is further reduced by H₂ gas in presence of Pd catalyst to give back 2-Ethylanthraquinol.

Properties of hydrogen peroxide :

(i) Pure H2O2 is a very pale blue coloured liquid, having b.p. 272.4 K.

(ii) H₂O₂ is miscible in water and forms a hydrate (H₂O₂.H₂O)

(iii) Strength of aqueous solution of H₂O₂ is expressed in ‘volume’ units.

• The commercially marketed 30% (by mass) solution of H₂O₂ has volume strength of 100 volume. It means that 1 mL of 30% solution of H₂O₂ will give 100 mL oxygen at STP.

(iv) H₂O₂ acts as a mild oxidising as well as reducing agent.

• Oxidising action of H₂O₂ in acidic medium

2Fe₂⁺_(aq) + 2H⁺_(aq) + H₂O_2(aq) → 2Fe₃⁺_(aq) + 2H₂O_(l)

• b. Reducing action of H₂O₂ in acidic medium

2MnO₄ + 6H⁺ + 5H₂O₂ → 2Mn2⁺ + 8H₂O + 5O₂

Uses of hydrogen peroxide :

• Hydrogen peroxide is used as mouthwash, germicide, mild antiseptic, preservative for milk and wine and bleaching agent for soft materials due to its mild oxidising property.
• Hydrogen peroxide, due to its reducing property, is used as an antichlor to remove excess chlorine from fabrics which have been bleached by chlorine.
• Now a days it is also used in environmental chemistry for pollution control, restoration of aerobic condition to sewage water.

Lithium aluminium hydride (LiAlH₄) :

Lithium aluminium hydride is commonly abbreviated as LAH. It has chemical formula

LiAlH₄.

Prepartion : Lithium hydride is treated with aluminium chloride to give lithium aluminium hydride

4LiH + AlCl₃ → LiAlH₄ + 3LiCl

Properties: Lithium aluminium hydride is a colourless solid. It reacts violently with water and even atmospheric moisture.

Uses

(i) LAH is a source of hydride and therefore used as reducing agent in organic synthesis.

(ii) LAH is useful to prepare PH3(phosphine)

4PCl₃ + 3LiAlH₄ → 4PH₃ + 3AlCl3 + 3LiCl