chemistry the s-block elements - topperlearning

CHEMISTRY THE s-BLOCK ELEMENTS

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Important Compounds of s-Block

Important Compounds of Sodium

Sodium carbonate (washing soda), Na2CO3.10H2O:

Sodium carbonate is generally manufactured by the Solvay–ammonia process.

(a) Principle:

1) When carbon dioxide is passed through a brine (sodium chloride) solution saturated with

ammonia,

4 3Na ,NH ,Cl andHCO

radicals are present in the solution.

2) Of these various possible compounds, sodium bicarbonate is sparingly soluble and

hence crystallises out. It is then finally calcined to form sodium carbonate.

(b) Theory:

1) When carbon dioxide is passed through the brine solution saturated with ammonia,

sodium bicarbonate is formed.

3 2 3 2 4

4 2 3 4

3 3

4 4

NH H O NH H O NH OH

Na Cl NH OH CO Na HCO NH Cl

Na HCO NaHCO

NH Cl NH Cl

2) Because of the common ion effect by Na+ ions of sodium chloride, sodium bicarbonate

precipitates out. It is filtered and ignited to get sodium carbonate.

Heat

3 2 3 2 22NaHCO Na CO CO H O

3) Sodium carbonate thus obtained is contaminated with ammonium salt. It is purified by

passing CO2 through its aqueous solution. Sodium bicarbonate is formed, which is then

filtered and heated to give pure sodium carbonate.

2 3 2 2 3

3 2 3 2 2

Na CO CO H O 2NaHCO

2NaHCO Na CO CO H O

(c) Process:

(i) Ammoniation tower:



1) A saturated solution of brine is introduced from the top of the iron tower, and at the

same time, gaseous ammonia from the ammonia recovery tower is passed from

below.

2) Because of this process, brine gets saturated with ammonia, while calcium and

magnesium are present as impurities and precipitate as their respective insoluble

carbonates.

3 2 2 4 32

2 4 3 4 32

2 4 3 4 32

2NH CO H O NH CO

CaCl NH CO 2NH Cl CaCO

MgCl NH CO 2NH Cl MgCO

3) The ammoniated brine is then passed through filters to remove precipitated calcium

and magnesium carbonates and then passed through the carbonation tower.

Solvay–ammonia process for manufacture of sodium carbonate

(ii) Carbonation tower:

1) It is made of iron and fitted with several perforated horizontal partitions. The clear

ammoniated brine solution trickles down from the top of the tower, and carbon

dioxide from the lime kiln is injected from the bottom at a pressure of 1–2 atm.

2) Carbon dioxide rises through the small holes and reacts with ammoniated brine to

form sodium bicarbonate. The temperature is maintained at 300–310 K throughout

the process.

3 2 2 3 4NaCl NH CO H O NaHCO s NH Cl aq

(iii) Filtration:



1) The solution coming out from the carbonation tower contains tiny crystals of sodium

bicarbonate. These are filtered by passing through vacuum filters.

(iv) Calcination of sodium bicarbonate:

1) Sodium bicarbonate obtained is strongly heated in a kiln to form sodium carbonate.

3 2 3 2 22NaHCO Na CO CO H O

(v) Ammonia recovery tower:

1) The filtrate from the carbonation tower contains ammonium chloride and traces of

ammonium bicarbonate. It is made to flow down from the ammonia recovery tower,

while a current of steam is passed upwards.

2) Milk of lime is injected at a point slightly above the middle of tower. Ammonium

bicarbonate is accumulated by steam and ammonium chloride by milk of lime.

Heat

4 3 3 2 2

4 3 2 22

NH HCO NH CO H O

2NH Cl Ca OH 2NH CaCl 2H O

3) This recovered ammonia mixed with a small amount of carbon dioxide is used for the

saturation of brine.

(vi) Lime kiln:

1) Carbon dioxide is generated by heating limestone to about 1273 K.

1273K

3 2CaCO CaO CO

2) Carbon dioxide is brought to the carbonation tower and lime is slaked with water in a

tank known as slaker to give milk of lime. It is then pumped to the ammonia recovery

tower.

(d) Properties:

1) Sodium carbonate is a white crystalline solid with ten molecules of water of

crystallisation.

2) It is soluble in water.

3) Action of heat:

On heating it below 373 K, it loses 9 molecules of water of crystallisation to form

monohydrate. On heating above 373 K, monohydrate changes to anhydrous white

powder known as soda ash.



Below 373K

2 3 2 2 3 2 2

Above 373K

2 3 2 2 3 2

Na CO .10H O Na CO H O 9H O

Na CO .H O Na CO H O

4) Hydrolysis:

Sodium carbonate is a salt of a strong base and a weak acid; hence, when dissolved

in water, it undergoes hydrolysis.

2 3 2 2 3completely ionizedweakly ionized

2

3 2 3

Na CO 2H O H CO 2NaOH

CO H O HCO OH

5) Action of acid:

It reacts with dilute mineral acids with the evolution of carbon dioxide.

2 3 2 2Na CO 2HCl 2NaCl H O CO

6) Reaction with milk of lime:

It reacts with hot milk of lime to form sodium hydroxide.

2 3 32Ca OH Na CO CaCO 2NaOH

(e) Uses:



Sodium chloride (common salt), NaCl:

(a) Preparation:

1) The most powerful source of sodium chloride is seawater. It contains 2.7–2.9%

sodium chloride by mass. In India, common salt is obtained by the evaporation of

seawater.

2) Crude sodium chloride is generally obtained by crystallisation of brine solution. It

contains impurities of sodium sulphate, calcium sulphate, calcium chloride and

magnesium chloride.

3) Because of the presence of MgCl2 and CaCl2, common salt gets wet in the rainy

season as both are deliquescent.

4) To obtain pure sodium chloride, the crude salt is dissolved in a minimum amount of

water and filtered to remove insoluble impurities.

5) The solution is then saturated with hydrogen chloride gas to give crystals of pure

sodium chloride.

(b) Properties:

1) Sodium chloride melts at 1081 K.

2) Its solubility is 36 g per 100 g of water at 273 K.

Used for softening of hard water, laundry and

cleaning purposes

Used in paints and dyes

Used in the manufacture of soaps, glass, caustic soda,

sodium phosphate etc.

A mixture of Na2CO3 and

K2CO3 is used as fusion mixture

Used in paper and textile industries

Used in petroleum and metal refining

Used as a reagent in the laboratory



(c) Uses:

Sodium hydroxide (caustic soda), NaOH:

Sodium hydroxide is manufactured by the electrolysis of an aqueous solution of sodium chloride

using a mercury cathode and a graphite anode.

Electrolysis is carried out in a vessel called Castner and Kellner’s cell or mercury cathode cell.

As common salt or table salt for

domestic purposes In the preparation of sodium carbonate,

sodium hydroxide and sodium oxide



Castner and Kellner’s cell for manufacture of NaOH by electrolytic process

(a) Process:

1) It consists of a large rectangular tank divided into three compartments by slate partitions

which do not touch the bottom of the tank. They are suspended in mercury placed on the

base of the tank.

2) The anodes are made of graphite rods, and the cathodes are made of iron rods. The

mercury acts as an intermediate electrode, because the cathode is in the outer

compartment and the anode is in the central compartment.

3) The brine solution is taken in the side compartment, and a very dilute solution of NaOH is

taken in the central compartment. On passing electricity, electrolysis occurs in both

compartments as shown below:

Outer compartment:

Sodium chloride undergoes complete ionisation.

NaCl Na Cl

Water is a weak electrolyte and hence slightly ionises.

2H O H OH

Cl− and OH

− ions migrate towards the graphite anode, and Na

+ and H

+ ions migrate

towards the mercury cathode.

Oxidation of Cl− occurs at the anode, and Cl2 is evolved. It is then ejected through

outlets in outer compartments, while OH− ions remain in the solution.



2Cl Cl e ; Cl Cl Cl

At the cathode, reduction of Na+ occurs and Na is discharged, while H+ ions remain

in the solution.

2 xSodium amalgum

Na e Na

2Na xHg Na Hg

In this cathodic process, sodium amalgam is formed.

Central compartment:

Sodium hydroxide is a strong electrolyte; hence, it undergoes complete

dissociation, while water dissociates slightly.

2

NaOH Na OH

H O H OH

H+ and Na

+ ions migrate towards the cathode, while OH

− ions migrate

towards the mercury anode.

Reduction of H+ ions occurs at the cathode, and hydrogen is liberated.

2H e H ; H H H

Oxidation of OH−

ions occurs at the mercury anode. OH−

ions combine with

Na+ ions to form NaOH.

2 x

OH OH e

Na Hg 2OH 2NaOH xHg

When the concentration of NaOH solution is increased to 20%, the

concentrated NaOH is withdrawn, evaporated, fused and cast into sticks.

(b) Properties:

1) NaOH is a white crystalline solid with a melting point of 591 K.

2) It is highly soluble in water and gives a strong alkaline solution which is bitter in

taste, corrosive and soapy to touch.

3) Crystals of NaOH are deliquescent.



4) Sodium hydroxide solution reacts with atmospheric carbon dioxide to form crystals of

sodium carbonate.

2 2 3 22NaOH CO Na CO H O

(c) Uses:

Sodium hydrogen carbonate (baking soda), NaHCO3:

Sodium hydrogen carbonate is generally known as baking soda. It decomposes on heating with

evolution of CO2.

(a) Preparation:

It is prepared by saturating a solution of sodium carbonate with carbon dioxide. It gives a

white crystalline powder of sodium hydrogen carbonate.

2 3 2 2 3Na CO CO H O 2NaHCO

(b) Uses:

In the manufacture of soap, paper and artificial silk

In petroleum refining and in purification of bauxite

For mercerising cotton fabrics in the textile industry

For the preparation of pure fats and oils

As a laboratory reagent



Some Important Compounds of Calcium

Calcium oxide (quick lime), CaO:

(a) Preparation:

1) Calcium oxide is prepared by heating limestone in a rotator kiln at 1070–1270 K.

1070 1270K

3 2CaCO CaO CO ; H 179.9kJ

2) The reaction is reversible; hence, carbon dioxide should be removed as soon as it

forms.

3) The temperature should be maintained at or below 1270 K; otherwise, silica present

as an impurity will react with CaO to form calcium silicate.

Above 1270K

2 3calcium silicate

CaO SiO CaSiO

(b) Properties:

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

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

3) When exposed to air, it absorbs moisture and carbon dioxide to form slacked lime

and calcium carbonate, respectively.

2 2 32moisture calcium carbonateslaked lime

CaO H O Ca OH ; CaO CO CaCO

4) Quick lime slacked with caustic soda gives a solid called soda lime.

5) Because it is a basic oxide, it reacts with acids and acidic oxides at high temperature

forming salts.

1

2 2CaO H O Ca OH ; H 64.5kJmol

6) When heated with coke in an electric furnace at 2273 K, it forms calcium carbide.

As mild antiseptic for skin infection

As an antacid in making digestive powders for removing acidity of the stomach



2273K

2CaO 3C CaC CO

7) When heated with ammonium salt, it liberates ammonia gas.

4 2 3 2CaO 2NH Cl CaCl 2NH H O

(c) Uses:

Calcium hydroxide (slacked lime), Ca(OH)2:

(a) Preparation:

1) From quick lime:

Calcium hydroxide is prepared on a large scale by adding water to quick lime. This

process is known as slaking of lime.

2 2CaO H O Ca OH

During this process, lumps of quick lime grind to a fine powder.

2) From calcium chloride:

It is also obtained by treating calcium chloride with caustic soda.

2 2CaCl 2NaOH Ca OH 2NaCl

(b) Properties:

An important primary material

and is the cheapest form of alkali

In the purification of sugar and in the manufacture of

dyestuff

In large-scale building

construction

In the preparation of ammonia and

soda lime

As a flux in metallurgy

For dying alcohols and non-acidic gases



1) It is a white amorphous powder slightly soluble in water.

2) The suspension of slaked lime in water is called milk of lime, while the filtered and

clear solution is known as lime water.

3) Action of heat:

Slaked lime loses water only at a temperature greater than 700 K.

700K

22Ca OH CaO H O

4) Reaction with chloride:

Slaked lime reacts with chlorine to give calcium hypochlorite, which is commonly

known as bleaching powder.

2 2 22 2Bleaching powder

2Ca OH 2Cl CaCl Ca OCl 2H O

5) Reaction with carbon dioxide:

Lime water turns milky when carbon dioxide is passed through it because of the

formation of insoluble calcium carbonate.

2 3 22milkiness

Ca OH CO CaCO H O

6) Slaked lime is a strong base; hence, it reacts with acids and acidic gases to form

salts.

2 2 3 4 22 2Ca OH 2HCl CaCl H O ; Ca OH SO CaSO H O

However, it does not react with dilute sulphuric acid because calcium sulphate is

sparingly soluble in water.

(c) Uses:



Calcium carbonate, CaCO3:

It is commonly known as limestone. It occurs in nature in the form of chalk, marble and corals.

(a) Preparation:

1) From slaked lime:

It is prepared by passing a known amount of carbon dioxide through slaked lime.

2 3 22Ca OH CO CaCO H O

2) From calcium chloride:

It is prepared by adding an aqueous solution of sodium carbonate to calcium

chloride.

2 2 3 3CaCl Na CO CaCO 2NaCl

(b) Properties:

1) It is white solid and fairly soluble in water.

2) When heated to 1070–1270 K, it decomposes with the evolution of carbon dioxide.

1070 1270K

3 2CaCO CaO CO

3) It reacts with dilute acids liberating carbon dioxide.

As a building material in the form of mortar

In white washing because of its disinfectant properties

In the preparation of ammonia from ammonium chloride and in the softening of hard water

In the manufacture of bleaching powder

For the detection of carbon dioxide

For absorbing acid gases



3 2 2 2 3 2 4 4 2 2CaCO 2HCl CaCl CO H O ; CaCO H SO CaSO CO H O

(c) Uses:

Plaster of Paris, (CaSO4)2.H2O:

(a) Preparation:

1) It is prepared by heating gypsum to 393 K.

393K

4 2 4 2 2Gypsum Plaster of paris

2CaSO .2H O 2CaSO .1 2H O 3H O

2) The temperature should not be allowed to increase above 393 K because all the

water of crystallisation will be lost. The residue formed is anhydrous CaSO4 which is

known as dead burnt plaster.

393K

4 2 4 2Plaster of paris Dead burnt plaster

2CaSO .1 2H O 2CaSO H O

3) The gypsum should not be allowed to come in contact with carbon containing fuel

because it will reduce to calcium sulphate.

4 2 2 4 2

Plaster of paris

2CaSO .1 2H O 3H O 2CaSO .2H O

(b) Properties:

1) It is a white powder.

2) It has a property of setting with water. When mixed with one-third of its weight of

water, it forms a hard mass. Hence, it is called plaster. During this process, the

volume slightly expands. Thus, it can take up the shape of its mould.

(c) Uses:

Calcium carbonate along with

magnesium carbonate is

used as a flux in the

extraction of metals such

as iron.

Specially precipitated

CaCO3 is used in the

manufacture of high-quality

paper.

As an antacid, mild abrasive in toothpaste and a filter in cosmetics.

As a raw material for

manufacture of sodium carbonate.

As a building material in the form of marble

and in the manufacture of quick lime.



For producing moulds for pottery and ceramics and casts of statues

In building industry as well as

plasters

In surgical bandages, used

for plastering broken or

fractured bones of the body, for

preparing black board chalks

In dentistry

chemistry the s-block elements - topperlearning

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