Note on Alkali and Alkaline Earth Metals

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Alkali metals

Lithium, sodium, potassium, rubidium, cesium and Francium are called alkali metals because they react with water to give alkali.

They contain one electron in valence shell due to their similar valence shell electronic configuration (ns1). All alkali metals have similar chemical properties and is thus placed in the single vertical column which constitutes group IA of the periodic table.

Characteristics of Alkali metals (Group IA of the periodic table)

1) All alkali metals have single valence electron (ns1).

2) They are very reactive metals because they have high tendency to lose electrons due to low ionization energy.

3) They are a strong reducing agent because they lose the electron to form the monopositive ion.

4) They are silvery-white in color, light, soft and easily fused.

5) The action of air: When exposed to air, they get tarnished (losing brightness), due to the formation of their oxides on the surface of the metal.

6) Action of water: They react with water at ordinary temperature to form alkali with the liberation of hydrogen gas.

7) Action of hydrogen: They combine with hydrogen on heating to form hydrate.

SODIUM

Occurence

i) Sodium chloride (NaCl)

ii) Sodium carbonate (Na2CO3)

iii) Caliche (NaNO3)

Extraction of sodium - Down's process

- J.C. Down, an American chemist

Down's process is the modern method for the extraction of sodium metal by the electrolysis of fused (molten) sodium chloride.

In the beginning, Down's process had some technical difficulties as given below:

i) Sodium chloride melts at 8200C which is difficult to maintain.

ii) At such high temperature, sodium fCormed after electrolysis vaporizes and gets dispersed in the melt forming metallic fog and its isolation becomes a great problem.

iii) Na and Cl2 formed in the electrolysis corrodes the materials of the shell at this high temperature.

In order to overcome difficulties, anhydrous CaCl2 is added to solid NaCl which lowers the melting point of it to 6000C.

Principle:The mixture of solid NaCl and anhydrous CaCl2 melt at around 6000C. On electrolysis, sodium metal is formed at the cathode.

Calcium ion (Ca++) cannot reduce because it requires higher potential for the reduction and remains in the electrolyte.

Procedure

Down's process consists of iron vessel lined inside with firebricks. The anode is graphite rod passed int vessel form the bottom and stands inside in the middle. Cathodes are two iron rods passed into the vessel through the sidewalls. There is a wire gauge partition between cathode and anode so that 'Na' and 'Cl2' do not come in contact with each other. There is a dome shaped receiver above the anode.

The mixture of solid NaCl and calcium chloride (CaCl2) is melted in the iron vessel by the heat produced form the resistance of current. Under electrolysis, sodium is formed at the cathode in the molten state and rises in the iron pipe and is collected in the receiver. The chloride ion is oxidised at the anode and escapes out as chlorine gas through the dome shaped receiver.

Properties

i) Physical

a) It is a silvery whiteand soft metal

b) It is lighter than water

c) Melting point: 97.50C and Boiling point: 8800C

II) Chemical

a) The action of moist air: When sodium is exposed to moist air it gets tarnished due to the formation of sodium carbonate.

Thus, sodium is dissolved under kerosene oil or paraffin to protect it from the air.

When sodium is ignited, it produces golden yellow flame forming sodium oxide and sodium peroxide.

b) Action with hydrogen

Forms sodium hydride on heating

c) With halogen and sulphur

- Forms halides and sulphides respectively on heating

d) Action of water

It reacts with water violently at the oridinary temperature to form sodium hydroxide with the liberation of hydrogen gas.

e) Action of ammonia

When ammonia is passed over heated sodium, sodamide is formed.

f) Reducing action

It reduces certain oxides to metal

g) With mercury

It dissolves mercury to form the homogeneous mixture called an amalgam.

Uses

a) It is used as sodium vapor lamp.

b) It is used in the preparation of sodium peroxide and sodamide.

c) It is used as a reducing agent

d) As a reagent in the detection of heteroelements in organic compounds

COMPOUNDS OF SODIUM

I) Sodium Hydroxide (Caustic soda)

Brine: Concentrated aqueous solution of sodium chloride is called brine

Manufacture

Sodium hydroxide is manufactured by the electrolysis of concentrated solution of sodium chloride called brine using mercury as cathode either :

A = Castner's - Kellner's cell

B = Solvay - Kellner's Cell

A: Castner's - Kellner's process

Principle

When concentrated solution of sodium chloride called 'brine' is electrolyzed between a poll of mercury acting as cathode nad graphite as anode, sodium ion (Na+) are reduced on mercury cathode and dissolved in it to form sodium amalgam and chloride (Cl-) ions are oxidized at graphite anode with the liberation of chlorine gas.

Sodium amalgam, so formed reacts with water in central compartment to form sodium hydroxide solution with the liberation of hydrogen gas and mercury is regenerated.

Process

This process consists of an iron vessel divided into three compartments by slate partitions. The two outer compartments are fitted with graphite anode filled with brine. The central compartment is fitted with a number of iron cathode dipped into the dilute solution of NaOH. The bottom of the shell is covered with mercury layer which acts as a cathode in the outer compartment and as the anode in the central compartment.

On the electrolysis, chlorine is discharged at graphite anode and escapes out as Cl2 gas. Sodium is discharged on mercury cathode and dissolves in it to form sodium amalgam. Sodium amalgam formed in the outer compartment is carried into the central compartment by rocking motion of eccentric wheel where it reacts with water to form sodium hydroxide solution with the liberation of H2 gas. When sodium hydroxide solution is concentrated, it is evaporated to dryness to get solid sodium hydroxide or caustic soda.

B. Solvay - Kellner's Process

Principle

When concentrated solution of sodium chloride called 'brine' is electrolyzed between a poll of mercury acting as cathode nad graphite as anode, sodium ion (Na+) are reduced on mercury cathode and dissolved in it to form sodium amalgam and chloride (Cl-) ions are oxidized at graphite anode with the liberation of chlorine gas.

Sodium amalgam, so formed reacts with water in central compartment to form sodium hydroxide solution with the liberation of hydrogen gas and mercury is regenerated.

Process

This process consists of the big iron tank called electrolyser and contains a number of graphite nodes dipped into brine solution as the electrolyte. The bottom of the cell is covered by thin layer of flowing mercury acting as the cathode. When the electric current is passed through brine, Cl- ions are oxidized at graphite anode and escapes out as chlorine gas. Sodium ion (Na+) are reduced on mercury cathode and dissolves in it to form sodium amalgam. It flows out at the other end and is collected in an iron vessel containing water which is decomposed to give sodium hydroxide solution with the liberation of hydrogen gas. When sodium hydroxide solution is concentrated, it is evaporated to dryness to get solid sodium hydroxide (NaOH).

Properties

a) Physical

- It is crystalline white solid soluble in water

- Deliquescence: The property of a substance to absorb water from atmosphere is called deliquescence. NaOH is deliquescent(hygroscopic) in nature and absorbs water when exposed to air.

- Its solution is corrosive on skin and soapy to touch

- It is a strong alkali and turns red litmus into blue

b) Chemical

- The action of moist air: It reacts with moist air to form sodium carbonate.

- With metals ( Zn, Al) : Metals like Zn, Al when heated with NaOh solution evolve hydrogen gas.

- With Cl2: It reacts with Cl2 to give sodium hypochlorite and sodium chlorate under different condition.

- With phosphorous (P4) : When white phosphorous is heated with NaOH solution, sodium hypophosphite is formed and phosphine gas is evolved.

- With ammonium salts

Ammonia gas is evolved when ammonium salts are heated with NaOH.

- With acid and acidic oxides

Being a strong base, it neutralizes acids and acidic oxides to give sodium salts

- With carbon monoxides

Carbon monoxide (CO), when passed over heated NaOH under pressure forms sodium formate , an organic compound.

- Precipitation of hydroxides

It precipitates metal ions as hydroxides from the aqueous solution of certain salts.


Certain metal hydroxides being amphoteric dissolve in excess NaOH solution forming soluble salt after initial precipitation.

Uses

i) It is used in making soap, paper, and artificial milk.

ii) It is used in the preparation of hypochlorite and chlorate salt.

iii) It is used as a lab reagent

SODIUM CARBONATE ( Na2CO3)

Crystalline Na2CO3 containing 10 molecules of water of crystallization is called washing soda.

(Na2CO3.10H2O)

Manufacture by Solvay Ammonia Soda Process

Principle

This process is based on the fact that when CO2 is passed through brine solution, saturated with ammonia gas,ammonium bicarbonate first forms which further reacts with sodium chloride to form sodium bicarboate.

Sodium bicarboate is slightly soluble in water in the presence of NaCl and gets precipitated. The precipitate of sodium bicarbonate , on strong heating gives dsodium carbonate.

An aqueous solution of sodium carbonate, on crystallization, gives washing soda.

Procedure

CO2 is produced by heating limestone in the lime kiln.

Ammonia is generated (obtained) by heating ammonium chloride and slaked lime in ammonia generator.

Ammonia gas so obtained is passed into brine solution in brine tower. When brine solution is saturated with ammonia gas, this ammonical brine called ammoniated brine is pumped into carbonation tower where it reacts with upward stream of CO2 . Ammonium bicarbonate firs formed reacts with NaCl to form sparingly soluble sodium bicarbonate and is thus precipitated. The thick liquid obtained at the bottom of the carbonation tower is passed into vacuum filter. The ppt. of sodium bicarbonate is strongly heated to get sodium carbonate. The filtrate containing NH4Cl and CaO as biproduct of lime kiln are pumped into ammonia generator to generate ammonia.

Properties

a) Physical

- White solid containig 10 molecules of water of crystallization is called washing soda(Na2CO3.10H2O)

- Efflorescence: The property of a substance to lose its water of crystallization on exposure to air is called efflorescence. . Na2CO3 is an efflorescent substance and loses its 9 molecules of water when exposed to air

- It is soluble in water

b) Chemical

- Action of heat: When heated at 1000C, washing soda becomes anhydrous called soda ash

- Aqueous olution of sodium carbonate is alkaline due to hydrolysis

- It is decomposed by mineral acid to evolve CO2 gas

- Precipitatingagent: It precipitates Mg++, Ca++, Ba++ as carbonate from aqueous solution

of their salt.

Uses

- It is used in making soaps and detergents.

- It is used in softening the hardness of water

- It is used as a lab reagent

Biological importance of Na+ and K+

Na+ andK+ are common cations found in the body fluids which play a vital role in our body

i) In order to establish the ratio of Na+ andK+called concentration gradient in cell, work has to be done. The energy for this work is obtained by the metabolism inside body.

ii) The difficult ratio ofNa+ andK+inside and outside the cell controls the functioning of muscle and nerve cell

iii) The movement of glucose into cell is associated with Na+

ALKALINE EARTH METALS

Alkaline earth metals have two electrons in their valence shell. They belong to group IIA in the periodic table.

Example: Beryllium, Calcium, Magnesium, etc.

CALCIUM

Occurence

In nature, calcium occurs in the form of

i) Limestone or marble (CaCO3)

ii) Dolomite (CaCO3.MgCO3)

iii) Gypsum (CaSO4. 2H2O)

Compounds of calcium

I) Quick lime or lime or calcium oxide (CaO)

- Manufacture

Quick lime is manufactured by heating limestone or marble to 9000C.This reaction is reversible as CO2 is removed as soon it is formed to proceed the reaction in forward direction.

The pieces of limestone is placed in lime kiln and is heated by burning coal in firebox. Quick lime is removed in lime car.

- Properties

i) CaO is a white powder.

ii) It is stable to heat and produces intense white light called lime lightwhen heated in oxy acetylene flame

iii) Action of water

When CaO is in contact with water, a hissing noise is produced due to the liberation of heat energy and it forms dry powder of calcium hydroxide called slaked lime and the process is called slaking of lime.

- If a small quantity of CaO is added to water, Ca(OH)2 formed dissolves in water to give saturated solution of slaked lime called lime water.

- If large quantity of CaO is added to water to make a slurry mass, it looks like milk called milk of lime used as white wash.

4. With acid and acidic oxides

Being a base, it neutralizes acid and acidic oxides to form calcium salt

CaO + HCl→ CaCl2 + H2O

CaO + SiO2→ CaSiO3

5. With carbon

It forms calcium carbide on strong heating

Uses

a) In the manufacture of slaked lime, bleaching powder and calcium carbide.

b) In making glass, cement and white wash.

c) In agriculture to treat acidic soil and as disinfectant and fungicide.

d) As a dehydrating agent.

II) Bleaching powder (CaOCl2)

- It is the called calcium chloro-hypochlorite,

Preparation

Bleaching powder is manufactured by the action of chlorine on dry slaked lime

Process

The vessel consists of a vertical iron tower with a number of shelves like partition at equal height, the slaked lime is placed into iron tower by compressed air which moves down through the rotating rakes and reacts with chlorine gas which slowly moves upwards and reacts to form bleaching powder which is collected in the drum at the base. The unreacted chlorine is expelled through the outlet near the top of the plant by the hot air passed through the bottom.

Properties

i) It is a pale yellow powder.

ii) Action with excess dilute acid

It liberates the whole of its chlorine when treated with excess dilute mineral acid.

CaOCl2 + HCl→ CaCl2 + H2O +Cl2

The amount of chlorine liberated by 1 mole of bleaching powder when treated with excess dilute acid is called available chlorine.

A standard sample of bleaching powder contains 35 to 40% available chlorine.

iii) Action with atmospheric CO2

It reacts with atmospheric CO2 to liberate chlorine and thus decreases available chlorine.

CaOCl2 + CO2→ CaCO3 + Cl2

iv) Bleaching action

It bleaches colored substances and the bleaching action is due to the oxidizing action of nascent oxygen liberated when it is treated with limited dilute acid.

CaOCl2+ HCl→ CaCl2 + HOCl (hypochlorous acid)

HOCL→ HCl + [O]

Colored substance + [O]→ Colorless substance

v) Self oxidation

On long standing, it is converted into CaCl2 and chlorate which also decreases available chlorine

CaOCl2→ CaCl2+ Ca(ClO3)2

Uses

i) It is used as bleaching agent in textile and paper industries.

ii) As a disinfectant and germicide in the sterilization of water.

III Plaster of paris ( CaSO4. \( \frac{1}{2}\) H2O)

It is calcium sulphate hemihydrate.

Preparation

It is prepared by heating gypsum at 1200C.

Properties

i) It is white powder isoluble in water.

ii) Action of water

When plaster of paris comes in contact with water, it has a remarkable property of setting into hard substance due to the formation of gypsum.

(CaSO4)2.H2O → CaSO4. 2H2O

This property of plaster of paris is utilized for plastering fractured bones of our body for immobility.

iii) Action of heat

At 2000C, it loses water of hydration and becomes anhydrous CaSO4 which does not set like plaster of paris called dead burn plaster.

Uses

i) In making mould in the pottery industry.

ii) In plastering fractured bones of the body.

iii) In making statues and artificial marble.

MAGNESIUM

COMPOUNDS OF MAGNESIUM

a) Magnesia or magnesium oxide (MgO)

Preparation

i) By burning Mg in air

Mg + O2→ MgO

ii) By heating nitrate, carbonate or hydroxide of magnesium

Properties

a) It is white powder

b) It is stable to heat. So, it is refractory in nature.

c) When moist, it turns red litmus into blue.

d) With atmospheric CO2: When exposed to air, it reacts with CO2 to form magnesium carbonate

MgO + CO2→ MgCO3

e) With water: It reacts with boiling water slowly to form magnesium hydroxide.

MgO + H2O→ Mg(OH)2

f) Basic nature: Being base, it neutralizes acid to form magnesium slats

MgO + H2SO4→ MgSO4 + H2O

MgO + HNO3→ Mg(NO3)2+ H2O

Uses

i) In making crucible and refractory bricks for internal lining of furnace.

ii) In medicine as anti - acid laxative( to neutralize the acidity of stomach).

II) Magnesium sulphate (Epsom salt) = MgSO4.7H2O

Crystalline magnesium sulphate containing 7 molecules of water of crystallization is called Epsom salt. (MgSO4.7H2O)

Preparation

Magnesium sulphate is prepared by the action of dil. H2SO4 on magnesium, its oxide, hydroxide or carbonates.

Mg + H2SO4→ MgSO4 + H2

MgO + H2SO4→ MgSO4 + H2O

Mg(OH)2 + H2SO4→ MgSO4 + H2O

MgCO3 + H2SO4→ MgSO4 + CO2 +H2O

The solution so obtained is evaporated up to crystallization point to get crystal of Epsom salt

MgSO4 + 7H2O → MgSO4.7H2O

  • This process consists of an iron vessel divided into three compartments by slate partitions.
  • They contain one electron in valence shell due to their similar valence shell electronic configuration (ns1).
  • They react with water at ordinary temperature to form alkali with the liberation of hydrogen gas.
  •  When heated at 1000C, washing soda becomes anhydrous called soda ash
  • At 2000C, it loses water of hydration and becomes anhydrous CaSO4 which does not set like plaster of Paris called dead burn plaster.
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Sushil

No figures are there. So please put figure too


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prajwal rai

Action of sodium with hcl,h2so4and hno3


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Prasan Tamang

why group IIA elements are called alkaline earth metals?


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vibek

There is no figure in this note. So please provide figure too which makes us easy to understand. Thank you


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