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Magnesium Metal

Magnesium occurs as

Magnesite MgCO3,

Dolomite CaMg(CO3)2,

Epsomite (MgSO4.7H2O)

Carnalite K2MgCl4.6H2O and

Langbeinite K2Mg2(SO4)3 deposits.

The chloride and sulphate of magnesium occurs in sea water from which it being extracted on an increasing scale.

Extraction of Magnesium

(a) From magnesite or Dolomite

The ore is first calcined to form the oxide

MgCO3 → MgO + CO2

CaCO3.MgCO3 → CaO.MgO + 2CO2

The metal is obtained from the oxide or the mixed oxides as follows:

(i) From MgO : The oxide is mixed with carbon and heated in a current of chlorine gas.

MgO + C + Cl2 → MgCl2 + CO

The chloride thus obtained is subjected to electrolysis.

(ii) The mixed oxides [CaO.MgO] obtained from calcination of  Dolomite [CaCO3.MgCO3] are redcued by ferrosilicon under reduced pressure above 1273 K.

2CaO + 2MgO + FeSi → 2Mg + Fe + Ca2SiO4

(b) From Carnallite: The ore is dehydrated in a current of hydrogen chloride and the mixture of fused chloride is electrolysed.

(c) From Sea water: Sea water containing magnesium chloride is concentrated under the sun and is treated with calcium hydroxide Ca(OH)2. Mg(OH)2 is thus precipitated, filtered and heated to give the oxide.

The oxide so obtained is treated as in (a) (i) above and then electrolysed.

Electrolysis of Magnesium Chloride

MgCl2 obtained by any of the above methods is fused and mixed with additional mixture of NaCl and CaCl2 in the temperature range of 973 – 1023 K. The molten mixture is electrolysed. Magnesium is liberated at the cathode and chlorine is evolved at the anode.

At cathode:   Mg2+ + 2e- → Mg

At cathode:   2Cl- → Cl2 + 2e-

A stream of coal gas is blown through the cell to prevent oxidation of Mg metal. Mg metal is obtained in liquid state which is further distilled to give pure magnesium.

Physical Properties of Magnesium

Magnesium is a silvery white metal which soon becomes dull in air.

It is a light metal with a density of 1.74 g cm-3.

It is fairly malleable and ductile.

Chemical Properties of Magnesium

Action of oxygen or air : Magnesium does not react with dry air but slowly gets tarnished in most air due to the formation of a thin film of the oxide, MgO. It burns in oxygen or air with a dazzling light.
2Mg + O2  2MgO

Action of CO2 and SO2 : Because of its great affinity for oxygen magnesium keeps on burning even in CO2 or SO2.
2Mg + CO2  2MgO + C
2Mg + SO2  2MgO + S

Action of nitrogen: On heating magnesium combines with nitrogen to form magnesium nitride.
3Mg + N2  Mg3N2
Thus when magnesium burns in air both the oxide and the nitride are formed.

Action of halogens: Magnesium on heating with halogens readily forms the halides e.g.
Mg + Cl2  MgCl2

Action of water: Magnesium does not decompose water in cold but decomposes boiling water or steam.
Mg + H2O ( steam)  MgO + H2

Action of Acids : Dilute acids reacts with magnesium to produce dihydrogen.
Mg + 2HCI → MgCI2 + H2
Mg + H2SO4 → MgSO4 + H2
Mg + 2HNO3 → Mg ( NO3)2 + H2

However with conc. H2SO4, SO2 is produced
Mg + 2H2SO4 ——→ MgSO4  + SO2 + 2H2O

Reaction with alkyl halide: Magnesium reacts with alkyl halides in dry ether to form covalent compound called Grignard reagent.
Mg + C2H5I →  C2H5MgI

Uses of Magnesium

The chief use of magnesium is in the preparation of alloys with aluminum, zinc, manganese and tin.
Duralium (Al = 95%, Cu = 4%. Mn = 0.5%, Mg = 0.5%)
Mangnalium (Al = 90% & Mg = 10%)
Duralium being light, tough and durable is used for the manufacture of airplanes and automobiles parts. Magnalium being light, tough and hard is used for making balance beams.

Magnesium burns with an intense lights, therefore, Mg (as power or ribbon) is used in flash bulbs for photography, fireworks and signals fibres.

Mg is used for ignition of thermite charge in aluminothermy.

A suspension of magnesium hydroxide known as milk of magnesium is used as an antacid for patients suffering from acidity.

In preparation of Grignard reagent.

Being a reducing agent, magnesium is used in the extraction of boron and silicon from their respective oxides.
B2O3 + 3Mg ——→ 3 MgO + 2B
SiO2 + 2Mg ——→ 2MgO + Si

Illustrations

Q.1:

The chemistry of Lithium is very much similar to that of magnesium even though they are placed in different groups - Explain.

Solution:

The ratio of their charge to size is nearly same by which they show the diagonal relationship.

Q.2:

Name some minerals of magnesium.

Solution:

Minerals of magnesium are:

Dolomite

Magnesite

Epsom salt

Q.3:

Write the chemical formulae of the following ores:

(i)     Dolomite                    (ii)    Epsom salt

(iii)    Carnallite                   (iv)   Magnesite

Solution:

(i) Dolomite – MgCO3.CaCO3

(ii) Epsom salt – MgSO4.7H2O

(iii) Carnallite – KCl.MgCl2.6H2O

(iv) Magnesite – MgCO3

Question 1: Which of the folloiwng minerals is not an orde of magnesium?

a. Magnesite ,

b. Dolomite

c. Epsomite

d. Bauxite

Question 2: Which of the following statemenets is incorrect?

a. On heating magnesium combines with nitrogen to form magnesium nitride.

b. Magnesium does not react with dry air but slowly gets tarnished in most air due to the formation of a thin film of the oxide, MgO.

c. Magnesium does not decompose water in cold but decomposes boiling water or steam.

d. On heating magnesium combines with nitrogen to form magnesium nitrate.

Question 3: Mg + C2H5I →

a. C2H5MgI

b. MgI2 + C2H4

c. C2H5Mg

d. C2H4 + I2

Question 4: The chemical formulae of magnesite is

a. MgCO3

b. MgSO4.7H2O

c. MgCO3.CaCO3

d. KCl.MgCl2.6H2O

Q.1

Q.2

Q.3

Q.4

d

d

a

a

Related Resources

Look here for past year papers of IIT JEE to get an idea about the type of questions asked in exam.

You should also refer to the syllabus of chemistry for IIT JEE which would help you to plan your study.