Detailed Discussion On Magnesium

Magnesium Metal

Magnesium occurs as magnesite MgCO₃, dolomite CaMg(CO₃)₂, Epsomite (MgSO₄.7H₂O) and 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

(a) From magnesite or Dolomite

The ore is first calcined to form the oxide

MgCO₃ $\to$ MgO + CO₂

CaCO₃.MgCO₃ $\to$ 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 $\to$ 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 $\to$ 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: Mg⁺² + 2e^- $\to$ Mg

At cathode: 2Cl^- $\to$ Cl₂ + 2e^-

Electrolysis of Magnesium Chlorine

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.

Properties of Magnesium

Physical Properties

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

(ii) It is a light metal with a density of 1.74 g cm^-3.

(iii) It is fairly malleable and ductile.

Chemical Properties

(i) 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.

$2Mg+O_2\stackrel{\Delta }{\to }2MgO$

(ii) Action of CO₂ and SO₂

Because of its great affinity for oxygen magnesium keeps on burning even in CO₂ or SO₂.

$2Mg+C{O}_2\stackrel{\Delta }{\to }2MgO+C$

$2Mg+S{O}_2\stackrel{\Delta }{\to }2MgO+S$

(iii) Action of nitrogen

On heating magnesium combines with nitrogen to form magnesium nitride.

$3Mg+N_2\to M{g}_3{N}_2$

Thus when magnesium burns in air both the oxide and the nitride are formed.

(iv) Action of halogens

Magnesium on heating with halogens readily forms the halides e.g.

$Mg+C{l}_2\stackrel{\Delta }{\to }MgC{l}_2$

(v) Action of water

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

$Mg+H_{2}O\left(steam\right)\to MgO+H_2$

(vi) Action of Acids

Dilute acids reacts with magnesium to produce dihydrogen.

$Mg+2HCl\to MgC{l}_2+H_2$

$Mg+\underset{\left(dil.\right)}{H_{2}S{O}_4}\to MgS{O}_4+H_2$

$Mg+2HN{O}_3\to Mg{\left(N{O}_3\right)}_2+H_2$

However with conc. H₂SO₄, SO₂ is produced

$Mg+\underset{\left(conc.\right)}{2H_{2}S{O}_4}\to MgS{O}_4+S{O}_2+2H_{2}O$

(vii) Reaction with alkyl halide

Magnesium reacts with alkyl halides in dry ether to form covalent compound called Grignard reagent.

$Mg+C_2{H}_{5}I\stackrel{Dryether}{\to }{C}_2{H}_{5}MgI$

Uses of Magnesium

(i) The chief use of magnesium is in the preparation of alloys with aluminium, 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.

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

(iii) Mg is used for ignition of thermite charge in aluminothermy.

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

(v) In preparation of Grignard reagent.

(vi) Being a reducing agent, magnesium is used in the extraction of boron and silicon from their respective oxides.

$B_2{O}_3+3Mg\to 3MgO+2B$

$Si{O}_2+2Mg\to 2MgO+Si$

Illustration 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.

Illustration 2. Name some minerals of magnesium.

Solution: Minerals of magnesium are:

(i) Magnesite,

(ii) Dolomite

(iii) Epsom salt