Magnesium

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Magnesium belongs to group 2 of the periodic table along with other elements like Calcium (Ca), beryllium (Be), strontium (Sr), barium (Ba) and radium (Ra). These elements are called alkaline Earth metals. Due to their smaller size, the electrons are more tightly bound in them and so the first ionization energies are higher than those of the corresponding group 1 elements. The second ionization energies are almost twice the first ionization energies in these elements. Magnesium forms bipositive cations, but they are less electropositive than the alkali elements. In view of the smaller size of the atoms and ions, alkaline earth metals are denser and harder than the alkali metals and have higher melting points.

Occurrence

Magnesium being quite reactive does not occur in the free state in nature but in the combined state.

Minerals of magnesium

Magnesium is the eighth most abundant element found on the Earth. It occurs as,

About 0.13% of MgCl₂ is present is sea water.

Extraction of magnesium

Magnesium metal is strong reducing agent and therefore, cannot be obtained by chemical reduction method. It is extracted by electrolysis of fused anhydrous salts.

A number of methods are available for the extraction of magnesium metal from its ores. The most widely used method is based on the electrolysis of magnesium chloride obtained from the seawater.

Dow's process

This process consists of the following steps:

• Magnesium ions present in the sea water are precipitated as magnesium hydroxide by the addition of slaked lime, Ca(OH)₂ to the sea water.

• Magnesium hydroxide is separated and converted to magnesium chloride by treatment with hydrochloric acid.

The magnesium chloride is crystallized as MgCl₂.6H₂O

• Fused magnesium chloride for electrolysis is obtained as follows:

Magnesium chloride hexahydrate is partially dehydrated by passing a current of dry hydrogen chloride gas. The magnesium chloride thus obtained is added to a molten mixture of sodium chloride and calcium chloride (35 : 50 : 15). Magnesium chloride melts under this condition with the loss of water at about 1000 K (700 - 725°C).

The molten mixture of MgCl₂, NaCl and CaCl₂ is electrolyzed in an iron cell through which an inert gas (or coal gas) flows to avoid any reaction between the liberated metal and oxygen / nitrogen of the air. The cell wall acts as the cathode, while the carbon rod dipping into the melt acts as anode. Magnesium is obtained at the cathode and chlorine is evolved at the anode. This chlorine is used in making hydrochloric acid, which is required for obtaining magnesium chloride.

At cathode:

At anode:

The liberated metal being lighter than the electrolyte floats over the surface. Chlorine evolved at anode is obtained as a by-product. Metal of 99.9 per cent purity is obtained by this method.

Fig: 12.5 - Electrolysis of magnesium chloride

Physical Properties of Metals

• Magnesium is a silvery-white, soft and light metal (density = 1.75 g/mL).

• It melts at 651°C and boils at 1090°C. It sublimes in vacuum at 550°C.

• It is malleable and ductile.

• It is a good conductor of electricity and heat.

Chemical Properties of Magnesium

Action of air

Dry air has no effect on magnesium and calcium. In moist air, a thin layer of oxide gets formed on its surface. When heated in air, they burn with a dazzling light giving its oxide and nitride.

Action of water

Magnesium reacts very slowly with cold water, and slowly with boiling water to give hydrogen.

When heated with steam, magnesium burns to give MgO and H₂.

Action of acids

• Both magnesium and calcium react with all dilute mineral acids to produce hydrogen.

• With dilute nitric acid, magnesium gives ammonium nitrate, and magnesium nitrate.

Reducing action

Calcium and magnesium have very high affinity for oxygen. They reduce the oxides of metals and non-metals to the respective element.

With non-metals

Magnesium combines with non-metals on heating.

Displacement of less electropositive elements

Mg is highly electropositive, hence it displaces nearly all the metals from their salt solutions.

Formation of alkyl magnesium halides

Magnesium reacts with haloakanes in ether medium, to form alkyl magnesium halide, commonly called as the Grignard's reagent.

Grignard's reagents are very useful synthetic reagents in organic chemistry.

Uses

• In flash light photography, fire works and signal fires.

• As a reducing agent for the preparation of silicon and boron.

• As a fuse in the alumino-thermic process.

• In the manufacture of light and high-tensile alloys such as magnesium aluminium alloy.

• As anodes for corrosion prevention.

Problem

10. Why can't anhydrous magnesium chloride be obtained by heating the hydrated salt?

Solution

Anhydrous magnesium chloride cannot be obtained by heating the hydrated salt because when magnesium hydrate chloride

(MgCl₂.6H₂O) is heated strongly, it hydrolyses to yield magnesium oxide, which is refractory material and not the anhydrous salt.