Electrolysis in Industries

Electrolysis in industries

Extraction of metals

Purification of metals

Electroplating of metals

Extraction of metals :

(a) Aluminium

Aluminium is extracted from bauxite, which contains Al2O3.

Aluminium oxide is mixed with cryolite, Na3AlF6 – to lower the melting point of Al2O3 ( from 2045 ˚C to 900 ˚C ).

At cathode :

Al3+ ions are discharged to form Al metal.

Al3+(l) + 3e− → Al(l)

Liquid Al is denser than the electrolyte & will be collected at the bottom of the cell.

At anode :

oxide ions are discharged to form oxygen gas.

2O2−(l) → O2(g) + 4e−

Overall equation :

2Al2O3(I) → 4Al(l) + 3O2(g)

Oxygen gas liberated at anode will react with carbon electrode to produce CO2 gas.

C(s) + O2(g) → CO2(g)

Anode is corroded slowly & must be replaced from time to time.

(b) Extraction of Sodium

Sodium is extracted from molten NaCl.

CaCl2 is added to lower the melting point of NaCl (from 900 ºC to 600 ºC).

Cathode : steel anode : graphite.

At cathode, Na+ ions are discharged to form Na metal.

Na+(l) + e− → Na(l)

At anode, chloride ions are discharged to form chlorine gas.

Cl−(l) → Cl2(g) + 2e−

Overall equation :

2NaCl(I) → 2Na(l) + Cl2(g)

Purification of metals

In the purification of metals, the impure metal is used as anode & the pure metal is used as a cathode.

The electrolyte used is an aqueous salt solution of the metal.

Impurities (anode mud)

At anode ( impure Cu ) :

Electrode becomes thinner.

Cu(s) → Cu2+(aq) + 2e−

At cathode ( pure Cu ):

Electrode becomes thicker.

Cu2+(aq) + 2e− → Cu(s)

Electroplating of metals

Electroplating – process of depositing a layer of metal on another substance using electrolysis.

This is to protect the object from corrosion & to give an attractive appearance.

Cathode – object to be electroplated.

Anode – pure plating metal.

Electrolyte – aqueous solutions containing ions of the plating metal.

Electroplating of an iron spoon with copper :

Cathode : iron spoon.

Anode : pure copper.

Electrolyte : copper (II) sulphate solution

At anode : copper electrode becomes thinner.

Cu(s) → Cu2+(aq) + 2e−

At cathode : brown solid deposited on iron spoon.

Cu2+(aq) + 2e− → Cu(s)

To ensure good quality plating :

Metal object to be plated must be clean & free of grease.

The concentration of ions of plating metal must be low.

Electric current must be small.

Object to be plated must be turned steadily.

Benefits of electrolysis in industries

Reactive metals can be extracted.

A very thin layer of metal can be coated on the object.

To purify metals.

Disadvantages of electrolysis in industries

Pollution.

Expensive.

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~ 靜思語