The passage of an electric current via molten or aqueous solutions of electrolytes yields in chemical reactions. This is electrolysis, broadly employed in industrial methods. The method is costly due to the large quantities of electricity needed. In spite of this demerit it is employed in electroplating, metal extraction from their ores, preparation of chemical compounds and purification of the copper metal.
Corrosion of steel and iron that we call rusting is of particular economic significance.
Industrial Applications of Electrolysis:
Purification of copper (refining):
Copper takes place naturally as the impure element. Even whenever produced through smelting copper pyrites (CuFeS2 ) in a blast furnace the copper obtained is still not 100% percent pure. The copper should be further refined for numerous of its industrial uses specifically in the electrical engineering industry where copper of extremely high purity is required. The impure copper known as the blister copper is purified through electrolysis.
The impure copper is made the anode while the cathode is a thin sheet of pure copper. The electrolyte is copper (II) tetraoxosulphate (vi). At the cathode copper is preferentially discharged as it is lower than hydrogen on the electrochemical series.
Cu2+ (aq) + 2e → Cu (s)
At the anode neither the OH- nor SO42- is discharged however copper goes to the solution as it needs less energy than the discharge of the ions.
Cu(s) → Cu2+ (aq) + 2e
The total effect is that copper goes to the solution at the anode and it is deposited at the cathode. The anode dissolves away as the copper ions go into solution. The impurities are not dissolved and fall to the bottom of the tank. The copper get is of extremely high purity.
Electroplating includes forming an extremely thin coat of one metal on the surface of the other. This might be for protection or for decorative aims. The article to be coated is made the cathode and the metal which will form the coat, made the anode. The electrolyte is a solution of the salt of the metal which will form the coat. Example: silver plating a steel rod, the steel rod is made up of the cathode and pure silver is made the anode. The electrolyte is silver trioxonitrate (V) solution.
Fig: Silver plating a steel rod
Throughout electrolysis, silver is deposited at the cathode and silver goes to solution at the anode.
Ag+ + e → Ag (s)
Ag(s) → Ag+ + e
The concentration of electrolyte remains virtually constant all through the procedure. For good outcomes, the surface of the metal to be plated should be clean and grease free. In place of silver other metals could be employed.
Electroplating prevents rusting and improves the appearance of the finished product.
Isolation of elements or extraction of ores:
Reactive metals like aluminium, sodium and potassium are made up from their molten chloride or hydroxide via electrolysis - (we are familiar that these metals are high on the electrochemical series). Oxygen and chlorine gases are as well made up by electrolysis of their aqueous sodium salts.
Aluminium finds numerous significant uses as a structural metal due to its strength and light weight. The commercial availability of aluminium has been made possible via the method of electrolysis. The electrolytic method of extraction of aluminium makes utilization of a solution of aluminium oxide (alumina) in cryolite Na3AlF6. The electrolytic cell is the iron container lined by graphite. The graphite lining serves as the cathode and the anodes are graphite rods dipping to the electrolyte solution.
At cathode, Al is deposited,
Al3+ + 3e → Al(s) reduction
At anode, oxygen gas is released,
O2- → O + 2e
O + O → O2
The electrolyte should be kept at very high temperatures (950°C) via heating electrically. This method is thus very costly.
Preparation of sodium hydroxide:
Sodium hydroxide is the vital raw material in numerous industries. The soap, paper and chemical industries and numerous others make use of sodium hydroxide. Sodium hydroxide is prepared industrially through the electrolysis of brine by employing inert electrodes.
NaCl (aq) → Na+ + Cl-
H2O → H+ + OH-
At cathode, H2 (g) is released and at anode Cl2 (g), is given off. The total effect is that the brine solution becomes more and more alkaline. The total reaction is the formation of sodium hydroxide from brine.
The quantity or amount of products released or deposited at the electrodes throughout electrolysis is quantified and compared by employing Faraday's laws of electrolysis. Consider an electrolysis method in which the silver ions are discharged.
For a mole of silver to be deposited, it would signify that a mole of silver ions have picked a mole of electrons. The electronic charge is 1.602 x 10-19 coulomb (Remember the Millikans oil drop experiment). For 1 mole of electron the net charge is calculated from the product of the electronic charge and Avogadro's constant. This quantity of charge which deposits a mole of Ag+ and any other monovalent ion at the electrode is termed as the Faraday (F) after Michael Faraday who first observed this relation.
F = Ne
= 6.023 x 1023 x 1.602 x 10-19
≈ 96500 coulombs
Here, N = Avogadro constant and e = electronic charge
To release or liberate a mole of a divalent ion at the electrode, two Faradays of charge are needed and for a trivalent ion, three Faradays. This can be applied to both anions and cations and this is an application of the mole theory in electrolysis.
Faraday's laws are two. The first law defines:
1) The quantity or amount of substance deposited or released at the electrodes throughout electrolysis is directly proportional to the quantity of electricity passed.
m = E it
m = Mass deposited (g)
E = Electrochemical equivalent of the element
i = Current (in amperes)
t = Time (in seconds)
'E' is a constant for the specific ion discharged and is generally extremely small.
For silver E = 1.118 x 10-1 g C-1 and
For hydrogen E = 1.045 x 10-5 g C-1
For numerous computations 'E' is not needed. The mole concept is employed whenever the faraday constant is given.
Corrosion of Metals:
We are familiar that electrochemical series is employed to illustrate the preferential discharge of ions throughout electrolysis. Most of the metals react with water and oxygen in the air and are stated to corrode. Corrosion can be associated to the electrochemical series. The very reactive metals, sodium and potassium react quickly by oxygen and water and are generally stored in the liquid paraffin to prevent their corrosion.
Iron in the form of steel is a significant engineering material. This is employed in the construction of bridges, fly-over's and store houses. This is as well employed in the automobile industry. Iron and steel are extremely prone to corrosion. For iron and steel this is termed as rusting. Rusting is of special financial significance.
Rusting of Iron:
Rusting is mainly caused by the action of oxygen and water vapor on iron or steel. Rust is hydrated iron (III) oxide having varying composition and can be deduced as x Fe2O3. yH2O. Iron doesn't rust in the absence of oxygen and moisture. An iron nail place in a test tube where air and moisture are excluded will not rust. Rusting is electrolytic in character and is accelerated through the presence of dissolved salts in solution. This is why iron or steel parts of machineries rust faster close to the sea. Where the atmosphere is polluted by gases such as CO2, H2S and SO2 that can dissolve and increase electrolytic conduction, the rate of rusting is as well accelerated.
Methods Used to Prevent Corrosion:
There are numerous methods which are used to prevent iron and steel from rusting. All the processes attempt to prevent air and moisture from getting to the iron or steel surface. The methods are:
This can be through spraying, dipping or brushing. Given the paint surface is not broken the iron will not rust.
ii) Plastic Coating:
The plastic layer prevents oxygen and water from getting to the iron surface. This kind of protection is employed for draining racks in the kitchen. Whenever the plastic breaks rusting sets in.
iii) Greasing or Oiling:
This will as well prevent contact by air and water. This is a very good process for treating moving parts which are made up of iron.
The iron product is coated via zinc. The zinc coating can be through spraying or dipping. If the zinc coating is scratched rusting will not occur, rather than iron to rust, it is the zinc which is influenced because it consists of a higher discharge potential than iron.
Fe2+ + 2e → Fe (s) Eo = - 0.44v
Zn2+ + 2e → Zn (s) Eo = - 0.76v
v) Coating with tin:
This kind of coating is employed for food cans. Zinc coating can't be employed as zinc is poisonous. If the tin coating is scratched and the bare steel is exposed rusting will occur. This is due to the reason of the discharge potential of iron is higher than that of tin.
Fe2+ + 2e → Fe (s) Eo = - 0.44v Fe(s)
Sn2+ + 2e → Sn (s) Eo = - 0.14v Sn(s)
vi) Sacrificial cathodic protection:
This is much similar to what happens if iron is galvanized. This process is employed for protecting ship body from rusting. In the presence of dissolved salt and gases in sea water, rusting is much sooner. To prevent ship body from rusting, blocks of an appropriate metal are strapped to the steel body of the ship. The metals employed are more reactive than steel and are sacrificed in place of the iron.
By employing electrolysis a thin coating of nickel can be deposited on the iron. This might be followed by the other thin layer of chromium to provide a nice shining surface example: car bumpers and electric kettles.
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