Electrolysis
When electric current is passed through an electrolyte solution, the ions of electrolyte undergoes chemical changes at the respective electrodes.
The chemical reaction carried out by passing electricity is called as electrolysis.
Electrolytic process is useful in reduction as well as purification of metals.
Electrolytic reduction of metals are useful in the metallurgy of the highly reactive metals like sodium, potassium, cesium, calcium, Magnesium and Aluminum.
The ores are mostly fused before electrolysis is done as only fused ores will conduct electricity.
The metal is deposited on the respective cathode while in the anode the gases are liberated. This method is highly useful for the reduction of most reactive metals. Because they cannot be reduced by any other methods.
Example:
When electric current is passed through molten sodium chloride, sodium is produced at the cathode and chlorine is produced at the anode.
Let us discuss some examples
Electrolysis of molten sodium chloride
The molten NaCl has Na+ and Cl- ions
The reactions may be expressed as
At cathode
At anode
Electrolysis of water
Pure water is poor conductor of electricity. However when few drops of H2SO4 are added to water, it becomes electrolyte.
At cathode
Faraday’s Laws of electrolysis
Michael Faraday performed a large number of experiments on electrolysis and summarized the results in form of two laws known as Faraday’s laws of electrolysis.
1. Faraday’s first law of electrolysis
The amount of any substance obtained gives the amount of chemical reaction which occurs at any electrode during electrolysis.
Thus, if w gram of the substance is deposited on passing Q coulombs of electricity, then
If a current I is passed for t seconds, then
2. Faraday’s second law of electrolysis
"It states that when similar or equal quantity of electricity is passed through different electrolytic solutions connected in series, the weights of the substances that are produced at the electrodes are directly proportional to their chemical equivalent weights."
Example: When same quantity of electricity is passed through two electrolytic solutions, containing CuSO4 and AgNO3 connected in series, the weights of copper and silver deposited are
$\frac{Weight\ of\ Copper\ deposited}{Weight\ of\ Silver\ deposited}$ = $\frac{Eq\ wt\ of\ Copper}{Eq\ wt\ of\ Silver}$
Units of Charge and their relationship
SI unit of charge is Coulomb. Most common unit of charge used in electrolysis is Faraday.
1 Faraday = charge of 1 mole of electrons.
= 6.022 x 1023 x 1.6 x 10-19 Coulomb
= 96458 Coulomb or approximately 96500 Coulomb.
Electrolytic refining is a kind of process in which the pure metals are obtained from the impure metals.
This is considered as one of the most convenient and exclusive method of refining and gives a metal of a very high purity. This method is applicable to many metals such as Cu, Ag, Pb, Au, Ni, Sn, Zn etc. The blocks of impure metal form the anode and thin sheets of pure metal form the cathode. A solution of a salt of the metal is taken as an electrolyte.
On passing an electric current through the solution pure metal dissolves from the anode and deposits on cathode. By this process, more metal ions undergo reduction and pure metal is deposited at the cathode. The insoluble impurities either dissolve in the electrolyte or fall at the bottom and collect as anode mud.
For example, in the refining of copper, impurities like Fe and Zn dissolve in the electrolyte, while Au, Ag and Pt are left behind as anode mud. During the electrolytic refining of a copper, a thick block of impure copper is made anode, and thin plate of pure copper is made cathode. Copper sulphate solution is used as an electrolyte. Normally the pure metal to be refined is made as cathode and the impure metal is made as anode.
Let us examine the process of electrolysis in an electrolytic cell when the electrolyte used is molten sodium chloride.
When sodium chloride is formed the sodium atom donates an electron to the chlorine atom to form an electrovalent bond. Hence in the process
- Sodium atom loses an electron and becomes positively charged (Na+).
- Chlorine atom gains an electron and becomes negatively charged (Cl-).
- The electrolyte consists of sodium (Na+) and chloride ions (Cl-) is shown below.
Before the electric current is switched on, the ions in molten state, through mobile, have no direction of movement. They move only randomly. But when molten sodium chloride is electrolysed (the current is switched on), the ions follow a particular direction of movement.
The cell set up is shown below. The electrodes in this case are of carbon.
The electrolysis of molten sodium chloride is as follows
In solution or molten state, when dissociation takes place, the sodium ion has a positive charge and the chloride ion has a negative charge.
These ions are capable of carrying the current. The positively charged ions Na+ migrate to the cathode, and the negatively charged ions Cl- migrate to the anode. On reaching the respective electrodes these ions get involved in electrochemical reactions at the electrode and form their respective products.
At the cathode, the ions accept electrons. The sodium ion takes in one electron from the cathode, gets converted into a sodium atom, and gets deposited at the cathode. Hence reduction takes place at the negative electrode or cathode.
The product obtained at the cathode is sodium metal.
In this case, the chloride ion has one negative charge that it has gained from sodium. This electron is given up to the anode and chlorine is deposited in the form of bubbles of greenish yellow chlorine gas.
In other words, the product obtained at anode is chlorine gas.
1. Electroplating on metals
2. Electrorefining of metals
3. Electrometallurgy
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Solved Examples
Al3+ + 3e- → Al
Solution:
Solution:
