Electrolysis involves dissociation (lysis) of an electrolyte by using a direct electric current. In this process, electromotive force is used to carryout a non-spontaneous redox chemical reaction.
The setup used to serve this purpose is known as electrolytic cell.
An electrolytic cell is used to perform electrolysis. It is provided with two electrodes, which are connected to different ends of the DC electric source. The electrode connected to the positive end is referred to as anode and that which is connected to the negative end is referred to as cathode.
Electrodes help in the conduction of electrons into and out of the cell as well as provide the surface for electrode reactions. They are made up of electric conductors like metals or graphite. They may be in the form of rods or as a surface coating on the rods of other material or as a coating on the inside surface of the electrolytic cell.
They may be divided into two types depending on how they are involved in the electrolytic process.
i) Inert electrodes - which do not enter into the electrolytic chemical reactions. The electrodes made up of Noble metals like platinum are used as inert electrodes.
ii) Active electrodes - which take part in the reactions of electrolysis. These are either dissolved into the electrolyte or a substance is deposited on them.
The cell is filled with the melt or the aqueous solution of the electrolyte and is subjected to potential difference sufficient to drive the electrolysis. The electrolyte is the substance that furnishes free ions either in the molten state or in the aqueous medium.
During electrolysis, the free ions furnished by the electrolyte are migrated towards oppositely charged electrodes and are discharged under electric potential.
* Cations migrate towards cathode and are reduced.
* Anions migrate towards anode and are oxidized.
E.g. Let us take a hypothetical electrolyte, XY, which is ionized as follows:
XY -------> Xn+ + Yn-
The cation, Xn+ migrates towards the cathode and get discharged to X by reduction i.e. by gaining electrons. Whereas, the anion, Yn- move towards the anode and is oxidized to Y by losing electrons.
Xn+ + ne- -----------> X (reduction half reaction)
Yn- -----------> Y + ne- (oxidation half reaction)
The complete electrolytic reaction can be written as:
XY -------> X + Y
Thus electrolysis is a redox reaction. The reduction half reaction occurs at cathode, whereas oxidation half reaction occurs at anode. The entire process, otherwise non spontaneous, is driven by the electromotive force.
However, sometimes the solvent molecules or the electrodes may also participate in the electrolysis instead of the ions furnished by the electrolyte.
The electrolytes are ionic compounds, which furnish free ions in the molten state. When ample electromotive force is applied, these free ions start migrating towards the electrodes and get discharged.
For example, when molten KCl is subjected to electrolysis using inert electrodes, the K+ ions migrate towards cathode and get reduced to potassium metal, K by gaining electrons. The Cl- ions migrate towards the anode and get oxidized by losing electrons to liberate Cl2 gas.
Ionization of electrolyte:
KCl -------------------> K+ + Cl-
K+ + 1e- --------------> K (reduction half reaction)
Cl- ---------------------> ½Cl2 (oxidation half reaction)
Complete electrolytic reaction:
KCl -------------------> K + ½Cl2
When the electrolyte is dissolved in water, the ions furnished will be in the hydrated condition; and hence may or may not be discharged at the electrodes depending on the nature of ions, concentration of solution, nature of electrodes etc.
In general, the ions of active metals like alkali and alkaline earth metals do not undergo reduction in presence of water. Instead, water molecules undergo reduction to liberate dihydrogen gas, H2.
For example, when a moderately concentrated aqueous solution of KCl is subjected to electrolysis using inert electrodes, only Cl- ions get discharged at the anode. Whereas, the K+ ions cannot be reduced in presence of water. Instead, water molecules undergo reduction to give hydrogen gas.
However, the ions of less reactive transition metal series can be discharged in the aqueous medium.
For example, the electrolysis of aqueous CuCl2, copper is deposited at cathode along with the liberation of chlorine gas.
Some stable oxo anions like sulfate, phosphate, carbonate, nitrate etc., also do not participate in the electrolytic reactions. Alternatively, the water molecules undergo oxidation and thus by liberating dioxygen gas, O2.
When aqueous solution of AgNO3 is electrolyzed, the silver ion, Ag+ is discharged at the cathode. But the nitrate ion, NO3- cannot be oxidized at anode. In place of it, water molecule is oxidized to liberate dioxygen gas, O2.
Regardless of the nature of the ions, only water molecules undergo electrolysis when dilute solutions are used.
For example, when a very dilute aqueous solution of NaCl is subjected to electrolysis, dihydrogen and dioxygen gases are liberated at cathode and anode respectively. In this case, NaCl helps in improving the electrical conductivity of water, which was otherwise a poor conductor.
The electrodes become active when the metal used in one or either of them and the cation in the electrolyte belong to same element. The electrodes now enter into the electrolytic reaction. At anode, the metal undergoes oxidation and dissolves in the electrolyte and the metal is deposited on cathode due to reduction of metal ions.
For example, when an aqueous solution of silver nitrate, AgNO3 is electrolyzed using silver electrodes, some amount of silver in the anode is dissolved into the solution and the same amount of silver is deposited on cathode.
Author: Aditya vardhan Vutturi