ICSE Class 10 Chemistry Revision Notes Chapter 6 - Electrolysis

When a compound is in a fluid form and when you pass a direct current then the process of decomposing ionic compounds into the form of elements takes place. The cations get reduced to cathode and anions basically get oxidized at the anode. Electrolytes, electrodes, and some form of external power source are the main components that are required for conducting the process of electrolysis. As an optional, a partition as an ion-exchange membrane or a salt bridge is also applied. But they are completely optional. It is important to apply them to keep the products from diffusing near the opposite electrode.

When we pass the electricity to their original elements which are hydrogen and oxygen, the salt-containing water or acidification can be decomposed. Molten sodium chloride gets decomposed into the form of sodium and chlorine atoms.

An electrolytic cell is utilized for the process of electrolysis because it contains two electrodes and they are cathode and anode and they are connected to a direct source of current and an electrolyte which goes through a process of decomposition because it is an ionic compound and that happens in either molten form or in a dissolves state in a suitable solvent which is required. Only electrodes that are made up of metal, graphite, and semiconductor materials, are only used for this process. The choice of a suitable electrode is completely based on the chemical reactivity between the electrode and electrolyte as well as the manufacturing cost that you get.

Electrolytic Process

When the process of electrolysis takes place then there is an interchange of ions and atoms and that is due to the addition or removal of electrons from the external circuit which happens because of the passing of current, cautions move to the cathode and take electrons from the cathode and get discharged in the neutral atom. If the neutral atom is in the solid form then it gets deposited on the cathode and if in the form of gas then it moves upwards, and therefore this is a reduction process and the cation gets reduced at the cathode. Anions at the same time produce extra electrons to the anode and are oxidized to neutral atoms at the anode. Electrons get released by the anions and they travel across the electrical circuit and reach the cathode for completing the circuit. Electrolysis also involves simultaneous oxidation reaction at anode and the reduction reaction occurs at the cathode. Just like when an electric current passes through the molten sodium chloride then the sodium ion gets attracted by the cathode from which it takes an electrode and converts into a sodium atom. Chloride ion reaches the anode and produces electrons and convert them into chlorine atoms to form chlorine molecules.

Na+(in electrolyte) + e–(from cathode) → Na …. At Cathode

Cl–(from electrolyte) → e– + Cl → Cl2 …. At Anode

The process of electrolysis is useful and helps in getting the elemental forms from compounds as well as it is indirectly also used in the metallurgy of alkali and alkaline earth metals, deposition of metals, purification of metals, etc.

Cell Potential or Voltage

The ability of the individual ions to absorb or release electrons actually decides how much voltage or cell potential is required for the process of electrolysis. This is also sometimes known as Decomposition Potential as well as Decomposition Voltage which actually refers to the minimum voltage which is required for electrolysis. Thermodynamically the decomposition voltage at which this process of electrolysis is preferred is the actual difference of the electrode potentials as calculated by using the Nernst equation. The application of additional voltage is known as overpotential and can lead to an increase in the rate of reaction and is mostly above the thermodynamic value. Therefore it is indispensable that the process of electrolysis involves gases such as oxygen, hydrogen, and chlorine. This ability is considered and measured as an electrode potential of the ions which is generally present in the electrolytic cell. The cell potential or voltage is basically the sum of the potential which is required for the reduction as well as an oxidation reaction. There is an involvement of potential in various redox reactions is available in literature as standard reduction potential. So, the reaction with the positive redox cell potential or voltage will be feasible as per thermodynamic Gibbs free energy (or standard potential). So, the electrolysis process is thermodynamically controlled.

In the process of electrolysis the potential which is equal to or slightly more than is generally applied externally. If the ions are stable and reacting then they need to undergo reaction in the presence of the externally applied cell potential or voltage. Thus, external potential leads to unfavorable reactions to occur.

Thus, it is generally seen in the process of electrolysis that the chemical bonds which connect atoms are either made or broken and therefore, electrolysis leads to the conversion of electrical energy into chemical energy.