Electrolysis SS2 Chemistry Lesson Note

ELECTROLYSIS

The chemical decomposition of a substance can be brought about by heating. Similarly, an electrical effect can also be used to bring about the chemical decomposition of substances. The Effect of electricity on matter is studied under electrolysis.

DEFINITION OF TERMS:

1. Electrolysis: This is defined as the chemical decomposition of a compound (electrolyte) brought about by the passage of direct current through either a solution or the molten form of the compound.
2. Electrolyte: An electrolyte is a compound which conducts electricity and is decomposed in the process. To behave as an electrolyte, the compound must be in a liquid form either as a molten compound or an aqueous solution of the compound. A non-electrolyte does not conduct electricity in this manner.
3. Electrodes: Electrodes are conductors in the form of wires, rods or plates through which an electric current leaves or enters the electrolyte.
4. Anode: Anode is the positive electrode by which the electrons leave an electrolyte (or by which conventional current enters the electrolyte). It is the electrode which is joined to the positive terminal of the direct current supply.
5. Cathode: This is the negative electrode by which electrons enter the electrolyte. It is the electrode which is joined to the negative terminal of the electric supply.
6. Electrolytic Cell: An electrolytic cell is an assembly of two electrodes in an electrolyte used for the electrolysis of a substance.

In an electrolytic cell, oxidation occurs at the anode (positive electrode) and reduction at the cathode (negative electrode).

MECHANISM OF ELECTROLYSIS 

The electrolysis of a given electrolyte can be carried out in the electrolytic cell in two forms:

1. Using the molten form of the electrolytes
2. Using the solution form of the electrolyte

1. Using the molten (fused) form of the electrolyte: Only two opposite ions from the electrolyte are present. For example, molten NaCl contains Nat and CI ions only. Na+ ions migrate to the cathode to accept electrons and become discharged to produce neutral Na atoms.

Na+(s) + e > Na(s)

While chloride ions migrate to the anode to give up electrons and become discharged to produce Cl atoms which pair up to form chlorine gas, Cl2.

CI > Cl + e > Cl +Cl > Cl2(g)

There is no competition for the discharge of ions at the electrodes.

2. Using the solution form of the electrolysis, ions are produced from the electrolytes and the solvent, usually water, H2O. Two opposite ions from the electrolyte e.g. Na+ and Cl from NaCl and two from water, H+ and OH. In such cases, the cations and anions of both the electrolyte and the solvent will migrate to the cathode and the anode respectively where they will compete with one another to be discharged. The products formed at the electrodes depend on which ions are preferentially discharged, the ions from the electrolytes or the solvent.