Electrolysis – SS2 Chemistry Lesson Note (First Term)

Electrolysis is the chemical decomposition of a compound (an electrolyte) by passing direct current through its solution or its molten form. The current enters and leaves through electrodes placed in the liquid. Positive ions go to the cathode and negative ions go to the anode, where they are discharged.

Behavioural objectives

By the end of the lesson, students should be able to:

• define electrolysis, electrolyte, electrode, anode, cathode and electrolytic cell;
• state what happens at the anode and at the cathode of an electrolytic cell;
• describe the electrolysis of a molten electrolyte such as NaCl;
• list the ions present when an electrolyte is dissolved in water;
• explain why ions compete to be discharged during the electrolysis of a solution.

What is electrolysis?

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

What are the key terms in electrolysis?

1. Electrolysis: 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: 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 as an aqueous solution of the compound. A non-electrolyte does not conduct electricity in this manner.
3. Electrodes: conductors in the form of wires, rods or plates through which an electric current leaves or enters the electrolyte.
4. Anode: the positive electrode by which electrons leave an electrolyte (or by which conventional current enters the electrolyte). It is the electrode joined to the positive terminal of the direct current supply.
5. Cathode: the negative electrode by which electrons enter the electrolyte. It is the electrode joined to the negative terminal of the electric supply.
6. Electrolytic cell: 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 occurs at the cathode (negative electrode).

How does electrolysis take place?

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

1. using the molten form of the electrolyte;
2. using the solution form of the electrolyte.

Electrolysis of a molten (fused) electrolyte

Only two kinds of ions, one positive and one negative, are present in the electrolyte. For example, molten NaCl contains Na⁺ and Cl⁻ ions only. Na⁺ ions migrate to the cathode to accept electrons and become discharged to produce neutral Na atoms.

Na⁺(l) + e⁻ → Na(l)

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.

Cl⁻ → Cl + e⁻

Cl + Cl → Cl₂(g)

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

Electrolysis of a solution of an electrolyte

In the solution form of electrolysis, ions are produced from the electrolyte and from the solvent, usually water, H₂O. For NaCl solution, two ions come from the electrolyte (Na⁺ and Cl⁻) and two come from water (H⁺ and OH⁻).

The cations and anions of both the electrolyte and the solvent migrate to the cathode and the anode respectively, where they compete with one another to be discharged. The products formed at the electrodes depend on which ions are preferentially discharged, the ions from the electrolyte or the ions from the solvent.

Evaluation

1. Define electrolysis and electrolyte.
2. State the difference between the anode and the cathode.
3. Which process, oxidation or reduction, occurs at the anode?
4. Why is there no competition for discharge when molten NaCl is electrolysed?
5. List the four ions present in a solution of NaCl in water.

Assignment

1. Write the equations for the discharge of Na⁺ at the cathode and Cl⁻ at the anode during the electrolysis of molten NaCl.
2. Explain why the electrolysis of an aqueous solution is more complicated than the electrolysis of a molten electrolyte.