Electrolytic Cell – IGCSE Chemistry Definition

An electrolytic cell is the apparatus used to carry out electrolysis. It consists of three essential components: a direct current (d.c.) power supply, two electrodes (an anode and a cathode), and an electrolyte. When the power supply is switched on, it forces a non-spontaneous chemical reaction to occur by pushing electrons around the circuit and through the electrolyte via ion movement.

Components

The d.c. power supply provides the energy needed to drive the reaction. Its positive terminal connects to the anode and its negative terminal connects to the cathode.

The electrodes are solid conductors that dip into the electrolyte. They can be inert (such as carbon/graphite or platinum), meaning they do not react, or active (such as copper in electroplating), meaning they participate in the reaction. The anode is the positive electrode where oxidation occurs (anions lose electrons). The cathode is the negative electrode where reduction occurs (cations gain electrons).

The electrolyte is the ionic compound, either molten or dissolved in water, that contains free-moving ions to carry the current through the liquid. In the solid state, the ions cannot move and electrolysis cannot occur.

How it works

When the cell is operating, cations in the electrolyte migrate towards the cathode, where they gain electrons and are discharged (reduction). Anions migrate towards the anode, where they lose electrons and are discharged (oxidation). The electrons flow through the external circuit from anode to cathode via the power supply.

Exam context

Paper 4 questions may ask candidates to draw and label an electrolytic cell for a specific electrolysis, such as molten lead bromide or aqueous copper sulfate. Marks are awarded for correct labelling of the anode, cathode, electrolyte, d.c. supply, and direction of ion movement.

Worked exam question

Draw a labelled diagram of the apparatus used to electrolyse molten zinc chloride. Label the anode, cathode, electrolyte, and d.c. supply. State the product at each electrode. (4 marks)

Diagram should show a container of molten ZnCl₂ (electrolyte) [1], two electrodes dipping in, connected to a d.c. supply with the positive terminal to the anode and negative to the cathode [1]. Zinc is produced at the cathode [1]. Chlorine is produced at the anode [1].

Common mistakes

Candidates often confuse the direction of ion movement with the direction of electron flow. Ions move through the electrolyte (cations to cathode, anions to anode). Electrons move through the external wire, not through the electrolyte. Another mistake is labelling the anode as negative, which is wrong in an electrolytic cell (though it is negative in an electrochemical/voltaic cell).