9.5 - Electrolytic cells
9.5.1 - Describe, using a diagram, the essential components of an electrolytic cell. The diagram should include the source of electric current and conductors, positive and negative electrodes and the electrolyte.
An electrolytic cell is the apparatus used to pass electricity through an electrolyte (a conducting liquid, either a molten ionic compound or an ionic solution)
The electrolytic cell has two (usually inert) electrodes to pass the electric current into the electrolyte.
The negative electrode is called the cathode and the positiive electrode is called the anode.
The inert electrodes are usually made of graphite or platinium wire.
The reactions occur at the surface of the electrodes.
The power supply is usually symbolised by a short fat line (negative) and a long thin line (positive)
9.5.2: State that oxidation occurs at the positive electrode (anode) and reduction occurs at the negative electrode (cathode)
Negative ions arrive at the anode, attracted by electrostatic forces, where they lose electrons. Loss of electrons is defined as oxidation.
The reverse process occurs at the negative electrode, where positive ions pick up electrons. They are consequently reduced.
9.5.3: Describe how current is conducted in an electrolytic cell.
Conduction within an electrolytic cell
Current passes around the external circuit to and from the battery in the normal way i.e. by the movement of electrons. However, in the cell itself there is a very different process occurring.
Positive ions from the electrolyte pick up electrons at the cathode and use them to perform reduction of the ion (reduction = addition of electrons). At the same time negative ions migrate to the positive electrode (anode) to drop off electrons and get oxidised (oxidation = loss of electrons).
Overall, there are ions picking up electrons from one electrode (the cathode) and DIFFERENT IONS dropping off different electrons at the other electrode (anode). The net effect is as if electrons are jumping from one electrode to the other. It should be stressed that at no time do electrons cross the electrolyte. The battery, however, cannot distinguish between electrons and to all intents and purposes, as a current passes around the external circuit, it seems also to pass through the electrolyte.
As an analogy one could consider the processes occurring in a bank. Certain clients deposit money and other clients withdraw money. To the bank it appears as though there is a constant flow of money in and out. What happens to this money outside the bank is immaterial. In the electrolytic cell certain ions deposit electrons onto an electrode and other ions withdraw electrons from an electrode. The electrodes are attached to the battery that performs the accountancy. The battery sees electrons leaving one side and arriving at another - it detects the flow of current in and out.
9.5.4: Deduce the products of the electrolysis of a molten salt. Half equations showing the formation of products at each electrode will be assessed.
Example: Electrolysis of molten sodium chloride
Molten sodium chloride contains Na+ ions and Cl- ions
The positive sodium ions are attracted by electrostatic forces to the cathode (negative electrode). Once they get there they encounter many available electrons and each sodium ion can accept one electron and be reduced to a sodium atom:
Na+ + 1e Na
The negative chloride ions are attracted to the positive electrode by elecrostatic forces and migrate there. Once there, the negative chloride ions encounter an anode seriously lacking in elecrtrons with positive 'holes'. The chloride ions 'drop off' their electrons into the positive 'holes' and become chlorine atoms:
Cl- - 1e Cl
(The chlorine atoms produced then pair up to become chlorine molecules)
The overall effect for the circuit is that for each Na+ and Cl- reaction at the electrodes, one electron has been removed from the cathode and another electron has been deposited on the anode.
The battery detects electrons flowing around the circuit as an electric current and the bulb lights.
2Na+ + 2Cl- 2Na + Cl2