Cambridge IGCSE Chemistry Notes 4: Electrochemistry
Cambridge IGCSE Chemistry 0620 and 0971 notes on electrolysis, electrode products, half-equations, electroplating and hydrogen-oxygen fuel cells.
Topic 4 of Cambridge IGCSE Chemistry 0620 and 0971 explains how electrical energy drives non-spontaneous chemical change and how a hydrogen-oxygen fuel cell produces electrical energy. Official sections 4.1 and 4.2 require confident electrode identification, ion movement, product prediction, half-equations, electroplating and a balanced evaluation of vehicle fuel cells.
Electrolysis and the electrolytic cell
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by passage of an electric current.
The electrolyte is the molten or aqueous substance that undergoes electrolysis. It must contain mobile ions. A solid ionic compound does not conduct because its ions are fixed in a lattice.
In an electrolytic cell:
- the anode is the positive electrode
- the cathode is the negative electrode
- cations move through the electrolyte to the cathode
- anions move through the electrolyte to the anode
The external power supply pushes electrons towards the cathode and draws electrons away from the anode.
Charge transfer at the electrodes
Reduction is gain of electrons. It occurs at the cathode. Oxidation is loss of electrons. It occurs at the anode.
A cation reaches the cathode and gains electrons. An anion reaches the anode and loses electrons. Half-equations show this transfer while conserving atoms and charge.
For copper deposition:
Cu²⁺(aq) + 2e⁻ → Cu(s)
For chlorine formation:
2Cl⁻(aq) → Cl₂(g) + 2e⁻
Electrons travel in the wires and electrodes. Ions move in the electrolyte. Electrons do not cross an aqueous solution as the charge carriers in this model.
Predict products from a molten binary compound
A molten binary ionic compound contains only its positive and negative ions. The positive ion forms its element at the cathode and the negative ion forms its element at the anode.
Molten lead(II) bromide contains Pb²⁺ and Br⁻:
- cathode:
Pb²⁺ + 2e⁻ → Pb - anode:
2Br⁻ → Br₂ + 2e⁻
The observations are grey molten lead at the cathode and orange-brown bromine vapour at the anode. Water is absent, so hydrogen and oxygen are not competing products.
Named aqueous electrolysis: concentrated sodium chloride
Concentrated aqueous sodium chloride contains Na⁺, Cl⁻ and ions associated with water.
Hydrogen forms at the cathode rather than sodium because sodium is much more reactive than hydrogen. Chlorine forms at the anode because chloride is present at high concentration.
