Electrolysis and Electrolytic Processes
What Is Electrolysis?
Electrolysis is the process of using an electric current to break down an ionic compound. The compound being broken down is called the electrolyte, and it must be either molten or dissolved so that the ions are free to move.
The electrolyte is connected to two solid conductors called electrodes. The cathode is the negative electrode; the anode is the positive electrode. The memory aid PANIC helps: Positive Anode, Negative Is Cathode.
Electrolysis: positive ions (cations) move to the cathode (negative) and gain electrons — this is reduction. Negative ions (anions) move to the anode (positive) and lose electrons — this is oxidation. PANIC: Positive Anode, Negative Is Cathode.
Electrolysis of Molten Ionic Compounds
When a molten ionic compound is electrolysed, only the ions from that compound are present. For molten lead(II) bromide (\(PbBr_2\)):
- Cathode: \(Pb^{2+}\) ions gain electrons → lead metal deposited. \(Pb^{2+} + 2e^- \rightarrow Pb\)
- Anode: \(Br^-\) ions lose electrons → bromine gas released. \(2Br^- \rightarrow Br_2 + 2e^-\)
These equations showing what happens at each electrode are called half-equations. The electrons lost at the anode must equal the electrons gained at the cathode.
Electrolysis of Aqueous Solutions
When an ionic compound is dissolved in water, \(H^+\) and \(OH^-\) ions from water are also present. This means there is competition at each electrode between different ions:
At the cathode:
- If the metal cation is less reactive than hydrogen (e.g. Cu²⁺, Ag⁺) → the metal is deposited.
- If the metal cation is more reactive than hydrogen (e.g. Na⁺, Zn²⁺) → hydrogen gas is produced instead.
At the anode:
- If a halide ion is present (Cl⁻, Br⁻, I⁻) → the halogen is produced (e.g. chlorine gas).
- If no halide is present → oxygen is produced from the discharge of \(OH^-\) ions.
Aluminium Extraction by Electrolysis
Aluminium is too reactive to be extracted by reduction with carbon, so it is extracted by electrolysis of molten aluminium oxide (\(Al_2O_3\)).
Pure aluminium oxide has a very high melting point (over 2000 °C). To reduce energy costs, it is dissolved in molten cryolite, which lowers the melting point to around 950 °C.
Aluminium extraction: electrolysis of \(Al_2O_3\) dissolved in molten cryolite (lowers melting point). At cathode: \(Al^{3+} + 3e^- \rightarrow Al\). At anode: oxygen released reacts with the carbon anodes, burning them away — they need regular replacement.
Cathode (reduction): \(Al^{3+} + 3e^- \rightarrow Al\)
Anode (oxidation): \(2O^{2-} \rightarrow O_2 + 4e^-\)
The oxygen produced at the anode reacts with the carbon (graphite) anodes at high temperature, forming carbon dioxide. This means the anodes gradually burn away and must be replaced regularly — a significant running cost.
Core Practical: Electrolysis of Copper Sulfate Solution
This core practical investigates the electrolysis of aqueous copper sulfate solution, comparing the effect of inert electrodes (carbon) versus active electrodes (copper).
With inert (carbon) electrodes:
- Cathode: copper metal deposits (Cu²⁺ + 2e⁻ → Cu). The cathode gains mass.
- Anode: oxygen gas is produced (no halide present). The copper sulfate solution gradually becomes paler as Cu²⁺ ions are removed.
With copper electrodes (active):
- Cathode: copper still deposits — cathode gains mass.
- Anode: the copper anode dissolves (Cu → Cu²⁺ + 2e⁻). The anode loses mass. The Cu²⁺ concentration in solution remains roughly constant, making this the basis of copper purification and electroplating.
To test the gas at the anode: a glowing splint would relight if it were oxygen. To test for copper, observe the reddish-brown deposit at the cathode. Weigh the electrodes before and after to quantify mass changes.
Common Misconceptions and Exam Pitfalls
A frequent error is assuming that the positive electrode always produces oxygen — this only applies when no halide ions (such as chloride) are present in solution. When chloride ions are present in higher concentration, chlorine gas forms at the anode instead. Likewise, students sometimes confuse electrolytic cells with electrochemical cells: in an electrolytic cell, electrical energy is put in to drive a non-spontaneous chemical change, whereas in a galvanic cell, chemical energy is converted out to electrical energy. Always identify the type of electrode material and the ions present in solution before predicting products.