Electrolysis
What Is Electrolysis?
Electrolysis is the process of using electricity to break down an ionic compound into its elements. The compound being broken down is called the electrolyte and must be either molten or dissolved in water so that the ions are free to move.
The electrolyte is placed between two electrodes: the cathode (negative electrode) attracts positive metal ions, and the anode (positive electrode) attracts negative non-metal ions.
Electrolysis: positive ions (cations) move to the cathode (negative electrode), negative ions (anions) move to the anode (positive electrode). At the cathode, metals are deposited. At the anode, non-metals are released.
At the cathode, positive metal ions gain electrons (reduction) to form metal atoms. At the anode, negative ions lose electrons (oxidation) to form atoms or molecules. Remember: PANIC — Positive Anode, Negative Is Cathode.
Electrolysis of Molten Compounds
When a molten binary ionic compound (e.g. lead bromide, \(PbBr_2\)) is electrolysed:
- Cathode: \(Pb^{2+} + 2e^- \rightarrow Pb\) (lead metal is deposited — reduction).
- Anode: \(2Br^- \rightarrow Br_2 + 2e^-\) (bromine gas is released — oxidation).
When an ionic compound is dissolved in water, there are additional \(H^+\) and \(OH^-\) ions from the water. The rules for predicting products at each electrode are:
Cathode: If the metal is more reactive than hydrogen, hydrogen gas is produced. If less reactive than hydrogen, the metal is deposited.
Anode: If a halide ion is present, the halogen is produced. If no halide is present, oxygen is produced (from \(OH^-\) ions).
Extraction of Aluminium
Aluminium is too reactive to be extracted by reduction with carbon, so it is extracted by electrolysis of molten aluminium oxide (\(Al_2O_3\)). The aluminium oxide is dissolved in molten cryolite to lower the melting point and reduce energy costs.
At the cathode: \(Al^{3+} + 3e^- \rightarrow Al\) (aluminium is deposited).
At the anode: \(2O^{2-} \rightarrow O_2 + 4e^-\) (oxygen is released). The carbon anodes gradually burn away because the oxygen reacts with them at high temperature, so they need regular replacement.
Aluminium extraction: electrolysis of \(Al_2O_3\) dissolved in cryolite. Al deposited at cathode, \(O_2\) at anode. Carbon anodes burn away and need replacing. The process uses large amounts of electricity, making it expensive.
Required Practical 3 involves electrolysing aqueous solutions using inert (carbon or platinum) electrodes. You observe the products at each electrode and test them: hydrogen burns with a squeaky pop, chlorine bleaches damp litmus paper, and oxygen relights a glowing splint.