4.3.5 Representation of reactions at electrodes as half equations
Half equations show reduction at the cathode and oxidation at the anode
Definition
Half equation
An equation that shows the electrons lost or gained by a single species in a redox reaction.
Definition
Electrolysis
The decomposition of an ionic compound, when molten or in solution, by passing an electric current through it.
- At the cathode, the reaction is reduction, because ions gain electrons.
- At the anode, the reaction is oxidation, because ions lose electrons.
- Every half equation must balance both the atoms and the total charge.
Key Idea
- The number of electrons equals the total charge being neutralised at that electrode.
- The same rules apply whether the electrolyte is molten or aqueous.
Cathode half equations across molten and aqueous electrolytes
Definition
Cathode
The negative electrode during electrolysis, where positively charged ions gain electrons.
- Hydrogen from an aqueous solution: 2H++2e−→H22\text{H}^{+} + 2\text{e}^{-} \rightarrow \text{H}_22H++2e−→H2
- Copper from copper chloride solution: Cu2++2e−→Cu\text{Cu}^{2+} + 2\text{e}^{-} \rightarrow \text{Cu}Cu2++2e−→Cu
- Sodium from molten sodium chloride: Na++e−→Na\text{Na}^{+} + \text{e}^{-} \rightarrow \text{Na}Na++e−→Na
- Aluminium from molten aluminium oxide: Al3++3e−→Al\text{Al}^{3+} + 3\text{e}^{-} \rightarrow \text{Al}Al3++3e−→Al
Exam technique
- Add one electron per unit of positive charge to the left of a cathode half equation.
- Balance a diatomic product such as H2\text{H}_2H2 before counting the electrons.
Anode half equations across molten and aqueous electrolytes
Definition
Anode
The positive electrode during electrolysis, where negatively charged ions lose electrons.
- Chlorine from a chloride solution: 2Cl−→Cl2+2e−2\text{Cl}^{-} \rightarrow \text{Cl}_2 + 2\text{e}^{-}2Cl−→Cl2+2e−
- Bromine from a bromide: 2Br−→Br2+2e−2\text{Br}^{-} \rightarrow \text{Br}_2 + 2\text{e}^{-}2Br−→Br2+2e−
- Oxygen from the hydroxide ions in solution: 4OH−→O2+2H2O+4e−4\text{OH}^{-} \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^{-}4OH−→O2+2H2O+4e−
- Oxygen from molten aluminium oxide: 2O2−→O2+4e−2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^{-}2O2−→O2+4e−
Self review
- Which type of reaction happens at the anode?
- Write the cathode half equation for copper ions.
- Write the cathode half equation for sodium ions.
- Write the anode half equation for chloride ions.
- Write the anode half equation for hydroxide ions forming oxygen.
