Electrochemistry, Oxidation and Reduction
Core ideas
Electrolysis uses electrical energy to decompose an ionic compound. The cathode is negative and attracts positive ions (cations), where reduction occurs (gain of electrons). The anode is positive and attracts negative ions (anions), where oxidation occurs (loss of electrons). Oxidation is loss of electrons or gain of oxygen; reduction is gain of electrons or loss of oxygen. A redox reaction involves both.
In electrolysis of molten ionic compounds, the ions are discharged. In aqueous solutions, competition occurs: at the cathode, hydrogen may be produced instead of a metal if the metal is reactive; at the anode, oxygen may form unless a halide is concentrated. Electroplating uses electrolysis to coat an object. Oxidation numbers help identify redox changes.
Traps to watch
- Cathode = reduction (both start with consonants); anode = oxidation.
- In aqueous solutions, water competes with ions, so predictions must consider reactivity.
- Electrolysis requires molten or dissolved ionic compounds, not solids.
- Oxidation numbers are not always equal to ionic charge in covalent compounds.
Rule of thumb
Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Worked example
In the electrolysis of molten lead(II) bromide, Pb²⁺ goes to the cathode and is reduced to lead; Br⁻ goes to the anode and is oxidised to bromine.