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Electrolysis and half-equations
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Electrolysis uses electricity to break down ionic compounds. Half-equations show what happens at each electrode.
Why ionic compounds conduct
Covalent compounds do not conduct: they have no ions or free electrons to carry charge.
Ionic compounds conduct only when molten or in aqueous solution, because then the ions are free to move. In a solid the ions are held in fixed positions.
Cations are positive ions and go to the cathode (negative). Anions are negative ions and go to the anode (positive). Inert electrodes, such as graphite or platinum, are used.
Ionic compounds conduct only when molten or in aqueous solution, because then the ions are free to move. In a solid the ions are held in fixed positions.
Cations are positive ions and go to the cathode (negative). Anions are negative ions and go to the anode (positive). Inert electrodes, such as graphite or platinum, are used.
Half-equations
At the cathode, ions gain electrons (reduction):
Pb2+ + 2e− → Pb
2H+ + 2e− → H2
Cu2+ + 2e− → Cu
At the anode, ions lose electrons (oxidation):
2Br− → Br2 + 2e−
2Cl− → Cl2 + 2e−
4OH− → O2 + 2H2O + 4e−
Pb2+ + 2e− → Pb
2H+ + 2e− → H2
Cu2+ + 2e− → Cu
At the anode, ions lose electrons (oxidation):
2Br− → Br2 + 2e−
2Cl− → Cl2 + 2e−
4OH− → O2 + 2H2O + 4e−
Aqueous solutions
Write the half-equation for chloride ions at the anode.
- Chloride ions are Cl−.
- Two chloride ions lose one electron each to form a chlorine molecule.
Answer: 2Cl− → Cl2 + 2e−
Ionic compounds conduct when molten or dissolved because ions can move. Cations go to the cathode and gain electrons; anions go to the anode and lose electrons. In solutions, hydrogen forms at the cathode unless the metal is less reactive (copper); oxygen forms at the anode unless a halide is present.
The interactive lesson includes the diagrams for this topic.
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