Describe the reactions in electrolysis of aqueous solutions.
Predict the products at electrodes during electrolysis.
(Higher tier) Write half equations for reactions at cathode and anode.
Key Concepts
Electrolysis: The process where electrical energy is used to drive a non-spontaneous chemical reaction.
Aqueous Solution: A solution where water is the solvent.
Electrolysis of Aqueous Solution
Water Ionization: Water molecules ionize to form hydrogen ions (H+) and hydroxide ions (OH-).
In an aqueous solution, the ions from the solute and from water must be considered during electrolysis.
Example: Copper Sulfate Solution
Formula: CuSO₄(aq)
Ions present: Copper ion (Cu²⁺), sulfate ion (SO₄²⁻), hydrogen ion (H⁺), hydroxide ion (OH⁻).
Electrolysis Process
At the Cathode (Negative Electrode)
Attracted ions: Cu²⁺ and H⁺.
Reactivity Series Rule:
Hydrogen is produced if the metal is more reactive than hydrogen.
In this case, copper is less reactive than hydrogen, so copper is produced at the cathode.
At the Anode (Positive Electrode)
Generally, oxygen gas is produced at the anode from OH⁻ ions, unless otherwise specified.
Inert Electrodes:
Electrodes do not react with the chemicals in electrolysis.
Platinum is often used due to its unreactive nature.
Half Equations (Higher Tier)
Cathode Reaction: Reduction occurs as copper ions gain electrons.
Cu²⁺ + 2e⁻ → Cu (reduction)
Anode Reaction: Oxidation of hydroxide ions to form oxygen and water, releasing electrons.
4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation)
Additional Resources
Practice questions on electrolysis of aqueous solutions available in the vision workbook.
Further exploration of exceptions in oxygen production at the anode will be covered in the next session.
Conclusion
By understanding the reactions and rules of electrolysis, you can predict the products at the electrodes and write half equations for various scenarios.