Electrolysis Lecture Notes

Introduction to Electrolysis

• Electrolysis: Using electricity to cause a chemical reaction.
• Requires a DC power supply connected to two electrodes placed in an electrolyte solution.
• Electrolyte: Water with dissolved ionic compound.
• Electrodes:
  • Cathode: Negative electrode attached to negative terminal of the battery.
  • Anode: Positive electrode attached to the positive terminal.
• Opposite charges attract ions in the solution to the respective electrodes.

Electrodes and Electrolyte

• Electrodes are typically made from graphite (carbon) or metal.
• Carbon electrodes are inert (do not change/react during electrolysis).
• Electrolyte example: Sodium chloride solution.
• Dissociation in water: NaCl forms Na+ and Cl- ions; Water dissociates into H+ and OH-.

Process of Electrolysis

• Positive ions (cations) are attracted to the cathode.
• Negative ions (anions) are attracted to the anode.
• Opposites attract: cations gain electrons at the cathode (reduction), anions lose electrons at the anode (oxidation).

Rules for Electrolysis

At the Cathode

• Less reactive cation is reduced.
• Example: Hydrogen is less reactive than sodium, so H+ ions are reduced to form H2 gas.

At the Anode

• Halide ions (e.g., Cl-, Br-, I-) are oxidized to form gases.
• Non-halide ions remain in solution, and oxygen gas is produced instead.
• Example: In sodium chloride solution, Cl- is oxidized, forming Cl2 gas.

Examples of Electrolysis

• Sodium Chloride Solution:

  • Cathode: H+ reduced to H2 gas.
  • Anode: Cl- oxidized to Cl2 gas, leaving Na+ and OH- in solution, forming NaOH.

• Copper Sulfate Solution:

  • Cathode: Cu2+ reduced to copper metal.
  • Anode: Oxygen gas produced as no halide ions are present.
  • Result: Formation of sulfuric acid (H2SO4).

Electrolysis of Water

• Produces hydrogen gas at the cathode and oxygen gas at the anode.

Electrolysis of Molten Compounds

• Ionic compounds can be electrolyzed in molten state to extract pure metals.
• Example: Aluminium from aluminum oxide using cryolite to lower melting point.
• Carbon anodes need replacement due to reaction with oxygen.

Purifying Metals via Electrolysis

• Used for purifying impure metals such as copper.
• Anode: Impure copper loses electrons, oxidizes, and copper ions enter solution.
• Cathode: Copper ions are reduced to copper metal, increasing the cathode's mass.

Conclusion

• Electrolysis is a powerful method for chemical reactions, metal purification, and gas production.
• Requires understanding of reactivity series and specific rules governing electrode processes.

• Practice Problem: Predict outcomes in electrolysis of copper sulfate solution.
• Further Study: Watch related videos for deeper understanding and practice with half-equations.