Electrolysis of Water: Key Concepts and Processes

Introduction to Electrolysis

• Definition: Electrolysis is the process of using electricity to break down a compound into its component molecules.
• Example: Breaking down water (H₂O) into hydrogen gas (H₂) and oxygen gas (O₂).

Basic Setup for Electrolysis of Water

• Materials Required:
  • A beaker filled with water.
  • Two electrodes: anode and cathode (carbon-based graphite electrodes).
  • A battery (e.g., 9-volt battery).
  • Sodium hydroxide (NaOH) dissolved in water.

Electrochemical Cell

• Electrode Connections:
  • Positive terminal of the battery connects to the anode.
  • Negative terminal connects to the cathode.
• Electron Flow:
  • Electrons flow from anode (positive) to cathode (negative).
  • Oxidation occurs at the anode; reduction occurs at the cathode.

Half Reactions

• Anode (Oxidation):
  • Hydroxide ions (OH⁻) are oxidized to oxygen gas (O₂) and water, releasing electrons.
• Cathode (Reduction):
  • Water is reduced to hydrogen gas (H₂), gaining electrons and forming hydroxide ions.

Chemical Reactions

• Anode Reaction:
  • 4 OH⁻ → O₂ + 2H₂O + 4e⁻
• Cathode Reaction:
  • 4 H₂O + 4e⁻ → 2 H₂ + 4 OH⁻

Cell Potential

• Calculation:
  • Anode potential: -0.4 V
  • Cathode potential: -0.83 V
  • Overall cell potential: -1.23 V
• Implication: A negative cell potential indicates a non-spontaneous reaction.

Sodium and Water Reaction

• Non-Favorable Reaction:
  • Sodium metal formation at the cathode is highly unfavorable due to a negative cell potential of -2.71 V.
  • If formed, sodium would react explosively with water to form NaOH and H₂.

Orientation of Water Molecule

• Electric Field Influence:
  • Water molecules will orient such that the oxygen faces the anode and the hydrogen faces the cathode.

Conclusion

• Electrolysis Outcome:
  • Hydrogen gas is produced at the cathode.
  • Oxygen gas is produced at the anode.
• Reversal: The spontaneous reverse reaction (H₂ + O₂ → H₂O) has a positive cell potential (1.23 V).

Additional Notes

• Non-Spontaneity: Electrolysis requires external energy since water is stable.
• Catalysis: A spark or increased temperature may be necessary to initiate the reverse reaction efficiently.