Hoffmann Electrolysis

Overview

• A little bit of soap helps visualize the gases produced.
• Important: Do not forget to add the electrolyte!

Reactions

Cathode Reaction (Reduction)

• At the negatively charged cathode, a reduction reaction occurs:
  Cathode (reduction): 2 H⁺ (aq) + 2 e⁻ → H₂ ​(g)

Anode Reaction (Oxidation)

• At the positively charged anode, an oxidation reaction occurs:
  Anode (oxidation): 2 H2​O (l) → O2 ​(g) + 4 H⁺ (aq) + 4e⁻

Alternate Half Reactions

• The same half reactions can also be balanced with base:

Cathode Reaction (with Base)

Cathode (reduction): 2 H2​O (l) + 2e⁻ → H₂ ​(g) + 2 OH⁻ (aq)

Anode Reaction (with Base)

Anode (oxidation): 4OH⁻ (aq) → O2​ (g) + 2 H2​O (l) + 4e⁻

Overall Reaction

• Combining either pair of half reactions yields the same overall decomposition of water into oxygen and hydrogen:
  
• Overall reaction: 2 H2​O (l) → 2 H2​ (g) + O2​ (g)

Key Observations

• The number of hydrogen molecules produced is twice that of oxygen molecules.
• Assuming equal temperature and pressure, the produced hydrogen gas occupies twice the volume of the produced oxygen gas.
• The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules.