Introduction
Electrolysis of water (H₂O) and brine (NaCl solution) are two common examples used in laboratories and industries. They demonstrate key redox concepts and the role of ion discharge preference. These processes are essential in manufacturing gases like hydrogen, chlorine, and compounds like sodium hydroxide.

🔹 1. Electrolysis of Water

Water is a weak electrolyte. To make it conduct electricity, a small amount of dilute sulfuric acid (H₂SO₄) is added.

Ions present:

• H⁺, OH⁻ (from water dissociation)

• H⁺ comes from acid

• OH⁻ from water auto-ionization

At cathode (reduction):

At anode (oxidation):

✅ Products:

• Cathode: Hydrogen gas (H₂)

• Anode: Oxygen gas (O₂)

🧪 Gas ratio:
Hydrogen:Oxygen = 2:1

🧠 Tip:

• Electrodes used are usually inert (e.g., platinum or graphite)

• Collect gases using Hofmann voltameter or test tubes

🔹 2. Electrolysis of Brine (NaCl solution)

Brine = concentrated aqueous sodium chloride
This process is called the chloralkali process.

Ions present:

• Na⁺, Cl⁻, H⁺, OH⁻

At cathode (reduction):

(H⁺ is discharged in preference to Na⁺)

At anode (oxidation):

(Cl⁻ discharged in preference to OH⁻)

✅ Products:

• Cathode: Hydrogen gas (H₂)

• Anode: Chlorine gas (Cl₂)

• In solution: Sodium hydroxide (NaOH) remains

🧪 Summary of the Chloralkali Process

🔹 3. Testing the Gases

🧠 NECTA Tips

• Balance equations carefully

• Explain preferential discharge clearly

• Always state products with test observations

• Use correct ion selection and electrode reactions

• Know industrial names: chloralkali process, Hofmann voltameter

✅ Summary

• Electrolysis of water yields H₂ and O₂ in a 2:1 ratio

• Electrolysis of brine produces H₂, Cl₂, and NaOH

• Discharge depends on ion reactivity and concentration

• Applied in chlorine production, hydrogen fuel, and alkali manufacture

• Key topic in NECTA theory and industrial chemistry