📖 In-Depth Theory
Why Aqueous Solutions Are More Complex
When an ionic compound is DISSOLVED in water, the solution contains:
Ions from the ionic compound, AND
H⁺ and OH⁻ ions from water (water partially ionises: H₂O ⇌ H⁺ + OH⁻).
At each electrode, there are COMPETING IONS — more than one type of ion that could be discharged.
The ion that is actually discharged depends on:
1. POSITION IN THE REACTIVITY SERIES — less reactive ions are discharged preferentially.
2. CONCENTRATION — a high concentration of an ion favours its discharge.
At the CATHODE (reduction):
If the metal is BELOW hydrogen in the series → metal is deposited.
If the metal is ABOVE hydrogen in the series → hydrogen gas (H₂) is produced (from H⁺ ions).
At the ANODE (oxidation):
If Cl⁻ ions are present in HIGH concentration → chlorine gas (Cl₂) produced.
If no Cl⁻ (or low concentration) → oxygen gas (O₂) produced (from OH⁻ ions from water).
Electrolysis of Key Aqueous Solutions
DILUTE SULFURIC ACID (H₂SO₄(aq)) with inert electrodes:
Ions: H⁺, OH⁻, SO₄²⁻.
Cathode: H⁺ + e⁻ → H₂ (hydrogen produced — SO₄²⁻ not discharged, too stable)
Anode: 4OH⁻ → O₂ + 2H₂O (oxygen produced)
Ratio: 2 volumes H₂ for every 1 volume O₂ (2:1 ratio)
CUPROUS SULFATE SOLUTION (CuSO₄(aq)) with inert electrodes:
Ions: Cu²⁺, SO₄²⁻, H⁺, OH⁻.
Cathode: Cu²⁺ preferred over H⁺ (copper is below hydrogen, less reactive) → copper deposited.
Anode: OH⁻ discharged → oxygen gas.
CONCENTRATED SODIUM CHLORIDE (brine: NaCl(aq)) with inert electrodes:
Ions: Na⁺, Cl⁻, H⁺, OH⁻.
Cathode: H⁺ preferred over Na⁺ (Na is far above hydrogen, very reactive) → hydrogen gas produced.
Anode: Cl⁻ in high concentration → CHLORINE gas produced (not O₂ — Cl⁻ concentration effect).
Remaining solution: Na⁺ and OH⁻ → sodium hydroxide (NaOH) — important industrial product.
This is the CHLOR-ALKALI process — produces chlorine, hydrogen and sodium hydroxide — all valuable chemicals.
Products Summary and Rules
CATHODE RULE (what's produced at the negative electrode):
Metal ions below hydrogen in series (Cu²⁺, Ag⁺, Au³⁺) → METAL deposited.
Metal ions above hydrogen in series (Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe²⁺) → HYDROGEN gas (H₂) produced.
ANODE RULE (what's produced at the positive electrode):
If halide ions present (Cl⁻, Br⁻, I⁻) AND in high concentration → HALOGEN gas (Cl₂, Br₂, I₂).
Otherwise → OXYGEN gas (O₂) from OH⁻ ions in water.
IMPORTANT INDUSTRIAL PRODUCTS from electrolysis:
CHLORINE — from brine (NaCl solution). Used in PVC, disinfectants, bleach.
HYDROGEN — from brine. Used in Haber process, fuel cells.
SODIUM HYDROXIDE — from brine. Used in paper making, soap, cleaning products.
ALUMINIUM — from molten Al₂O₃ in cryolite.
OXYGEN — from water/dilute sulfuric acid.
⚠️ Common Mistake
In aqueous solutions, WATER provides H⁺ and OH⁻ ions that compete with the ions from the dissolved salt. At the cathode: metals above hydrogen → H₂ is produced (not the metal). At the anode: if no halide ions present → O₂ is produced (from OH⁻). Students often predict the wrong product for Na⁺ at the cathode — sodium is so reactive it is NEVER deposited from aqueous solution; hydrogen is produced instead.