Electrolysis of Aqueous NaCl & H₂SO₄
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Concentrated Aqueous Sodium Chloride (Brine)
Concentrated aqueous NaCl (brine) is electrolysed using inert electrodes. Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
- Cathode: sodium is far more reactive than hydrogen, so H⁺ is discharged instead: 2H⁺ + 2e⁻ → H₂ - colourless gas bubbles (hydrogen)
- Anode: Cl⁻ is a halide and is present in high concentration, so it is discharged: 2Cl⁻ → Cl₂ + 2e⁻ - greenish-yellow gas forms (chlorine)
Na⁺ and OH⁻ are left behind in solution, so the solution becomes sodium hydroxide.
The Chlor-Alkali Industry
Electrolysing brine on an industrial scale produces three valuable chemicals at once:
- Chlorine (Cl₂) at the anode - water treatment, PVC plastics, bleach
- Hydrogen (H₂) at the cathode - margarine hydrogenation, ammonia manufacture
- Sodium hydroxide (NaOH) left in solution - soap, paper, detergents
Dilute Sulfuric Acid
Dilute H₂SO₄ is electrolysed using inert electrodes. Ions present: H⁺, SO₄²⁻, plus the small amount of H⁺ and OH⁻ from water.
- Cathode: 2H⁺ + 2e⁻ → H₂ - colourless gas (hydrogen)
- Anode: SO₄²⁻ is not a halide, so OH⁻ is discharged: 4OH⁻ → O₂ + 2H₂O + 4e⁻ - colourless gas (oxygen)
Overall, water is being decomposed (2H₂O → 2H₂ + O₂), so the sulfuric acid becomes more concentrated as electrolysis proceeds. Because 2 mol of H₂ forms for every 1 mol of O₂, the volume ratio of gases collected is H₂ : O₂ = 2 : 1.
| Gas | Test | Result |
|---|---|---|
| Hydrogen | lit splint | burns with a squeaky pop |
| Oxygen | glowing splint | relights |
| Chlorine | damp litmus paper | bleaches it white |
Molten vs. Dilute vs. Concentrated: Comparing Sodium Chloride
The state and concentration of sodium chloride changes the products, especially at the cathode and anode independently.
Whether water is present changes the cathode product; the concentration of a dissolved halide changes the anode product
Worked Example: Cathode Product in Three Forms of NaCl
- Question: name a suitable inert non-metal and inert metal electrode; then complete a table showing the product formed at the cathode when electrolysing (i) molten NaCl, (ii) dilute aqueous NaCl, (iii) concentrated aqueous NaCl. [5]
- Answer - electrodes: non-metal = graphite; metal = platinum [2]
- Answer - molten NaCl: only Na⁺ and Cl⁻ ions present (no water) - cathode product is sodium [1]
- Answer - dilute aqueous NaCl: Na⁺ is far more reactive than hydrogen, so H⁺ is discharged - cathode product is hydrogen [1]
- Answer - concentrated aqueous NaCl: same reasoning as dilute - concentration affects the anode, not the cathode - cathode product is still hydrogen [1]
| Form of NaCl | Cathode product | Anode product |
|---|---|---|
| Molten | Sodium | Chlorine |
| Dilute aqueous | Hydrogen | mainly Oxygen |
| Concentrated aqueous | Hydrogen | mainly Chlorine |
The cathode product depends only on whether water is present at all (molten vs. aqueous). The anode product for a dissolved halide instead depends on concentration: in dilute solution OH⁻ tends to be discharged (oxygen), while in concentrated solution the halide ion, now present in much greater excess, is discharged instead (the halogen).
Common Mistakes
- ❌ Assuming concentration changes the cathode product
- ✓ for common metal salts, the cathode product depends on the reactivity comparison with hydrogen, not on concentration - only the anode product for a halide changes with concentration
- ❌ Reporting the volume ratio of H₂ to O₂ in dilute sulfuric acid as 1:1
- ✓ the ratio is 2:1 (H₂:O₂), matching the equation 2H₂O → 2H₂ + O₂
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