Electrolysis of Aqueous NaCl & H₂SO₄
Section: 4. Electrochemistry | Syllabus: Cambridge AS Level Physics 9702
What's Different About Aqueous Electrolysis? Aqueous electrolysis is the electrolysis of an ionic compound dissolved in water, rather than molten. The key difference from molten electrolysis: Water molecules are also present in the solution Water can break down: H₂O → H⁺ + OH⁻ We now have four types of ions present (not just two) This creates competition at each electrode Products are less predictable than with molten compounds Ions Present in Aqueous Solutions Example: Aqueous sodium chloride (NaCl solution) From compound From water Na⁺ (sodium ions) H⁺ (hydrogen ions) Cl⁻ (chloride ions) OH⁻ (hydroxide ions) At the cathode: Na⁺ and H⁺ compete At the anode: Cl⁻ and OH⁻ compete Which Ion Wins at the Cathode?
At the cathode (negative electrode), REDUCTION occurs Rule: If the metal is MORE reactive than hydrogen → H⁺ is discharged (hydrogen gas forms) Rule: If the metal is LESS reactive than hydrogen → Metal ion is discharged (metal forms) Reactivity series (partial): K > Na > Ca > Mg > Al > (Carbon) > Zn > Fe > HYDROGEN > Cu > Ag > Au Metals above hydrogen: H⁺ wins (hydrogen gas produced) Metals below hydrogen: Metal ion wins (metal produced) Which Ion Wins at the Anode?
At the anode (positive electrode), OXIDATION occurs Simple rule: If a halide ion (Cl⁻, Br⁻, I⁻) is present → halogen gas forms If NO halide present (e.g., sulfate SO₄²⁻, nitrate NO₃⁻) → oxygen gas forms from OH⁻ Electrolysis of Aqueous Sodium Chloride (NaCl solution) Setup: Electrolyte: Sodium chloride solution (brine) Electrodes: Inert (graphite or platinum) Ions present: Na⁺, Cl⁻, H⁺, OH⁻ At the cathode: Competition: Na⁺ vs H⁺ Sodium is MORE reactive than hydrogen H⁺ wins → hydrogen gas forms 2H⁺ + 2e⁻ → H₂ Observation: Bubbles of colourless gas (hydrogen) At the anode: Competition: Cl⁻ vs OH⁻ Chloride (halide) is present Cl⁻ wins → chlorine gas forms 2Cl⁻ → Cl₂ + 2e⁻ Observation: Bubbles of greenish-yellow gas (chlorine) What's left in solution: Na⁺ and OH⁻ remain This is sodium hydroxide (NaOH) solution!
Industrial Production from Brine The Chlor-Alkali Industry Electrolysis of concentrated sodium chloride solution (brine) produces three important industrial chemicals: Chlorine gas (Cl₂) at the anode - for water purification, plastics (PVC), bleach Hydrogen gas (H₂) at the cathode - for margarine production, ammonia manufacture Sodium hydroxide solution (NaOH) remains - for soap, paper, detergents This is one of the most important industrial processes!
Electrolysis of Dilute Sulfuric Acid (H₂SO₄) Setup: Electrolyte: Dilute sulfuric acid Electrodes: Inert (graphite or platinum) Ions present: H⁺, SO₄²⁻, H⁺, OH⁻ (from water) Note: Extra H⁺ from the acid, plus H⁺ and OH⁻ from water At the cathode: H⁺ ions attracted Hydrogen gas forms 2H⁺ + 2e⁻ → H₂ Observation: Bubbles of colourless gas At the anode: Competition: SO₄²⁻ vs OH⁻ Sulfate is NOT a halide OH⁻ wins → oxygen gas forms 4OH⁻ → O₂ + 2H₂O + 4e⁻ Observation: Bubbles of colourless gas (oxygen) Overall effect: Water is being decomposed: 2H₂O → 2H₂ + O₂ Sulfuric acid concentration increases (water is removed) Volume Ratio in Dilute Sulfuric Acid When electrolyzing dilute sulfuric acid: Volume ratio: Hydrogen : Oxygen = 2 : 1 Why?
Overall equation: 2H₂O → 2H₂ + O₂ 2 moles of hydrogen produced for every 1 mole of oxygen Same volume ratio (at same temperature and pressure) Test: Collect gases in separate test tubes - you should get twice as much hydrogen as oxygen!
Testing the Gases Produced Gas Test Result Hydrogen (H₂) Lit splint Burns with squeaky pop Oxygen (O₂) Glowing splint Relights the splint Chlorine (Cl₂) Damp litmus paper Bleaches it white Comparison: NaCl vs H₂SO₄ Electrolysis Feature NaCl solution Dilute H₂SO₄ Cathode product H₂ (hydrogen) H₂ (hydrogen) Anode product Cl₂ (chlorine) O₂ (oxygen) Solution after NaOH (alkaline) More concentrated H₂SO₄ Industrial use Chlor-alkali industry Producing pure water samples Why Hydrogen Always Forms at Cathode For common salt solutions (Na⁺, K⁺, Ca²⁺, Mg²⁺, etc.): These metals are all MORE reactive than hydrogen H⁺ ions are preferentially discharged The metal ions stay in solution Hydrogen gas always produced at cathode Exception: Copper sulfate solution (Cu²⁺) Copper is LESS reactive than hydrogen, so copper metal forms at the cathode instead!
Summary of Discharge Rules At the CATHODE (negative): If metal is MORE reactive than hydrogen → H₂ forms If metal is LESS reactive than hydrogen → Metal forms At the ANODE (positive): If HALIDE present (Cl⁻, Br⁻, I⁻) → Halogen gas forms (Cl₂, Br₂, I₂) If NO halide (SO₄²⁻, NO₃⁻, etc.) → O₂ forms from OH⁻ Common Mistakes to Avoid Forgetting water ions: Always remember H⁺ and OH⁻ from water are present Wrong reactivity comparison: Compare metal with hydrogen, not with water Ignoring halides: Halides always discharge at anode (not OH⁻) Wrong gas tests: Squeaky pop for H₂, relights for O₂, bleaches for Cl₂ Forgetting NaOH remains: In NaCl electrolysis, sodium hydroxide solution is left Wrong volume ratio…
Interactive revision notes, videos and practice questions load below.