Electrolysis of Dilute Sulfuric Acid
15 Questions · Observations, Gas Ratios & Ion Selection · 32 Marks Total
🔬 The Scene
You are a lab technician running an electrolysis experiment. Two platinum electrodes are submerged in dilute sulfuric acid (H₂SO₄). You connect the power supply — and bubbles begin rising from both electrodes. But something is different: one tube fills with gas TWICE as fast as the other. Your job today is to explain EVERYTHING that is happening — at the atomic level.
⚡ Core Half-Equations Reference
Cathode (−) — REDUCTION — Gain electrons
2H⁺(aq) + 2e⁻ → H₂(g)
Anode (+) — OXIDATION — Lose electrons
4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻
Products of Electrolysis
Name the product formed at the cathode during electrolysis of dilute sulfuric acid.
Name the product formed at the anode during electrolysis of dilute sulfuric acid.
State which electrode undergoes oxidation during electrolysis of dilute sulfuric acid. Give a reason for your answer.
Write the half-equation for the reaction that occurs at the cathode during electrolysis of dilute sulfuric acid.
Write the half-equation for the reaction that occurs at the anode during electrolysis of dilute sulfuric acid.
Volume Ratios & Gas Testing
Two gas collection tubes are set up during electrolysis of dilute sulfuric acid as shown. Tube X collects twice the volume of gas as Tube Y. Which tube is at the cathode? Explain your answer.
Apparatus Diagram — Electrolysis Gas Collection
State the test for the gas produced at the anode. Give the expected result.
State the test and result for the gas produced at the cathode during electrolysis of dilute H₂SO₄.
During electrolysis of dilute sulfuric acid, 20 cm³ of gas is collected at the anode. Calculate the expected volume of gas collected at the cathode. Show your working.
Explain why the volume of gas at the cathode is greater than at the anode during electrolysis of dilute sulfuric acid.
Ion Selection & Preferential Discharge
During electrolysis of dilute sulfuric acid, oxygen is produced at the anode rather than sulfur dioxide. Explain why.
State the two ions present in dilute sulfuric acid that could be discharged at the anode. Name the ion that is actually discharged and give a reason.
Explain what would happen to the product at the anode if concentrated sulfuric acid was used instead of dilute sulfuric acid.
Electrolysis of dilute sulfuric acid is used to produce hydrogen for fuel cells. (a) Give one use of hydrogen fuel cells. (b) State one advantage of using hydrogen fuel cells over burning fossil fuels. (c) Name the only product when hydrogen is used in a fuel cell.
[Extended Response] Describe the electrolysis of dilute sulfuric acid. In your answer, include: (i) what happens at each electrode, (ii) the half-equations at each electrode, (iii) why oxygen is produced at the anode rather than sulfur dioxide, and (iv) the relative volumes of gas collected.
🏆 Examiner's Final Summary
Cathode = Hydrogen (H₂) — reduction — H⁺ gains electrons
Anode = Oxygen (O₂) — oxidation — OH⁻ loses electrons
Volume ratio H₂:O₂ = 2:1 — cathode always has double the gas
OH⁻ discharged over SO₄²⁻ because it requires less energy in dilute acid
Gas tests: Glowing splint (O₂ relights) · Lighted splint (H₂ pops)
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