Chemical changes · GCSE Chemistry
Electrolysis of solutions
GCSE Chemistry revision on electrolysis of aqueous solutions: hydrogen versus metal at the cathode, halogen versus oxygen at the anode, and the brine cell.
Water adds H⁺ and OH⁻ to the competition. At the cathode, hydrogen forms if the metal is more reactive than hydrogen. At the anode, a concentrated halide gives the halogen; otherwise you get oxygen.
The important bits
What you need to know
- 1
In aqueous solutions, H⁺ and OH⁻ from water compete with the ions from the dissolved salt. Products are not always the metal and the non-metal of the salt.
- 2
Cathode rule: if the metal is more reactive than hydrogen (K, Na, Ca, Mg, Al, Zn, Fe), hydrogen is discharged: 2H⁺ + 2e⁻ → H₂ (or 2H₂O + 2e⁻ → H₂ + 2OH⁻). If the metal is less reactive (Cu, Ag, Au), the metal is deposited.
- 3
Anode rule: if halide ions (Cl⁻, Br⁻, I⁻) are present in reasonable concentration, the halogen is produced: 2Cl⁻ → Cl₂ + 2e⁻. If the only anion that can be discharged is OH⁻ or a dilute sulfate/nitrate, oxygen is produced: 4OH⁻ → O₂ + 2H₂O + 4e⁻.
- 4
Copper(II) sulfate with inert electrodes: copper at the cathode (Cu²⁺ + 2e⁻ → Cu), oxygen at the anode, and the blue colour fades as Cu²⁺ is used up.
- 5
Copper(II) sulfate with copper electrodes: copper still plates at the cathode, but the copper anode dissolves (Cu → Cu²⁺ + 2e⁻). Solution concentration stays roughly constant — this is purification of copper.
- 6
Electrolysis of brine (concentrated NaCl(aq)): hydrogen at the cathode, chlorine at the anode, sodium hydroxide left in solution. All three products are useful industrially.
- 7
Aqueous sodium chloride does not produce sodium metal. Sodium ions stay in solution; hydrogen is easier to discharge at the cathode.
- 8
Test products: H₂ squeaky pop, O₂ relights a glowing splint, Cl₂ bleaches damp litmus, Br₂ orange vapour/solution, I₂ brown solution or purple vapour.
Quotations worth analysing
Short evidence. Real method.
“2H⁺(aq) + 2e⁻ → H₂(g) 2Cl⁻(aq) → Cl₂(g) + 2e⁻”
Hydrogen, not sodium, at the cathode. Chlorine, not oxygen, at the anode because Cl⁻ is concentrated. Na⁺ and OH⁻ remaining make NaOH(aq).
“Cu²⁺(aq) + 2e⁻ → Cu(s) 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻”
Copper is less reactive than hydrogen, so copper plates. Sulfate is not discharged; hydroxide from water is, so oxygen gas is made.
“Cu(s) → Cu²⁺(aq) + 2e⁻ (copper anode)”
The anode is not inert. Copper atoms lose electrons and enter the solution as the cathode gains an equal mass of copper. Impurities fall as anode sludge.
Go deeper
Learn the decision tree, then write one half equation per electrode
List all ions, including H⁺ and OH⁻ from water. Cathode: compare the metal with hydrogen. More reactive metal → hydrogen gas. Less reactive metal → metal coating. Anode: is there a concentrated halide? Yes → halogen. No → oxygen. Then write the matching half equation. Dilute NaCl can give oxygen at the anode because [Cl⁻] is low; concentrated brine gives chlorine. If the question says “concentrated sodium chloride solution”, it wants Cl₂, H₂ and NaOH. If it says molten sodium chloride, it wants Na and Cl₂. Those two questions are designed to catch students who only memorised one set of products.
Go deeper
Brine is an industrial trio, not just a school demo
Hydrogen is a fuel and a feedstock. Chlorine is used in water treatment and plastics (PVC). Sodium hydroxide is used to make soap, paper and other chemicals. The membrane or diaphragm cell keeps chlorine away from the sodium hydroxide so they do not reverse-react. Evaluation: useful products, but electricity cost and the need to handle toxic chlorine. You do not need the full industrial diagram on every specification, but you do need the three products and which electrode makes which. Sodium metal is not one of them. If the electrolyte is dilute rather than concentrated brine, oxygen may form at the anode instead of chlorine.
Go deeper
Copper electrodes change the anode story
With graphite, oxygen comes from OH⁻ and the solution loses Cu²⁺, so the blue fades. With a copper anode, copper itself is oxidised and replaces the Cu²⁺ that plates out, so the colour stays and the anode loses mass while the cathode gains mass. That is how impure copper is refined: the pure copper sticks to the cathode and impurities drop off the anode. Half equations still obey OIL RIG. Students mix the two copper sulfate experiments. Always state whether the anode is inert or copper before you predict the anode product.
See the idea in action
Predict the products of electrolysing concentrated potassium iodide solution with inert electrodes. Ions: K⁺, I⁻, H⁺, OH⁻. Potassium is more reactive than hydrogen, so at the cathode 2H⁺ + 2e⁻ → H₂. Iodide is a halide in concentration, so at the anode 2I⁻ → I₂ + 2e⁻ (brown iodine). Potassium ions remain with OH⁻ as KOH(aq). If the electrolyte were molten KI, the cathode product would be potassium metal instead of hydrogen.
Exam technique
Turn knowledge into marks
Write every ion present, including from water, then apply the cathode reactivity rule and the anode halide rule. State electrode material. For brine, name H₂, Cl₂ and NaOH. Never produce sodium from aqueous NaCl.
Common mistakes
Do not give these marks away
- 01
Predicting sodium metal from aqueous sodium chloride.
- 02
Giving oxygen at the anode of concentrated brine, or chlorine from copper sulfate.
- 03
Forgetting that copper electrodes change the anode reaction in CuSO₄(aq).
What are the products of electrolysing concentrated sodium chloride solution with inert electrodes?
ASodium at the cathode and chlorine at the anode
BHydrogen at the cathode, chlorine at the anode, and sodium hydroxide in solution
CHydrogen and oxygen only
DSodium at the cathode and oxygen at the anode
Show the answer
Hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in solution. Na is more reactive than hydrogen, so H₂ forms at the cathode. Concentrated Cl⁻ gives Cl₂ at the anode. Remaining Na⁺ and OH⁻ make NaOH(aq).
Quick questions
If this is the bit you searched
Why is hydrogen produced instead of sodium in aqueous NaCl?
Sodium is more reactive than hydrogen, so H⁺ from water is discharged at the cathode in preference to Na⁺. Sodium ions stay in the solution.
When is oxygen produced at the anode?
When there is no concentrated halide. Hydroxide ions from water are discharged: 4OH⁻ → O₂ + 2H₂O + 4e⁻. Sulfate and nitrate are not discharged.
What is produced in the electrolysis of brine?
Hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in the solution. Brine is concentrated sodium chloride solution.
How is copper purified by electrolysis?
An impure copper anode and a pure copper cathode in copper(II) sulfate solution. Copper dissolves from the anode and plates onto the cathode. Impurities fall as sludge.