Predict electrolysis products from the electrolyte
Use whether an electrolyte is molten or aqueous, the redox series and concentration to predict products at each electrode.
Section 24 · Electrochemistry
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 24. Treat all tabulated electrode potentials as reduction potentials; state conditions, phases, signs and units. Written practice is self-checked
Before you begin
- Use AS redox half-equations, oxidation states, mole ratios, current and charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which process occurs at the cathode during electrolysis?
What you will learn
- Distinguish molten from aqueous electrolysis
- Use reduction tendencies to select cathode products
- Use anion identity and concentration to predict anode products
Start with the state and the electrode reaction
At the cathode, reduction occurs: positive ions or water gain electrons. At the anode, oxidation occurs: negative ions or water lose electrons. In a molten ionic compound, only its ions are available for discharge; in aqueous solution, water also competes.
Use the supplied redox series or electrode potentials to compare competing species. At the cathode the more favourable reduction is selected; at the anode compare the competing oxidations and apply the stated concentration conditions.
Concentration can change the anode product
For aqueous sodium chloride with inert electrodes, hydrogen forms at the cathode because Na⁺ is not preferentially reduced in water. Concentrated chloride solution can produce chlorine at the anode; in dilute chloride solution, oxygen from water/hydroxide is favoured under the stated syllabus conditions.
Do not apply aqueous rules to molten salts. For molten PbBr₂, Pb²⁺ forms Pb at the cathode and Br⁻ forms Br₂ at the anode. Always name the state and electrode material if it affects the competition.
Pause and explain
Why can concentrated and dilute aqueous chloride give different anode products?
Worked example
Predict the products of electrolysis of molten PbBr₂ and concentrated aqueous NaCl using inert electrodes.
Show the worked solution
- In molten PbBr₂, the ions present are Pb²⁺ and Br⁻.
- At the cathode Pb²⁺ gains electrons; at the anode Br⁻ loses electrons.
- For concentrated aqueous NaCl, account for water and chloride concentration.
Answer Molten PbBr₂: Pb forms at the cathode and Br₂ at the anode. Concentrated aqueous NaCl: H₂ forms at the cathode and concentrated Cl⁻ is oxidised to Cl₂ at the anode; Na⁺ remains in solution with OH⁻.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict the cathode and anode products when molten PbBr₂ is electrolysed.
Give me a hint
Only ions from the molten salt are present; assign reduction to the cathode.
Compare my reasoning
- Pb²⁺ moves to the cathode and gains electrons.
- Br⁻ moves to the anode and loses electrons.
- Balance the atom and charge changes.
Pb forms at the cathode; Br₂ forms at the anode. The half-equations are Pb²⁺ + 2e⁻ → Pb and 2Br⁻ → Br₂ + 2e⁻.
Look for these in your work
- Molten ions identified
- Electrode roles correct
- Products balanced
With inert electrodes, predict the products of concentrated aqueous NaCl electrolysis and explain why sodium metal does not form.
Give me a hint
Compare Na⁺ reduction with the competing reduction of water in aqueous solution.
Compare my reasoning
- Water is present and competes at the cathode.
- Hydrogen forms rather than sodium metal.
- Concentrated chloride is oxidised at the anode.
H₂ forms at the cathode and Cl₂ at the anode under the stated concentrated-brine conditions. Na⁺ remains in solution; water is reduced instead of Na⁺ being discharged.
Look for these in your work
- Aqueous competition considered
- Cathode product correct
- Concentrated chloride anode product correct
A learner predicts oxygen at the anode for concentrated NaCl(aq) and chlorine for dilute NaCl(aq). Correct the concentration comparison.
Give me a hint
Keep the electrode fixed and reverse which anion/water oxidation is favoured under concentrated versus dilute chloride conditions.
Compare my reasoning
- Concentrated chloride favours chlorine formation.
- Dilute chloride favours oxygen from water/hydroxide under the syllabus rule.
- State the aqueous condition with the predicted product.
For the syllabus comparison, concentrated chloride solution gives Cl₂ at the anode; dilute chloride solution gives O₂ from oxidation of water/hydroxide.
Look for these in your work
- Concentration effect reversed
- Both products assigned
- Aqueous condition retained
Common mistakes
- Using molten-salt products for an aqueous electrolyte
- Ignoring water as a competing species
- Forgetting chloride concentration when predicting anode products
- Reversing cathode reduction and anode oxidation
What can you explain now?
Which process occurs at the cathode during electrolysis?
Compare with the explanation
Reduction
The cathode receives electrons, so a species gains electrons.
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Make it stick
How do electrolyte state, competing species, redox position and concentration determine the two electrolysis products?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.