For the standard SEC electrolysis comparison using inert electrodes, dilute aqueous sodium chloride gives oxygen at the anode and concentrated aqueous sodium chloride gives chlorine. Hydrogen is predicted at the cathode in both cases.
The 2027 SEC G3 Chemistry K324 syllabus includes how concentration affects selective discharge. The example holds the salt and electrodes constant so you can see which prediction changes when the chloride concentration changes.
- Check whether the electrolyte is aqueous or molten.
- Identify the electrode whose product is requested.
- Use the stated concentration for the aqueous case.
- Check the atoms and charge in your half-equation.
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Keep the conditions attached to the answer
The electrolyte here is sodium chloride dissolved in water. Sodium ions and chloride ions are present, and water can participate in electrode reactions. The electrodes are inert. Read those conditions before predicting any products.
Oxidation happens at the anode and electrons appear on the product side of its half-equation. In the aqueous school model, chloride oxidation competes with oxygen formation.
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Worked example | Change the concentration only
Case A uses dilute aqueous sodium chloride. The predicted anode product is oxygen. One way to represent oxygen formation is 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻. There are four oxygen atoms and four hydrogen atoms on each side, with total charge −4 on each side.
Case B uses concentrated aqueous sodium chloride. Under the stated school model, chloride is preferentially discharged and chlorine forms. Write 2Cl⁻(aq) → Cl₂(g) + 2e⁻. The two chloride ions have total charge −2 and the two electrons on the right account for that charge.
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Change the state and redo the prediction
In molten sodium chloride there is no water present. Chloride forms chlorine at the anode and sodium ions form sodium at the cathode. The same salt name appears in all three cases, but its state and the presence of water affect the predictions.
Use these as written questions. Any practical involving possible chlorine production belongs in a properly supervised laboratory.
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Frequently asked questions
What happens at the cathode in both aqueous cases?
Hydrogen forms from the aqueous system. Sodium metal is the cathode product in the molten case.
Why should I mention “inert” in my answer?
A participating electrode can change the reaction. State the electrode condition before applying the model in the example.