During the electrolysis of dilute sodium chloride solution, also known as brine, electrical energy is used to decompose the solution into its constituent elements and compounds. The primary products formed at the electrodes are hydrogen gas, chlorine gas, and sodium hydroxide solution is left behind.
When an electric current is passed through the dilute sodium chloride solution, several ions are present: Na⁺(aq), Cl⁻(aq), H⁺(aq), and OH⁻(aq) from the dissociation of NaCl and water. At the electrodes, a competition occurs to determine which ions are discharged.
Reactions at the Electrodes
The electrodes are where the chemical reactions take place:
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At the Cathode (Negative Electrode): This is where reduction occurs (gain of electrons). Both Na⁺ and H⁺ ions are attracted to the cathode. However, because water (or H⁺ ions) is more easily reduced than sodium ions in dilute solution, water is preferentially reduced.
- Reaction: 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
- Alternatively, considering H⁺: 2H⁺(aq) + 2e⁻ → H₂(g)
- Hydrogen gas (H₂) is produced.
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At the Anode (Positive Electrode): This is where oxidation occurs (loss of electrons). Both Cl⁻ and OH⁻ ions are attracted to the anode. In dilute sodium chloride solution, the concentration of Cl⁻ is relatively low compared to OH⁻ (from water). Therefore, water (or OH⁻ ions) might be expected to be oxidized. However, the overpotential for oxygen formation at certain electrode materials (like platinum or carbon) is higher than that for chlorine. Consequently, chloride ions are preferentially oxidized, especially if the concentration is not extremely dilute.
- Reaction: 2Cl⁻(aq) → Cl₂(g) + 2e⁻
- Chlorine gas (Cl₂) is produced.
If the solution were very dilute, oxygen might also be produced at the anode from the oxidation of water:
- Alternative Reaction: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻
Formation of Sodium Hydroxide
As the electrolysis proceeds, hydrogen and chloride ions are removed from solution whereas sodium and hydroxide ions are left behind in solution. This means that sodium hydroxide is also formed during the electrolysis of sodium chloride solution. The Na⁺ ions (spectator ions at the cathode) remain in the solution, and as OH⁻ ions are produced at the cathode (or from the oxidation of water producing H⁺ which then reacts with OH⁻ in the bulk), they combine with the remaining Na⁺ ions to form sodium hydroxide (NaOH) in the solution.
Summary of Products
The overall results of the electrolysis of dilute sodium chloride solution are:
- At the Cathode: Hydrogen gas (H₂)
- At the Anode: Chlorine gas (Cl₂) (primarily in dilute solutions, oxygen can also be formed)
- In Solution: Sodium hydroxide (NaOH)
| Electrode | Reaction Type | Species Reacting | Product(s) |
|---|---|---|---|
| Cathode | Reduction | Water (or H⁺) | Hydrogen gas (H₂) |
| Anode | Oxidation | Chloride ions | Chlorine gas (Cl₂) |
| Solution | - | Sodium & Hydroxide ions | Sodium Hydroxide (NaOH) |
This process is industrially important for producing these valuable chemicals, although concentrated brine is more commonly used for higher efficiency and specific product yields (like less oxygen at the anode).
