What change will occur during the electrolysis of brine using carbon electrodes?

Analyze the question: Brine is concentrated sodium chloride solution (NaCl in water). We need the correct statement about its electrolysis with inert carbon electrodes.

Concept: selective (preferential) discharge

Ions present in the solution:

• From NaCl: Na⁺ and Cl⁻
• From water: H⁺ and OH⁻

Which ions get discharged depends on:

1. Position in the electrochemical series
2. Concentration of the ions
3. Nature of the electrode

Step-by-step solution

At the cathode (negative electrode): Na⁺ and H⁺ are attracted. H⁺ is lower in the electrochemical series than Na⁺, so it is discharged preferentially: 2H⁺ + 2e⁻ → H₂

At the anode (positive electrode): Cl⁻ and OH⁻ are attracted. Because brine is concentrated, Cl⁻ is present in a much higher concentration, so it is discharged in preference to OH⁻. Carbon is inert, so it does not react: 2Cl⁻ → Cl₂ + 2e⁻

Solution left behind: Na⁺ and OH⁻ remain, forming NaOH, so the solution becomes alkaline.

Checking each option

• A. Chlorine will be given off at the anode. True, for concentrated brine.
• B. Oxygen at the anode. False. Oxygen would form only if the solution were dilute.
• C. Sodium deposited at the cathode. False. Sodium is deposited only with a mercury cathode (Castner-Kellner cell), where it forms an amalgam. With carbon, hydrogen is released.
• D. Solution becomes acidic. False. It becomes alkaline because NaOH forms.
• E. Equal volumes of H₂ and Cl₂. In theory the mole ratio is 1:1. In practice, however, some chlorine dissolves in water and reacts with the NaOH, so the volumes collected are not exactly equal. The standard answer treats this as not the best choice, since A is the definite, unambiguous change.

**Answer: A.

Exam tip: Remember the brine rule: concentrated brine gives Cl₂ at the anode, H₂ at the cathode, and NaOH is left in solution. If the NaCl is very dilute, O₂ forms at the anode instead. If the cathode is mercury, Na is discharged.