Q.Assertion: Electrolysis of $NaCl$ solution gives chlorine at anode instead of $O_2$.
Reason: Formation of oxygen at anode requires overvoltage.

Concept: Faraday's Laws of Electrolysis — selective discharge of ions depends on electrode potential and overvoltage.

1. In the electrolysis of aqueous NaCl, the possible anode reactions are: $2Cl^- \rightarrow Cl_2 + 2e^-$ (standard potential $E^\circ = +1.36$ V) $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ (standard potential $E^\circ = +1.23$ V) …

1. The core idea: thermodynamics vs. kinetics in electrolysis.

   When you electrolyse an aqueous solution of NaCl, you have two possible oxidation reactions at the anode (the positive electrode where oxidation occurs):

   • Oxidation of chloride ions: $2Cl^- \rightarrow Cl_2 + 2e^-$
   • Oxidation of water: $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$

   To decide which happens, you first look at the standard electrode potentials (thermodynamics). The reaction with the lower (less positive) reduction potential is easier to oxidise — or equivalently, the one with the more negative oxidation potential occurs preferentially.

2. Compare the standard potentials.

   Standard reduction potentials (at 25°C, 1 M, 1 atm):

   • $Cl_2 + 2e^- \rightarrow 2Cl^-$: $E^\circ = +1.36\ \text{V}$
   • $O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$: $E^\circ = +1.23\ \text{V}$

   For oxidation, we reverse the sign. So:

   • Oxidation of $Cl^-$: $E^\circ_{\text{ox}} = -1.36\ \text{V}$
   • Oxidation of $H_2O$: $E^\circ_{\text{ox}} = -1.23\ \text{V}$

   Since $-1.23\ \text{V} > -1.36\ \text{V}$, water oxidation is thermodynamically more favourable — it requires a smaller applied voltage. So, based on standard potentials alone, oxygen should be produced at the anode, not chlorine.

3. Enter overvoltage — the kinetic barrier.

   The reason oxygen does not appear is overvoltage (also called overpotential). This is the extra voltage needed beyond the thermodynamic value to make a reaction proceed at a noticeable rate, due to kinetic hurdles (slow electron transfer, intermediate formation, etc.).

   • Oxygen evolution on common anode materials (like platinum or graphite) has a high overvoltage — often around 0.4–0.6 V.
   • Chlorine evolution has a much lower overvoltage — typically negligible on these electrodes.

   So the actual potential required to evolve oxygen is roughly:

While for chlorine:

Now, chlorine requires a lower applied voltage than oxygen in practice. Hence, chlorine is produced preferentially at the anode.

4. Why the reason is correct and explains the assertion. …

Method: Overvoltage Analysis in Electrolysis

Concept: In aqueous electrolysis, the actual product at an electrode depends not only on standard reduction potentials but also on overvoltage — the extra voltage required beyond the theoretical value for a gas to evolve.

Steps to solve:

1. Identify the possible reactions at the anode

   For electrolysis of aqueous $NaCl$:

   • Chloride oxidation: $2Cl^- \rightarrow Cl_2 + 2e^-$ ( $E^\circ = -1.36 \, V$ )
   • Water oxidation: $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ ( $E^\circ = -1.23 \, V$ )

2. Compare standard potentials

   Theoretically, $O_2$ evolution ( $E^\circ = -1.23 \, V$ ) is easier than $Cl_2$ evolution ( $E^\circ = -1.36 \, V$ ), so oxygen should form first.

3. Apply the overvoltage concept

   • Oxygen evolution has a high overvoltage on common anode materials (like graphite or platinum).
   • This means the actual voltage needed for $O_2$ evolution is much higher than $-1.23 \, V$ (often around $-1.6$ to $-1.8 \, V$).
   • Chlorine evolution has a low overvoltage, so its actual required voltage remains close to $-1.36 \, V$.

4. Determine the actual product …

Here’s a breakdown of the common mistakes students make on this concept and how to avoid each.

Common Mistake #1: Confusing the reason for chlorine being produced instead of oxygen

What students do wrong:

Many students think the reason is simply that “chloride ions are easier to oxidise than water” — and then they accept the given reason (“overvoltage”) as correct without checking the actual standard electrode potentials.

Why it’s wrong:

The standard reduction potentials (at 25°C, 1 M) are:

• $E^\circ_{\text{Cl}_2/\text{Cl}^-} = +1.36 \, \text{V}$
• $E^\circ_{\text{O}_2/\text{H}_2\text{O}} = +1.23 \, \text{V}$

Based on these values, oxygen should be produced first (lower potential means easier oxidation). But in practice, chlorine is produced because the oxidation of water to oxygen requires a high overvoltage at the anode (especially on inert electrodes like platinum or graphite). This overvoltage makes the effective potential for oxygen evolution higher than that for chlorine, so chlorine is favoured.

How to avoid:

Always check the standard potentials first. If the expected product (oxygen) is not formed, look for kinetic factors — overvoltage is the key here. Memorise that overvoltage is the extra voltage needed to overcome the activation energy barrier for gas evolution (especially O₂ and H₂).

Common Mistake #2: Thinking the reason is false because “overvoltage” sounds unfamiliar

What students do wrong:

Students who haven’t studied overvoltage in detail often mark the reason as false, assuming it’s a made-up term.

Why it’s wrong:

Overvoltage is a real electrochemical phenomenon. For oxygen evolution, the overvoltage is significant (~0.6 V on platinum), making the effective potential for O₂ evolution around $1.23 + 0.6 = 1.83 \, \text{V}$, which is higher than the $1.36 \, \text{V}$ for chlorine. Hence, chlorine is produced.

How to avoid:

Learn the concept of overvoltage — it’s the extra voltage required to overcome the slow kinetics of gas evolution at an electrode. It is especially high for O₂ and H₂. This is a standard topic in electrochemistry for JEE/NEET.

Common Mistake #3: Confusing the correct option (A vs B)

What students do wrong:

Some students correctly identify that both assertion and reason are true, but then choose option (ii) — “reason is not the correct explanation” — because they think overvoltage is unrelated.

Why it’s wrong:

The reason is the correct explanation. The overvoltage for oxygen is the direct reason why chlorine is produced instead of oxygen. Without overvoltage, oxygen would be the product.

How to avoid: …