Q.While charging the lead storage battery ______________.

The key idea is that NCERT describes the lead storage battery's charging process using FIXED electrode labels carried over from discharge: "anode" always refers to the Pb plate, and "cathode" always refers to the PbO2 plate — the textbook states, "PbSO4(s) on anode and cathode is converted into Pb and PbO2 respectively" on charging.

1. At the anode (Pb plate), $PbSO_4$ gains electrons (reduction) to re-form $Pb$ and $SO_4^{2-}$: $PbSO_4(s) + 2e^- \rightarrow Pb(s) + SO_4^{2-}(aq)$. …

Why this approach works

A lead storage battery is a secondary cell — it can be recharged by passing a current opposite to the direction of its discharge current. NCERT's own chapter text describes this explicitly: "On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode is converted into Pb and PbO2 respectively." This sentence is the key to answering the question correctly — it tells us the textbook keeps "anode" as a FIXED name for the Pb plate and "cathode" as a FIXED name for the PbO2 plate, in both the discharge and charge cycles, rather than re-assigning the names based on which reaction occurs where during charging.

(Note: this differs from the strict, universal IUPAC convention, under which "anode" always means "wherever oxidation occurs at this moment" — under that stricter convention, the Pb plate's role during charging would technically be reduction, so it would be called the "cathode" for that half-cycle. NCERT does not use this stricter convention for describing the lead storage battery; it keeps the plate labels fixed. Exam answers for this question should follow the source textbook's own convention.)

Step-by-step reasoning

1. Recall the discharge reactions (spontaneous, defines the fixed plate names):

   • Anode (Pb plate): $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$ (oxidation)
   • Cathode (PbO2 plate): $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$ (reduction)

   After discharge, both plates are coated with $PbSO_4$.

2. Charging reverses both reactions, but the plate LABELS stay the same as during discharge:

   • At the anode (Pb plate):

 PbSO4 is **reduced** back to spongy Pb.

• At the cathode (PbO2 plate):

 PbSO4 is **oxidised** back to PbO2. …

Method: Electrode Reaction Analysis During Charging

This method uses the reversal of discharge reactions — during charging, the battery acts as an electrolytic cell, so oxidation and reduction swap electrodes compared to discharge.

Step-by-step reasoning:

1. Recall the discharge reactions (spontaneous):

   • Anode (oxidation): $\text{Pb} + \text{SO}_4^{2-} \rightarrow \text{PbSO}_4 + 2e^-$
   • Cathode (reduction): $\text{PbO}_2 + 4\text{H}^+ + \text{SO}_4^{2-} + 2e^- \rightarrow \text{PbSO}_4 + 2\text{H}_2\text{O}$

2. Reverse for charging (non-spontaneous, external voltage applied):

   • At the electrode that was the anode during discharge (now becomes cathode during charging): $\text{PbSO}_4 + 2e^- \rightarrow \text{Pb} + \text{SO}_4^{2-}$ → Reduction of $\text{PbSO}_4$ to $\text{Pb}$. …

Common Mistakes in Lead-Acid Battery Recharging

Students often confuse the discharging and charging processes, and also get confused about which naming convention NCERT uses for the electrodes. Let's break down the key errors.

Mistake 1: Applying the strict IUPAC anode/cathode swap to this NCERT question

The error: Thinking that because charging reverses current direction, the electrode names "anode" and "cathode" must also swap (i.e. concluding that the plate which was the anode during discharge becomes the cathode during charging, and vice versa).

Why it's wrong (in this context): While it is true that, by the strict universal IUPAC definition, oxidation always occurs at whichever electrode is called "anode" at that moment, the NCERT textbook itself describes the lead storage battery using FIXED electrode names — "anode" always refers to the Pb plate and "cathode" always refers to the PbO2 plate, regardless of whether the cell is discharging or charging. The NCERT text states: "On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode is converted into Pb and PbO2 respectively." This means, in the NCERT-based (and hence exam-expected) convention: PbSO4 on the anode (the Pb plate) is reduced back to Pb, and PbSO4 on the cathode (the PbO2 plate) is oxidised back to PbO2.

How to avoid: For NCERT-based exam questions on the lead storage battery, always use the FIXED plate labels (anode = Pb plate, cathode = PbO2 plate) as given in the textbook's own description, rather than re-deriving "true" IUPAC anode/cathode identity for the charging half-cycle.

Mistake 2: Mixing Up Oxidation & Reduction at Each (fixed-label) Electrode

The error: Saying $PbSO_4$ is oxidised at the anode or reduced at the cathode during charging.

Correct reactions during charging (using the fixed plate labels):

• At the anode (Pb plate):

(Reduction — PbSO4 is reduced back to spongy Pb)

• At the cathode (PbO2 plate):

(Oxidation — PbSO4 is oxidised back to PbO2)

How to avoid: Memorise the NCERT wording directly: on charging, PbSO4 on the anode is converted to Pb, and PbSO4 on the cathode is converted to PbO2.

Mistake 3: Picking option (iv) — "PbSO4 anode is oxidised to PbO2" …