Which of the following reaction occurs at anode during discharging of lead storage battery?

\(Pb(s) + SO_4^{2-} (aq) \longrightarrow PbSO_4 (s) + 2e^-\)

The correct answer is option 2. \(Pb(s) + SO_4^{2-} (aq) \longrightarrow PbSO_4 (s) + 2e^-\).

Let us break down the discharging process of a lead storage battery in more detail, focusing on the anode reaction.

Overview of a Lead Storage Battery:

A lead storage battery consists of:

Anode: Made of lead (Pb).

Cathode: Made of lead dioxide (PbO₂).

Electrolyte: Dilute sulfuric acid (H₂SO₄).

During discharging, the battery generates electricity by converting chemical energy into electrical energy. At this time, both the anode and cathode undergo redox (reduction and oxidation) reactions, and the goal is the conversion of lead and lead dioxide into lead sulfate (\(PbSO_4\)) at both electrodes.

Anode Reaction (Oxidation):

At the anode, lead metal (\(Pb\)) undergoes oxidation. This means it loses electrons and forms lead sulfate (\(PbSO_4\)).

The oxidation reaction can be written as:

\(Pb(s) + SO_4^{2-}(aq) \longrightarrow PbSO_4(s) + 2e^-\)

Steps involved:

i. Lead (Pb), in its solid metallic form, comes into contact with the sulfate ion \((SO_4^{2-})\) from the sulfuric acid in the electrolyte

ii. Oxidation occurs, where lead (\(Pb\)) loses two electrons and reacts with the sulfate ion to form lead sulfate (\(PbSO_4\)).

iii. The two electrons (\(2e^-\)) released during the oxidation of lead flow through the external circuit, generating electricity.

This is the anodic reaction because:

Oxidation always occurs at the anode (loss of electrons).

Lead (Pb) is being oxidized into lead sulfate (\(PbSO_4\)).

Role of Sulfate Ion (\(SO_4^{2-}\)):

The sulfate ion (\(SO_4^{2-}\)) comes from the sulfuric acid in the electrolyte. It reacts with lead at the anode to form lead sulfate. Sulfate ions are necessary for the reaction because they provide the anionic part of the lead sulfate formed during discharging.

Cathode Reaction (Reduction):

At the cathode, the reaction involves the reduction of lead dioxide (PbO₂) to lead sulfate (\(PbSO_4\)). This is a separate process and complements the anode reaction during discharging.

Summary of Battery Reactions during Discharging:

Anode (oxidation):

\(Pb(s) + SO_4^{2-}(aq) \longrightarrow PbSO_4(s) + 2e^-\)

Lead metal is oxidized to lead sulfate, releasing electrons.

Cathode (reduction):

\(PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \longrightarrow PbSO_4(s) + 2H_2O(l)\)

Lead dioxide is reduced to lead sulfate.

Both reactions result in the formation of lead sulfate at both electrodes, which is a hallmark of the discharging process in a lead storage battery.

Why Option 2 is Correct:

The oxidation of lead metal at the anode is what happens during discharging. The correct reaction at the anode is:

\(Pb(s) + SO_4^{2-}(aq) \longrightarrow PbSO_4(s) + 2e^-\)

This reaction describes the loss of electrons (oxidation) from the lead metal and the formation of lead sulfate, matching the behavior of the anode during discharge.

This makes option 2 the correct answer.