Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

When a lead storage battery is discharged (a) lead is formed (b) lead sulphate is consumed (c) \(\mathrm{SO}_{2}\) is evolved (d) sulphuric acid is consumed

Step-by-Step Solution

Answer

(d) sulphuric acid is consumed.

Step 1: Understanding a Lead Storage Battery

A lead storage battery, also known as a lead-acid battery, consists of lead dioxide ( ext{PbO}_2) as the positive plate, sponge lead ( ext{Pb}) as the negative plate, and dilute sulfuric acid ( ext{H}_2 ext{SO}_4) as the electrolyte. These components participate in a reversible chemical reaction during the battery's use.

Step 2: Chemical Reaction During Discharge

When discharging, both the lead dioxide on the positive plate and the sponge lead on the negative plate react with sulfuric acid to form lead sulfate ( ext{PbSO}_4). The chemical equations representing the reactions for discharge are:1. Positive Plate Reaction: \( ext{PbO}_2 + ext{SO}_4^{2-} + 4 ext{H}^+ + 2 ext{e}^- ightarrow ext{PbSO}_4 + 2 ext{H}_2 ext{O}\) 2. Negative Plate Reaction: \( ext{Pb} + ext{SO}_4^{2-} ightarrow ext{PbSO}_4 + 2 ext{e}^-\)

Step 3: Analyze the Outcome of Discharge

During the discharge of the battery, lead sulfate is formed on both the positive and negative plates. Simultaneously, sulfuric acid is consumed in the process as it participates in the chemical reactions. There is no formation of lead metal or evolution of ext{SO}_2 gas in this process.

Step 4: Determine the Correct Answer

From the analysis, the correct aspect of what happens when a lead storage battery is discharged is that sulfuric acid is consumed (option (d)). Lead sulfate is formed, not consumed, contradicting (b); neither lead (option (a)) is formed, nor is ext{SO}_2 gas (option (c)) evolved during discharge.

Key Concepts

Lead-Acid Battery

A lead-acid battery is a type of rechargeable battery often used in vehicles like cars and trucks. Its main components include lead dioxide \(\text{(PbO}_2\) as the positive electrode plate, sponge lead \(\text{(Pb)}\) as the negative electrode plate, and a solution of sulfuric acid \(\text{(H}_2\text{SO}_4)\) acting as the electrolyte.The battery operates based on a straightforward principle:

• The lead dioxide and sponge lead react with the sulfuric acid when the battery is in use.
• It is a reversible process, which means that it can discharge and recharge.

This versatility in charging makes lead-acid batteries a practical energy storage solution for a variety of applications.

Chemical Reaction During Discharge

During the discharge phase of a lead-acid battery, specific chemical reactions occur at the electrodes. Here’s how it works:

• At the positive electrode (lead dioxide plate), the reaction involves the lead dioxide, sulfate ions \(\text{(SO}_4^{2-})\), hydrogen ions \(\text{(H}^+)\), and electrons to form lead sulfate \(\text{(PbSO}_4)\) and water \(\text{(H}_2\text{O)}\).
• At the negative electrode (sponge lead plate), the lead reacts with sulfate ions to form lead sulfate and releases electrons.

These processes ensure the flow of electrical energy, enabling the battery to power devices. The involvements of electrons in these reactions are crucial as they travel through the circuit, providing electrical energy.

Sulfuric Acid Consumption

As the lead-acid battery discharges, sulfuric acid is a vital participant in the reactions at both electrodes. It combines with lead and lead dioxide to form lead sulfate, which entails a consumption of sulfuric acid. Here are some key points about sulfuric acid in the discharge process:

• The concentration of sulfuric acid decreases as the battery discharges.
• The strength and efficiency of the battery depend on the availability of sulfuric acid.

In essence, the discharge process gradually depletes the sulfuric acid, signaling a reduction in the battery’s ability to deliver power until it is recharged.

Lead Sulfate Formation

Lead sulfate forms as an end product during the discharge of a lead-acid battery. Both the lead dioxide and the sponge lead at opposite sides of the cell plates convert into lead sulfate. Lead sulfate's role can be summarized in three important behaviors:

• Formation on both electrodes during discharge.
• Reveres to lead dioxide and lead upon recharging, a reversible process that restores sulfuric acid.
• Accumulation of lead sulfate can reduce the effectiveness if the battery is not regularly charged.

The formation and conversion of lead sulfate are crucial to the cyclical use of the battery, allowing it to be recharged and reused multiple times safely.