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

Sulfuric acid is consumed during discharge.

Step 1: Understand the Composition and Reaction

A lead storage battery, commonly used in vehicles, consists of lead dioxide (PbO₂) as the positive plate, sponge lead (Pb) as the negative plate, and sulfuric acid (H₂SO₄) as the electrolyte. During discharge, the battery converts chemical energy into electrical energy.

Step 2: Analyze the Discharging Reaction

When the battery discharges, lead dioxide (PbO₂) on the positive plate and sponge lead (Pb) on the negative plate are converted into lead sulfate (PbSO₄). The overall discharging reaction is: \[\text{PbO}_2 + \text{Pb} + 2\text{H}_2\text{SO}_4 \rightarrow 2\text{PbSO}_4 + 2\text{H}_2\text{O}\] This equation indicates that sulfuric acid (H₂SO₄) is used or consumed in the reaction, forming water (H₂O) as a byproduct.

Step 3: Identify the Correct Option

Based on the reaction: 1. Lead is not formed; rather, both electrodes convert lead to lead sulfate. 2. Lead sulfate (PbSO₄) is produced, not consumed. 3. There is no evolution of sulfur dioxide (SO₂) in the standard equation. 4. Sulfuric acid is indeed consumed as it reacts to form lead sulfate and water. Thus, the correct answer is that sulfuric acid is consumed during discharge.

Key Concepts

Chemical Reactions in Batteries

The lead storage battery is a prime example of converting chemical energy into electrical energy. Inside this battery, a chemical reaction takes place when it is either charged or discharged. During discharge, the two electrodes, made of lead dioxide (PbO₂) and sponge lead (Pb), react with the sulfuric acid electrolyte (H₂SO₄). The chemical equation for this discharge reaction is:\[\text{PbO}_2 + \text{Pb} + 2\text{H}_2\text{SO}_4 \rightarrow 2\text{PbSO}_4 + 2\text{H}_2\text{O}\]This equation shows that as the battery discharges, lead dioxide and sponge lead transform into lead sulfate (PbSO₄), and meanwhile, sulfuric acid gets consumed to form water. These reactions are what generate the electric current that powers your devices when the battery discharges.

Sulfuric Acid Consumption

Sulfuric acid plays a critical role in the functioning of a lead storage battery. As the battery discharges, sulfuric acid is gradually consumed in the conversion process to form lead sulfate and water. This consumption can be reflected in the specific gravity of the electrolyte solution. Here's why sulfuric acid consumption matters:

• Power Generation: The acid is crucial for the chemical reaction that generates electricity.
• Indicator of Power: A drop in sulfuric acid concentration indicates the state of charge of the battery.

When sulfuric acid is consumed, the ability of the battery to deliver power decreases, as there is less acid available to react with. Therefore, understanding sulfuric acid consumption is essential for assessing the battery's remaining capacity and ensuring its proper maintenance.

Lead Sulfate Formation

One of the key transformations during the discharge of a lead storage battery is the formation of lead sulfate (PbSO₄). This compound forms on both the positive and negative plates. Let's explore this process:

• Reaction Outcome: The lead from both electrodes binds with sulfate ions from sulfuric acid to become lead sulfate.
• Importance: This formation is critical for the electricity generation process.
• Reversible Process: When charged, lead sulfate is converted back to lead dioxide and sponge lead, ready to start another cycle.

However, if the battery is left discharged for extended periods, lead sulfate can harden and crystallize. This can decrease the efficiency and life of the battery since the hard crystals are more difficult to convert back to their original forms during recharging. Thus, regular cycling and maintenance are crucial for battery longevity.