Problem 47 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

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

Step-by-Step Solution

Answer

(d) sulphuric acid is consumed

Step 1: Understanding the Lead Storage Battery

A lead storage battery, commonly referred to as a lead-acid battery, consists of lead dioxide (PbO2) as the positive electrode, sponge lead (Pb) as the negative electrode, and a sulfuric acid (H2SO4) solution as the electrolyte.

Step 2: Analyze the Discharge Reaction

When the battery discharges, both the positive and negative electrodes undergo chemical reactions with the sulfuric acid electrolyte to produce lead sulfate (PbSO4), water (H2O), and energy. The overall discharge reaction can be described by the equation: PbO2(s) + Pb(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)

Step 3: Identify the Correct Option

By analyzing the equation, we can identify the correct option regarding the discharge process. (a) is incorrect as sulfur dioxide (SO2) is not evolved; (b) is incorrect as lead sulfate is produced, not consumed; (c) is incorrect because lead is converted to lead sulfate, not formed as a pure element; (d) is correct as sulfuric acid is a reactant, meaning it is consumed during the discharge of the battery.

Key Concepts

Discharge Reaction

The discharge reaction in a lead storage battery is a crucial process where chemical energy is converted into electrical energy that we use to power devices. During this reaction, the lead dioxide (usually at the positive electrode) and the sponge lead (the negative electrode) react with sulfuric acid in the electrolyte to produce lead sulfate and water.

The chemical equation for this reaction is: \[\text{PbO}_{2}(s) + \text{Pb}(s) + 2\text{H}_{2}\text{SO}_{4}(aq) \rightarrow 2\text{PbSO}_{4}(s) + 2\text{H}_{2}\text{O}(l)\].

This equation shows that lead sulfate is a product of the reaction, not a reactant, which eliminates certain potential misconceptions. Comprehending this reaction is fundamental to understand how the battery generates an electric current when discharging.

Electrochemistry

Electrochemistry is the branch of chemistry that deals with the relationship between electrical energy and chemical reactions. In the context of a lead storage battery, electrochemistry explains how the battery operates. The transfer of electrons during the discharge reaction is what generates electricity. At the positive electrode, lead dioxide is reduced (gains electrons), and at the negative electrode, lead is oxidized (loses electrons).

These reactions at different electrodes, also known as half-reactions, occur simultaneously and are interconnected through the circuit the battery is attached to. The ability to control and harness these electrochemical reactions is what makes lead storage batteries reliable sources of direct current (DC) power in many applications.

Lead Sulfate

Lead sulfate (\(\text{PbSO}_{4}\)) is a product of the discharge reaction in a lead-acid battery, and its formation is a sign that the battery is being utilized. As the battery discharges, more and more lead sulfate is formed, coating both electrodes and preventing the flow of electrons. This process is part of what determines the battery's capacity and life expectancy.

Significance of Lead Sulfate in Battery Recycling

Lead sulfate is also an important consideration in the recycling of lead-acid batteries. The reclamation and proper management of lead sulfate are essential due to its environmental impact and the value of lead as a resource.

Sulfuric Acid Consumption

Sulfuric acid (\(\text{H}_{2}\text{SO}_{4}\)) plays a vital role as the electrolyte in a lead-acid battery. It participates actively in the discharge reaction, with its concentration decreasing as it is consumed. This consumption of sulfuric acid leads to a drop in the density of the electrolyte, a change that can be measured with a hydrometer to assess the state of charge in the battery.

Monitoring Sulfuric Acid Levels

Regular monitoring of the sulfuric acid concentration can provide information about battery health and remaining charge, making sulfuric acid consumption a key factor in battery maintenance and longevity.