Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

When an acid cell is charged, then (a) voltage of cell increases (b) electrolyte of cell dilutes (c) resistance of cell increases (d) none of these

Step-by-Step Solution

Answer

The voltage of the cell increases when an acid cell is charged.

Step 1: Understanding the Acid Cell Charging Process

An acid cell, commonly known as a lead-acid battery, consists of electrodes submerged in an electrolyte, often sulfuric acid. During charging, electrical energy is applied, causing a chemical reaction in the lead-acid cell that converts lead sulfate ( ext{PbSO}_4) on the electrodes back into lead dioxide ( ext{PbO}_2) and lead ( ext{Pb}). The electrolyte, which is a water-sulfuric acid mixture, becomes denser as the concentration of sulfuric acid increases.

Step 2: Analyzing Voltage Change

When charging a lead-acid cell, the chemical reactions restore the electrodes to their charged states. This process increases the potential difference between the electrodes, causing the voltage of the cell to increase. This increase in voltage is a typical indicator that a cell is being charged.

Step 3: Assessing Electrolyte Concentration

During charging, the reaction converts water and lead sulfate back to sulfuric acid and lead dioxide, increasing the concentration of sulfuric acid in the electrolyte. This means that instead of the electrolyte diluting, it actually becomes more concentrated, contrary to option (b).

Step 4: Evaluating Resistance Change

Charging typically decreases the internal resistance of the lead-acid cell. As the electrolyte concentration increases, the lead-acid cell's internal resistance decreases, allowing more efficient flow of current.

Step 5: Identifying the Correct Answer

Given the evaluated effects, option (a) is correct, as the voltage of the cell increases during charging. Neither dilution of the electrolyte nor an increase in resistance accurately describes the process, excluding options (b) and (c).

Key Concepts

Electrode Reactions

In a lead-acid battery, the "electrode reactions" are crucial to its functioning, especially during the charging process. During charging, electrical energy forces a chemical reaction at the electrodes, which are made of lead and lead dioxide. Here's what happens in a nutshell:

• The lead sulfate (\(\text{PbSO}_4\)) on the electrodes gets converted back to lead dioxide (\(\text{PbO}_2\)) at the positive electrode, and to pure lead (\(\text{Pb}\)) at the negative electrode.


• This restoration of the electrodes to their original materials is essential for the battery to store energy and work efficiently upon discharging.

Refreshing the electrodes in this way increases their potential difference, which is why we see an increase in voltage during the charging. Thus, the electrode reactions facilitate the battery's capability to recharge and deliver necessary voltage when needed.

Electrolyte Concentration

"Electrolyte concentration" plays a vital role in the performance of a lead-acid battery. At the heart of this process is the sulfuric acid solution that acts as the battery's electrolyte.

• When the battery charges, water and lead sulfate react to increase the amount of sulfuric acid present in the electrolyte.


• This results in a denser solution, indicating a higher concentration of sulfuric acid.

This increase in concentration is the opposite of dilution, which is why during charging, we do not observe a dilution of the electrolyte. Instead, the heightened concentration indicates a successful charging process, allowing for better conductivity and improved performance of the battery.

Internal Resistance of Battery

The "internal resistance of battery" is a crucial factor influencing how efficiently a battery can charge and discharge.

• During charging, as the sulfuric acid concentration increases, the battery's internal resistance tends to decrease.


• Lower internal resistance allows for more efficient current flow, decreasing energy loss and heating within the battery.

This decreased resistance is beneficial because it means the battery can be charged more rapidly and power devices more effectively when discharging. Hence, during the charging process of a lead-acid battery, instead of seeing an increase in resistance, you will typically observe a decrease, enhancing the battery's overall performance.