Question

When an acid cell is charged, then : (a) Voltage of cell increases (b) Resistance of cell increases (c) Electrolyte of cell dilutes (d) None of these

Short answer

During charging of an acid cell, (a) the voltage of the cell increases and (c) the electrolyte of the cell dilutes.

Step-by-step solution

Understanding the Charging of an Acid Cell

To solve this problem, one needs to understand what happens to an acid-based electrochemical cell, like a lead-acid cell, when it is being charged. During charging, positive ions move towards the negative electrode and negative ions move towards the positive electrode, which changes the concentration of the electrolyte, typically diluting it as water is formed from hydrogen and oxygen ions.

Identifying the Changes During Charging

The voltage of the cell does indeed increase as it charges, due to the buildup of potential energy in the form of chemical energy. However, as the concentration of acid decreases because more water is produced, the electrolyte dilutes.

Analyzing the Effect on Resistance

As the electrolyte dilutes, its conductivity decreases because there are fewer ions to carry the charge. This would mean that the resistance of the cell actually increases. However, because the voltage is rising more notably during the charge, the increase in cell resistance is typically a more gradual process and may not be immediately apparent.

Key concepts

Electrochemical Cells

Electrochemical cells are the foundational technology behind batteries, converting chemical energy into electrical energy through redox reactions.
An electrochemical cell is constituted of two different metal electrodes submerged in an electrolyte solution. When these cells are connected in a circuit, a chemical reaction takes place at the electrodes, causing one electrode to oxidize (lose electrons) and the other to reduce (gain electrons).
The movement of electrons from the oxidized electrode to the reduced one generates an electric current that can be harnessed to power electronic devices. In the context of lead-acid batteries, the electrolyte solution is typically a sulfuric acid solution, playing a crucial role in the electrochemical reactions.

Lead-Acid Battery Operation

Lead-acid batteries are a type of rechargeable electrochemical cell that have widespread use in vehicles and backup power systems.

Charging Process

During charging, the battery undergoes a reverse reaction where lead sulfate, formed on the electrodes during discharging, is converted back into lead, lead dioxide, and sulfuric acid.
The process requires an external power source that forces the current to flow in the reverse direction. As a result, the concentrations of lead and sulfuric acid within the battery increase, revitalizing its ability to produce electricity for future use.

Discharging Process

The discharging process involves the conversion of the lead oxide and lead on the electrodes back into lead sulfate while releasing electrical energy to power connected devices.

Voltage Changes in Cells

Voltage in an electrochemical cell represents the electric potential difference between the two electrodes.
During charging, this potential difference increases as energy is stored in the cell, and the voltage rises. This is because energy is being forced into the cell, increasing the potential energy of the charged species.
Conversely, during discharging, the voltage decreases since the potential energy is being converted into electrical energy, which is then used to do work, such as starting a car engine or lighting a bulb.

Electrolyte Dilution

Electrolyte dilution occurs when the sulfuric acid concentration in the electrolyte of a lead-acid battery decreases.
This dilution is directly tied to the charging process. As the lead-acid battery charges, the sulfuric acid reacts with lead to form water and lead sulfate.
This increase in water content dilutes the electrolyte, decreasing its ionic concentration and consequently, its conductivity. A diluted electrolyte leads to a higher resistance within the cell, making it more difficult for the electric current to flow.
Regular monitoring and maintenance of the electrolyte's concentration are vital for the longevity and efficiency of lead-acid batteries.