Question

In a lead acid cell, hydrogen is liberated at

• A. positive plate
• B. negative plate
• C. both plates
• D. none of the plates

Hint:

In a lead-acid battery, hydrogen gas is released at the negative plate during the charging process due to reduction.

Answer 1

The Correct Option is B

Step 1: Understanding hydrogen evolution in lead-acid cells.
In a lead-acid battery, hydrogen gas is liberated at the negative plate during the charging process, where reduction occurs.
Step 2: Conclusion.
Thus, hydrogen is liberated at the negative plate, which corresponds to option (B).

Answer 2

When a lead-acid battery is overcharged (charged beyond full capacity), the excess energy causes electrolysis of the water in the electrolyte rather than useful chemical charging. In electrolysis, reduction reactions (gain of electrons) occur at the cathode, and oxidation reactions (loss of electrons) occur at the anode. During charging, the negative plate acts as the cathode, where hydrogen ions (H⁺) from the electrolyte are reduced to hydrogen gas: \[ 2H^+ + 2e^- \rightarrow H_2 \uparrow. \] Let's check each option against this electrode assignment.

1. Positive plate: The positive plate acts as the anode during charging, where oxidation occurs, releasing oxygen gas instead of hydrogen.
2. Negative plate: The negative plate acts as the cathode during charging, exactly where the reduction reaction producing hydrogen gas takes place.
3. Both plates: Hydrogen is specifically a reduction product, which only occurs at the cathode (negative plate); the positive plate instead releases oxygen, not hydrogen, so hydrogen is not produced at both.
4. None of the plates: Since gassing at both electrodes is a well-documented effect of overcharging, hydrogen is indeed released, ruling this out.

The electrode-reaction assignment during electrolysis confirms hydrogen is liberated at the negative plate.

Therefore, the correct answer is negative plate.