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

During overcharging, gassing occurs at both plates, but the two plates release different gases. Recognizing which gas comes from which plate answers this question directly.

1. Positive plate: This plate releases oxygen gas during overcharging, not hydrogen.
2. Negative plate: This plate releases hydrogen gas during overcharging.
3. Both plates: While both plates do release SOME gas, they release different gases (oxygen at positive, hydrogen at negative) — hydrogen specifically comes from only one of them.
4. None of the plates: Gassing is a real, observable effect during overcharging, so this option is incorrect.

Identifying that hydrogen specifically (not oxygen) is released at the negative plate confirms the answer.

Therefore, the correct answer is negative plate.

Answer 2

We can also reason from the direction of conventional current and ion migration during charging. During charging, positive hydrogen ions (H⁺) in the electrolyte migrate toward the plate connected to the negative terminal of the charging source, since opposite charges attract. At that negative plate, the H⁺ ions pick up electrons supplied by the external charging circuit and are neutralized into hydrogen gas. Let's check each option against this migration/neutralization picture.

1. Positive plate: Positive hydrogen ions are repelled from, not attracted to, the positive plate, so hydrogen does not accumulate and neutralize there.
2. Negative plate: Hydrogen ions migrate toward and are neutralized at the negative plate, consistent with the migration argument.
3. Both plates: Since H⁺ ions are directionally attracted only to the negative plate (not the positive plate, which attracts negative ions instead), hydrogen gas specifically forms only at the negative plate.
4. None of the plates: The migration and neutralization of H⁺ ions clearly does occur at the negative plate under overcharge conditions, ruling this out.

The ion-migration argument confirms hydrogen is liberated specifically at the negative plate.

Therefore, the correct answer is negative plate.