In an electrochemical cell, polarization is caused by

Analyze the question: The question asks what causes polarization in an electrochemical cell, such as a simple voltaic (Leclanché or Daniell-type) cell.

Explain the concept: Polarization is the build-up of a layer of hydrogen gas bubbles on the surface of the positive electrode (the copper plate in a simple voltaic cell). Here is how it happens:

1. In a simple cell with zinc and copper in dilute H₂SO₄, zinc is oxidized: Zn → Zn²⁺ + 2e⁻
2. The electrons travel through the external circuit to the copper plate.
3. At the copper plate, H⁺ ions from the acid pick up the electrons: 2H⁺ + 2e⁻ → H₂(g)
4. The hydrogen gas forms a film of bubbles on the copper. This film: • Insulates the electrode from the electrolyte, raising internal resistance • Sets up a reverse potential (the hydrogen electrode opposes the zinc), so the cell's e.m.f. drops

This drop in current and voltage is called polarization.

Remedy: A depolarizer is added. It oxidizes the hydrogen to water. For example, manganese(IV) oxide (MnO₂) is used in the Leclanché cell: 2MnO₂ + H₂ → Mn₂O₃ + H₂O

Checking the options:

• A. Oxygen: this is not the cause. Oxidizing agents act as depolarizers.
• B. Hydrogen: correct, since hydrogen gas bubbles cause the polarization.
• C. Tetraoxosulphate(IV) acid: this is the electrolyte, which supplies H⁺ ions, but it is not the direct cause.
• D. Chlorine: not involved in a simple cell using H₂SO₄.

Answer: B (Hydrogen)

Exam tip: Remember "Polarization = Pile-up of Hydrogen." If you see "polarization" or "depolarizer", think of hydrogen bubbles on the positive electrode and MnO₂ removing them.