A depolarizer used in dry cell batteries is — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Define Polarization and the Role of a Depolarizer In electrochemical cells, **polarization** is a phenomenon where the accumulation of reaction products (such as gases) on or near the electrode surfaces increases the internal resistance of the cell. This resistance opposes the flow of current and significantly reduces the cell's electromotive force (EMF). A **depolarizer** is a chemical substance added to a battery cell to prevent polarization. It does this by chemically reacting with and removing the insulating reaction products (usually hydrogen gas) from the active surface of the electrode. Step 2 - Understand the Chemical Components of a Dry Cell A standard dry cell (also known as a Leclanché cell) consists of the following components: * **Anode (Negative Electrode):** A zinc container ($\ce{Zn}$). * **Cathode (Positive Electrode):** A central carbon (graphite) rod surrounded by a paste of powdered manganese dioxide ($\ce{MnO2}$) and carbon. * **Electrolyte:** A moist paste of ammonium chloride ($\ce{NH4Cl}$) and zinc chloride ($\ce{ZnCl2}$). Step 3 - Analyze the Electrode Reactions and Polarization During the discharge of the dry cell, the following electrochemical half-reactions take place: * **At the Anode (Oxidation):** $$\ce{Zn(s) -> Zn^2+(aq) + 2e^-}$$ * **At the Cathode (Reduction):** Ammonium ions ($\ce{NH4^+}$) are reduced at the carbon electrode: $$\ce{2NH4^+(aq) + 2e^- -> 2NH3(g) + H2(g)}$$ The hydrogen gas ($\ce{H2}$) produced in this reduction step tends to deposit as a thin, insulating gaseous layer on the surface of the carbon cathode. This accumulation of gas bubbles blocks electrical contact between the electrode and the electrolyte, causing gaseous polarization of the cathode. This dramatically increases the internal resistance of the cell and causes its voltage to drop rapidly. Step 4 - Explain the Action of Manganese Dioxide ($\ce{MnO2}$) as a Depolarizer To combat this polarization, manganese dioxide ($\ce{MnO2}$) is packed around the carbon cathode. It acts as an oxidizing agent (depolarizer) that slowly oxidizes the accumulated hydrogen gas on the electrode surface into water: $$\ce{2MnO2(s) + H2(g) -> Mn2O3(s) + H2O(l)}$$ Through this chemical removal of hydrogen gas, the insulating layer is destroyed, keeping the cell depolarized and allowing it to maintain a stable voltage of about $1.5\text{ V}$. Step 5 - Evaluate and Explain the Options * **Option (A) is incorrect:** Ammonium chloride ($\ce{NH4Cl}$) is the main electrolyte in the dry cell. It provides the $\ce{NH4^+}$ ions that undergo reduction, which actually generates the polarizing hydrogen gas rather than removing it. * **Option (B) is correct:** Manganese dioxide ($\ce{MnO2}$) is the designated depolarizer because it chemically oxidizes the hydrogen gas to water. * **Option (C) is incorrect:** Potassium hydroxide ($\ce{KOH}$) is an alkaline electrolyte used in alkaline dry cells, but it does not act as a depolarizer. * **Option (D) is incorrect:** Sodium phosphate ($\ce{Na3PO4}$) plays no role in dry cell chemistry. $$\text{Correct Option: } \boxed{\text{B}}$$