In salt bridge, normally is used because — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Purpose and Mechanics of a Salt Bridge A salt bridge is a vital component of a galvanic cell that connects the anode and cathode compartments. It serves two main electrochemical functions: 1. It completes the internal electrical circuit of the cell by allowing the migration of ions. 2. It maintains electrical neutrality in both half-cells. As the redox reactions proceed, positive charge builds up at the anode (due to oxidation) and negative charge builds up at the cathode (due to reduction). The salt bridge neutralizes these excess charges by releasing anions into the anodic compartment and cations into the cathodic compartment. Step 2 - The Concept of Liquid Junction Potential If the cations and anions of the electrolyte in the salt bridge move at different velocities, they will not reach the respective half-cell compartments at the same time. This leads to a localized charge separation at the junction between the salt bridge and the half-cell solutions, creating an undesired electrical potential difference called the **liquid junction potential**. This potential opposes the natural electromotive force (EMF) of the cell, reducing its overall voltage and efficiency. To eliminate this junction potential, the cations and anions of the chosen electrolyte must migrate at nearly identical speeds. Step 3 - Properties of Potassium Chloride ($\ce{KCl}$) In an aqueous medium, potassium ions ($\ce{K^+}$) and chloride ions ($\ce{Cl^-}$) possess nearly identical ionic mobilities (and ionic velocities): $$\lambda^\circ(\ce{K^+}) \approx 73.5\text{ }\Omega^{-1}\text{ cm}^2\text{ mol}^{-1}$$ $$\lambda^\circ(\ce{Cl^-}) \approx 76.3\text{ }\Omega^{-1}\text{ cm}^2\text{ mol}^{-1}$$ Because their mobilities are almost equal, they migrate out of the salt bridge into their respective compartments at nearly the exact same rate. This simultaneous movement prevents the formation of a liquid junction potential, making $\ce{KCl}$ the ideal electrolyte for a salt bridge. Step 4 - Analyzing the Options * **Option (A) is incorrect:** While $\ce{KCl}$ is indeed a strong electrolyte, many other salts (like $\ce{NaCl}$ or $\ce{CaCl2}$) are also strong electrolytes but cannot be used effectively because of the mismatched velocities of their constituent ions. * **Option (B) is incorrect:** High electrical conductivity is a common feature of many soluble salts, but it does not prevent liquid junction potential if the ions move at different speeds. * **Option (C) is correct:** The specific, primary reason $\ce{KCl}$ is selected is because the potassium and chloride ions have nearly the same ionic mobility, which prevents liquid junction potential. * **Option (D) is incorrect:** Being an ionic compound is a general property of all salts and does not explain why $\ce{KCl}$ is preferred over other ionic compounds. $$\text{Correct Option: } \boxed{\text{C}}$$