Statement I: , , , etc., cannot be used in the salt bridge of a cell containing silver. Statement II — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Evaluate Statement I (Stability of the Salt Bridge in Silver Cells) A salt bridge is designed to maintain electrical neutrality between the two half-cells and complete the electrical circuit by allowing the migration of ions. However, the electrolyte selected for the salt bridge must be chemically inert and must not react with the ions present in either of the half-cells. If chloride-containing salts like $\ce{KCl}$, $\ce{NaCl}$, or $\ce{NH4Cl}$ are used in a cell containing silver ions ($\ce{Ag^+}$) (such as in a silver half-cell, $\ce{Ag \mid AgNO3}$), the chloride ions ($\ce{Cl^-}$) from the salt bridge will diffuse into the half-cell. There, they react with $\ce{Ag^+}$ ions to form an insoluble precipitate of silver chloride ($\ce{AgCl}$): $$\ce{Ag^+(aq) + Cl^-(aq) -> AgCl(s) \downarrow}$$ This precipitation of solid $\ce{AgCl}$ occurs at the junction where the salt bridge meets the electrode solution, which clogs the porous plug or capillary of the salt bridge. This clogging halts the migration of ions, drastically increases the cell's internal resistance, and ultimately stops the electrochemical cell from functioning. Hence, chloride-containing salts cannot be used in salt bridges for cells containing $\ce{Ag^+}$, $\ce{Pb^{2+}}$, or $\ce{Hg2^{2+}}$ ions. Therefore, **Statement I is correct**. Step 2 - Evaluate Statement II (General Composition and Ingredients of a Salt Bridge) A salt bridge typically consists of a U-shaped glass tube filled with a concentrated solution of an inert electrolyte such as $\ce{KCl}$, $\ce{KNO3}$, $\ce{NH4NO3}$, or $\ce{K2SO4}$. To prevent the liquid electrolyte from flowing out of the tube while still permitting ionic migration, the electrolyte is mixed with a hot gel-forming agent like **agar-agar** or gelatin. Upon cooling, the mixture solidifies into a semi-solid gel. An electrolyte is considered "inert" if its ions do not participate chemically in the redox reactions of the cell and have nearly equal ionic mobilities, ensuring that no liquid-junction potential is established. Therefore, **Statement II is correct**. Step 3 - Analyze the Relationship between the Two Statements * Statement I addresses a specific chemical incompatibility (precipitation) that limits the choice of electrolytes in cells containing silver ions. * Statement II provides a general definition of what a salt bridge is made of and lists some common electrolytes. Although both statements are individually true, Statement II does not provide the chemical reasoning behind the restriction mentioned in Statement I (which is the precipitation of insoluble $\ce{AgCl}$). Thus, Statement II is NOT the correct explanation of Statement I. $$\text{Correct Option: } \boxed{B}$$