The function(s) of salt bridge in a cell is/are — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Understand the Setup of a Galvanic Cell A standard galvanic (voltaic) cell is composed of two separate half-cells: 1. **Anodic Half-Cell:** Where oxidation occurs (e.g., metal atoms lose electrons to form cations, $\ce{M(s) -> M^{n+}(aq) + n e^-}$). 2. **Cathodic Half-Cell:** Where reduction occurs (e.g., metal cations in solution gain electrons to deposit as metal atoms, $\ce{M'^{m+}(aq) + m e^- -> M'(s)}$). If these two half-cells are connected externally only by a metallic conducting wire, a current cannot flow continuously because the circuit is open. To close the circuit and allow the cell to operate continuously, an internal connection must be established. This is accomplished using a **salt bridge**. Step 2 - Analyze Option (A): Maintaining standard electrode potential of cell constant The standard electrode potential ($E^\circ$) of an electrode is a fundamental thermodynamic quantity determined solely by the chemical nature of the redox couple, the standard temperature, and standard concentration/pressure states. It is defined as: $$E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$$ The salt bridge acts as a physical medium for ion migration but has no thermodynamic influence on the standard potentials of the electrode couples. Therefore, Statement (A) is incorrect. Step 3 - Analyze Option (B): Completing the electrical circuit For an electrical current to flow, there must be a continuous, closed loop. * **External Circuit:** Electrons flow through the metallic wire from the anode (oxidation site) to the cathode (reduction site). * **Internal Circuit:** The salt bridge permits the physical migration of ions (anions toward the anode and cations toward the cathode) through its gelatinous medium. This movement of ions completes the internal electrical circuit. Therefore, Statement (B) is correct. Step 4 - Analyze Option (C): Separating both solutions from each other If the anodic and cathodic electrolyte solutions were allowed to mix directly: * The reactants (such as $\ce{Zn(s)}$ and $\ce{Cu^{2+}(aq)}$ in a Daniell cell) would react directly at the interface of the solutions. * This direct redox reaction would release energy purely as heat rather than converting chemical energy into electrical work through the external circuit. The salt bridge physically separates ("departs") the two half-cell solutions from each other, preventing mechanical mixing and direct chemical reactions while still allowing ionic contact. Therefore, Statement (C) is correct. Step 5 - Analyze Option (D): Maintaining electrical neutrality of both solutions As the redox reaction proceeds in a running cell: * At the anode, positive metal ions accumulate in the solution due to oxidation: $$\ce{Zn(s) -> Zn^{2+}(aq) + 2e^-}$$ * At the cathode, positive metal ions are discharged, leaving behind a surplus of negative spectator anions in the solution: $$\ce{Cu^{2+}(aq) + 2e^- -> Cu(s)}$$ This accumulation of positive charge at the anode and negative charge at the cathode would instantly create an opposing electrical field, blocking further electron flow and causing the cell potential to drop to zero. The salt bridge contains inert ions (such as $\ce{K^+}$ and $\ce{Cl^-}$) that migrate to neutralize these charges: * Anions ($\ce{Cl^-}$) migrate into the anodic half-cell to neutralize excess positive charge. * Cations ($\ce{K^+}$) migrate into the cathodic half-cell to neutralize excess negative charge. Thus, the salt bridge maintains the electrical neutrality of both electrolyte solutions. Therefore, Statement (D) is correct. $$\text{Correct Options: } \boxed{\text{B, C, D}}$$