In an experimental set-up for the measurement of EMF of a half-cell using a reference electrode and — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Understand the Purpose and Function of a Salt Bridge In an electrochemical cell, a salt bridge plays several essential roles: 1. **Completing the Electrical Circuit:** It provides a physical path for the migration of ions between the anode and cathode half-cells, completing the internal circuit of the cell. 2. **Maintaining Electrical Neutrality:** As oxidation occurs at the anode, positive ions (cations) accumulate in the anode compartment. Simultaneously, reduction at the cathode removes cations from the solution, leading to an excess of negative ions (anions) in the cathode compartment. The salt bridge supplies anions to the anode compartment and cations to the cathode compartment, neutralizing this charge build-up. 3. **Preventing Liquid Junction Potential:** It minimizes the potential difference that arises at the junction of the two electrolyte solutions. Step 2 - Analyze the Effect of Removing the Salt Bridge When the salt bridge is physically removed from the experimental set-up: * **The internal electrical circuit is broken:** Electricity cannot flow unless there is a complete, closed loop. By removing the path of ionic transport, the internal circuit becomes open. * **Rapid accumulation of charges occurs:** Without the salt bridge, any initial transfer of electrons from the anode to the cathode immediately produces an unneutralized positive charge in the anode beaker and an unneutralized negative charge in the cathode beaker. This charge accumulation creates a counter-electromotive force that instantaneously opposes and halts further electron flow. * **Ionic current drops to zero:** Since ions can no longer migrate between the two half-cells to complete the circuit, the electrical current ($I$) becomes zero. Step 3 - Determine the Measured Cell Voltage According to Ohm's law and the principles of electrochemical cells, when the internal circuit is broken (the resistance of the circuit becomes infinite, $R \to \infty$ internally), no current can flow, and the electrical connection is severed. Consequently, the potential difference (voltage) measured across the terminals by a voltmeter drops immediately and completely to zero: $$V = 0\text{ V}$$ Step 4 - Evaluate and Explain the Options * **Option (A) is incorrect:** The voltage does not remain the same because the physical path for ion migration is destroyed, rendering the electrochemical cell non-functional. * **Option (B) is incorrect:** The voltage does not increase to a maximum. It drops because the circuit is broken. * **Option (C) is incorrect:** The voltage does not decrease to half its value. There is no partial circuit completion; it is a binary state of open vs. closed circuit. * **Option (D) is correct:** Removing the salt bridge opens the circuit, halting the flow of ions and electrons, causing the measured voltage to drop immediately to zero. $$\text{Correct Option: } \boxed{\text{D}}$$