Problem 32 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

In an experimental set-up for the measurement of EMF of a half-cell using a reference electrode and a salt bridge, when the salt bridge is removed, the voltage (a) remains the same (b) increases to maximum (c) decreases half the value (d) drops to zero

Step-by-Step Solution

Answer

The voltage drops to zero when the salt bridge is removed, as it interrupts the cell circuit.

Step 1: Understanding the Function of a Salt Bridge

The salt bridge in an electrochemical cell setup is used to maintain electrical neutrality within the internal circuit, allowing the flow of ions between the two compartments of the cell. It prevents the liquid junction potential that would arise if the solutions of different phases were in direct contact.

Step 2: Analyzing the Consequences of Removing the Salt Bridge

Removal of the salt bridge disrupts the circuit, stopping ion flow between the two half-cells. Since the flow of ions is necessary to balance the charge that flows due to the movement of electrons through the wire, removing the salt bridge effectively stops the overall reaction and the flow of electrons. This cessation of electron flow means there will be no current, and consequently, the measured voltage drops.

Step 3: Identifying the Correct Outcome

With the interruption of the ion flow due to the removal of the salt bridge, the electrochemical cell can no longer maintain a voltage. The correct answer is that the voltage drops to zero.

Key Concepts

Salt Bridge Function

A salt bridge is a crucial component in the world of chemistry, specifically when dealing with electrochemical cells. Its role is somewhat like a peacekeeper, ensuring that both sides of the cell play fair and maintain balance. Let's peek inside an electrochemical cell, which, in simple terms, is like a contained chemical reaction party. Electrons are excitedly moving from the guest of honor—the reducing agent—to the attendee that eagerly accepts them—the oxidizing agent. Meanwhile, the salt bridge oversees the party, making sure that the two separate halves of the cell don't get into a fight by accumulating too much positive charge in one and too much negative in the other.

It quietly exchanges ions between the two sides to neutralize the charge building up, preventing a potential ruckus that could fall into chaos. Think of it as the unsung hero of the electrochemical cell that, without fanfare, sustains the reaction and allows the transfer of electrons to continue unhindered. If you yank away this mediator—the salt bridge—then it's like turning off the music in the middle of the dance. Everything comes to a halt, and so does the current and voltage, as no more electron flow occurs because of the imbalance. This highlights the salt bridge's pivotal function: maintaining neutrality and keeping the electrochemical cell's reaction steady.

Electrochemical Cell

The electrochemical cell is like a tiny power generator on a molecular scale. You could imagine it as a battery, which, in principle, it is! It comes in different forms, but they all have a similar mission: to convert chemical energy into electrical energy through a chemical reaction. The cell consists of two half-cells, each comprising an electrode and an electrolyte. These half-cells like to communicate through, you guessed it, the salt bridge! They're obligated to pass messages in the form of ions, thanks to the salt bridge's diplomatic efforts.

While this is happening, electrons are made to sprint through an external wire from one half-cell to the other—this is the electricity that we are so eager to measure and use. With the absence of a salt bridge, this communication breaks down; ions cannot parade across to each half-cell, and you've got yourself a cell that effectively ceases to produce energy. The realization here is that an electrochemical cell is a complex but beautifully arranged system, where each part plays a profound role in the generation of electric current.

Measurement of EMF

EMF, or electromotive force, might sound like something out of a sci-fi movie, but it's really just a fancy term for the voltage produced by an electrochemical cell when no current is drawn from it. Measuring the EMF is tantamount to understanding the strength of the cell—how much force it can exert to push electrons through the circuit. It's a scientist's way of peeking into the cell's potential before it begins its chemical reaction journey.

To measure this 'oomph' of the cell, we rely on precise and sensitive instruments like voltmeters. However, the cell itself has to be intact and in harmony, with the salt bridge in place to regulate ion flow and maintain charge balance. If we remove the salt bridge, as seen in our textbook's exercise, the EMF measurement doesn't just change; it plummets to zero, because the communication system within the cell collapses. This critical point shines a light on both the importance of a correct setup when measuring EMF and the interdependence of cell components for accurate readings and cell functionality.