Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is/are correct statement about salt bridge? (a) Ions of salt bridge discharge at electrode (b) Ions of salt bridge do not discharge at electrode (c) Velocity of ions of salt bridge are almost equal (d) Salt bridge complete the electric circuit.

Step-by-Step Solution

Answer

Statements (b), (c), and (d) are correct.

Step 1: Understand the Role of a Salt Bridge

A salt bridge is a device used in an electrochemical cell to connect its oxidation and reduction half-cells. It allows the flow of ions and maintains electrical neutrality within the internal circuit.

Step 2: Evaluate Discharging of Ions

Examine statement (a) and (b). In a salt bridge, ions do not discharge at the electrodes; their primary purpose is to maintain a balance in charge to prevent the build-up of excess charge that would stop the reaction. Thus, statement (b) is correct, and statement (a) is false.

Step 3: Assess the Velocity of Ions

Consider statement (c). Ideally, the ions in a salt bridge should have similar velocities to maintain charge neutrality effectively. Hence, it's reasonable to assert that the velocities of cations and anions are nearly equal. Therefore, statement (c) is correct.

Step 4: Confirm Electric Circuit Completion

Look at statement (d). The salt bridge is essential for completing the circuit as it allows ions to flow between the half-cells, facilitating continuous electron flow through the external circuit. This makes statement (d) correct.

Key Concepts

Electrochemical Cell

An electrochemical cell is a fascinating setup that converts chemical energy into electrical energy. This process involves two half-cells, each containing a different chemical reaction.
In one half-cell, oxidation occurs, meaning electrons are lost. In the other half-cell, reduction takes place, signifying a gain of electrons. These two reactions are connected by a salt bridge, which allows ions to flow and maintain balance.
This flow of electrons from one half-cell to the other generates a flow of electrical energy, which we can use to power devices. Without the salt bridge, the reactions would eventually stop because of charge imbalance, halting the flow of electrons.

Ion Flow

Ion flow is a crucial mechanism in the operation of electrochemical cells. As electrons move through the external circuit from the anode to the cathode, ions in the solution must simultaneously move within the cell to maintain balance.
The salt bridge is a key player in this process. It contains a neutral salt like potassium nitrate, whose ions move into the two half-cells.

• Cations (\(+\)) from the salt bridge move towards the cathode.
• Anions (\(-\)) from the salt bridge move towards the anode.

This interior movement of ions helps ensure that each half-cell remains electrically neutral, allowing the reactions to continue uninterrupted in the electrochemical cell.

Electrical Neutrality

Electrical neutrality is essential in electrochemical cells to keep the reactions going smoothly. Each half-cell must remain balanced in charge, even while electrons are moving between them.
As reactions at the electrodes occur, they consume ions from the solution or produce extra ions, creating an imbalance. Without an external mediator, this imbalance would quickly shut down the cell.
The salt bridge steps in to maintain this crucial neutrality. By transferring ions between the half-cells, it compensates for the charge differences created by electrode reactions, ensuring the cell functions effectively over time.

Velocity of Ions

The velocity of ions in a salt bridge is an important factor for maintaining the effectiveness of an electrochemical cell. For the cell to work properly, ions must travel swiftly across the bridge to stabilize charge differences.
Ideally, the cations and anions in the salt bridge move at similar speeds, helping maintain charge balance across the setup. When one type of ion moves faster than the other, it could temporarily disrupt the cell's operations.
Selecting a salt for the bridge where both ion types have similar velocities helps ensure this balance. This concept underscores the importance of choosing the right materials to construct efficient electrochemical cells.