Problem 43 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In a galvanic cell, the salt bridge (a) Does not participate chemically in the cell reaction (b) Stops the diffusion of ions from one electrode to another (c) Is necessary for the occurrence of the cell reaction (d) Ensures mixing of the two electrolytic solutions

Step-by-Step Solution

Answer

(a) and (c) are true.

Step 1: Understanding the Role of a Salt Bridge

A salt bridge is used in a galvanic cell to allow the flow of ions and prevent the solutions from mixing. It's essentially a pathway for ions to maintain electrical neutrality as electrons flow through the external circuit.

Step 2: Analyzing Each Option

Let's analyze each option given: (a) Salt bridges do not participate chemically in the reactions occurring at the electrodes. (b) The salt bridge stops the mixing of the solutions by preventing ions from freely diffusing into each other’s solutions through a direct path. (c) The cell reaction depends on electron flow, which indirectly relies on the salt bridge to maintain charge balance. (d) The salt bridge does the opposite of mixing; it helps keep the solutions separate.

Step 3: Identifying the Correct Statement

By examining each statement, we can determine: - Statement (a) is true because the salt bridge does not participate chemically. - Statement (b) is incorrect as the bridge encourages but regulates ions' diffusion to maintain charge balance, not stop it. - Statement (c) is correct because, without the salt bridge, the cell reaction would stop due to charge buildup. - Statement (d) is incorrect since it ensures separation, not mixing.

Key Concepts

Salt Bridge

A salt bridge plays a crucial role in the function of a galvanic cell. It connects the two half-cells, allowing for the movement of ions between them.
This movement is essential to maintain the overall balance of charge within the cell. The salt bridge usually contains a neutral salt, such as potassium nitrate (\( \text{KNO}_3 \)).

• It is composed of a gel or porous material.
• It prevents the mixing of different solutions in each half-cell.
• The salt bridge completes the electrical circuit by enabling ion flow, which complements electron movement through the external wire.

The ions in the salt bridge do not directly take part in the chemical reaction but are vital to keeping the system working by ensuring ionic balance.

Ion Flow

In a galvanic cell, ions have the important job of ensuring that charge neutrality is maintained as reactions happen. The movement of these ions through the salt bridge is critical when the electrons flow via an external circuit.

• The movement of electrons generates a negative charge build-up.
• Positive ions from the salt bridge shift towards the cathode to balance out the charge.
• Meanwhile, negative ions move to the anode to maintain balance.

This regulated flow of ions keeps the galvanic cell operating smoothly. Without it, the cell would quickly stop functioning due to the buildup of charges.

Cell Reaction

The primary purpose of a galvanic cell is to carry out a spontaneous redox reaction, producing electrical energy from chemical changes. The cell consists of two separate compartments, called half-cells, where these reactions occur.
In each half-cell, different reactions take place:

• Anode Reaction: Oxidation occurs here, losing electrons, and typically involves metals like zinc \( \left( \text{Zn} \rightarrow \text{Zn}^{2+} + 2e^- \right)\).
• Cathode Reaction: Reduction happens here, gaining electrons, often using ions like copper \( \left( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \right)\).

The electron flow from anode to cathode happens externally through a wire, while the associated ions move internally via the salt bridge to continue the reaction.

Electrical Neutrality

Electrical neutrality is a key factor in a galvanic cell to ensure the continuous flow of electrons and sustained electricity generation. It refers to the balance of positive and negative charges in the system.
  To maintain neutrality:

• The anode builds up positive charges as it loses electrons through oxidation.
• Without a balancing influx of negative ions, this positive charge would halt electron flow.
• Simultaneously, the cathode gains electrons, developing a negative charge that could stop the flow if unchecked.

Here, the salt bridge and its ions play a fundamental role. By allowing selective ion movement, it neutralizes charge imbalances, thereby enabling the galvanic cell to function effectively.