Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Saturated solution of \(\mathrm{KNO}_{3}\) is used to make 'salt bridge' because (a) velocity of \(\mathrm{K}^{\prime}\) is greater than that of \(\mathrm{NO}_{3}\) (b) velocity of \(\mathrm{NO}_{3}^{-}\)is greater than that of \(\mathrm{K}^{4}\) (c) velocities of both \(\mathrm{K}^{+}\)and \(\mathrm{NO}_{3}^{-}\)are nearly the same (d) \(\mathrm{KNO}_{3}\) is highly soluble in water

Step-by-Step Solution

Answer

Option (c) is correct: velocities of both \( \mathrm{K}^+ \) and \( \mathrm{NO}_3^- \) are nearly the same.

Step 1: Understanding the Concept

In this problem, we are asked why a saturated solution of \( \mathrm{KNO}_3 \) is used in a salt bridge. A salt bridge allows ions to flow between two half-cells in an electrochemical cell, maintaining electrical neutrality. The solution used should not interfere with the cell reactions and should ideally have ions that move at similar speeds.

Step 2: Analyzing the Options

Let's break down the provided options: (a) and (b) mention that the velocity of one ion is greater than the other, which could lead to an imbalance in charge migration, so these are not ideal for maintaining neutrality.(c) mentions that the velocities of both \( \mathrm{K}^+ \) and \( \mathrm{NO}_3^- \) are nearly the same, which is ideal for ensuring charge balance during the cell operation.(d) states high solubility, which is a property of \( \mathrm{KNO}_3 \) but concerns with neutrality are more pressing for salt bridges.

Step 3: Choosing the Correct Option

Given the need for the ions to move at similar speeds to maintain charge balance in the electrochemical cell, option (c) is most suitable because it implies that the migration of \( \mathrm{K}^+ \) and \( \mathrm{NO}_3^- \) will occur at nearly the same rate. This prevents any significant potential difference from developing across the salt bridge, which could disrupt the cell.

Key Concepts

Salt Bridge

The salt bridge is an essential component of an electrochemical cell. It connects two half-cells, enabling the flow of ions between them. This ensures that the charge balance is maintained throughout the system. By allowing ions to migrate, the salt bridge prevents the build-up of excessive charge in one half-cell over the other, which could halt the cell's reaction.

A good salt bridge should contain a solution of ions that do not react with the chemicals in the half-cells. Potassium nitrate (KNO₃) is frequently used because it is inert in many reactions. The ions it provides (\(\mathrm{K}^+\) and \(\mathrm{NO}_3^-\)) are also nearly equal in terms of mobility, allowing for balanced ion transfer.

• Prevents charge imbalance
• Ensures smooth ion flow
• Typically contains inert electrolytes like \(\mathrm{KNO}_3\)

Electrochemical Cell

An electrochemical cell is a device capable of generating electrical energy from chemical reactions or facilitating chemical reactions through the introduction of electrical energy. It consists of two half-cells, each containing an electrode and an electrolyte, where redox reactions take place.

The half-cells are where oxidation and reduction reactions occur, accompanied by electrons flowing through an external circuit. The movement of electrons from one electrode to another is what generates electricity.

• Comprised of two half-cells
• Redox reactions generate electron flow
• Connected by a salt bridge to maintain neutrality

Ion Velocity

Ion velocity refers to the speed at which ions traverse a solution. In the context of electrochemical cells, ion velocity is crucial because it affects how effectively a salt bridge can maintain electrical neutrality between two half-cells.

Ideally, ions with similar velocities are used in a salt bridge. This is because disproportionate ion speeds can lead to charge imbalances, creating a potential difference that disrupts the cell's operations. \(\mathrm{K}^+\) and \(\mathrm{NO}_3^-\) in \(\mathrm{KNO}_3\) solutions have nearly the same velocities, which makes them exemplary for maintaining equilibrium in the system.

• Determines efficiency of charge balancing
• Should ideally match for all ions in the salt bridge
• Affects the stability of the electrochemical reaction

Electrical Neutrality

Electrical neutrality within an electrochemical cell is a cornerstone for its functionality. This principle ensures that the total charge on either side of the cell remains balanced, preventing any excessive build-up of positive or negative charge.

A salt bridge aids in maintaining this neutrality by compensating for the continuous movement of electrons. Without the balance provided by the salt bridge, the cell reactions would be unable to proceed past an initial stage, as charge imbalances would halt the flow of electrons.

• Ensures uninterrupted cell reactions
• Salt bridges crucial for maintaining balance
• Prevents potential differences disrupting cell function