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 \(\mathrm{K}^{+}\)is greater than that of \(\mathrm{NO}_{3}^{-}\) (b) velocity of \(\mathrm{NO}_{3}^{-}\)is greater than that of \(\mathrm{K}^{+}\) (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

(c) Velocities of both 4K^+ 4 and 4NO_3^- 4 are nearly the same.

Step 1: Understand the Role of a Salt Bridge

A salt bridge is used in electrochemical cells to maintain electrical neutrality and complete the electrical circuit. It contains an electrolyte that allows for the flow of ions but prevents the mixing of different solutions.

Step 2: Consider the Properties of 4KNO_3 4

Potassium nitrate ( 4KNO_3 4) is commonly used in salt bridges as it is highly soluble in water and provides ions that do not participate in chemical reactions in the electrochemical cell.

Step 3: Compare Ion Velocities

In a salt bridge, the velocities of the ions involved are important. Ideally, the cation and anion should move at similar speeds across the bridge to maintain electrical neutrality effectively. This is a desirable property when choosing a salt for a salt bridge.

Step 4: Match the Properties with Options

Consider each option: - Option (a) implies unequal ion velocities which is not desirable. - Option (b) implies unequal ion velocities. - Option (c) implies that the velocities of 4K^+ 4 and 4NO_3^- 4 are nearly the same, which is preferred. - Option (d) states that 4KNO_3 4 is highly soluble. While useful, this is not the key reason for its use in salt bridges.

Step 5: Determine the Correct Answer

Given the requirement for balance and neutrality in electrochemical cells, option (c) is the best answer. The nearly equal velocities of 4K^+ 4 and 4NO_3^- 4 ions help to maintain the necessary conditions within the salt bridge.

Key Concepts

Electrochemical Cell

An electrochemical cell is a fascinating device that transforms chemical energy into electrical energy or vice versa. It consists of two electrodes dipped into electrolyte solutions, where chemical reactions occur. These electrodes are connected by a wire that allows electrons to flow, generating electricity. However, there's a twist: the circuit isn't complete until we add a salt bridge.

A salt bridge is an essential component. It connects the two electrolyte solutions, ensuring that ions can move between them without the solutions themselves mixing. This movement of ions maintains the electrical neutrality required for continuous electron flow through the wire.

If ions were to accumulate on one side or the other, it would disrupt the flow, bringing the reaction to a halt. Hence, the salt bridge helps keep everything balanced, allowing the electrochemical process to proceed smoothly.

• Transforms energy forms
• Uses electrodes and electrolyte solutions
• Includes a salt bridge for ion flow

Ion Velocity

Ion velocity is a critical factor to consider in electrochemical cells, especially within the context of a salt bridge. The term 'ion velocity' refers to how quickly the ions within the salt bridge move during the cell's operation.

For optimal performance, it's preferable that the cations and anions in the salt bridge travel at nearly equal speeds. This helps maintain the delicate balance of charge within the cell, ensuring neither side becomes overly positive or negative.

When ion velocities are mismatched, it leads to imbalances and uneven charges, causing inefficiencies and potentially stopping the cell's functions. This is why the selection of the right salt for a bridge, which provides ions with similar velocities, is crucial.

• Critical for maintaining balance
• Requires similar speeds for cations and anions
• Ensures efficient cell operation

Potassium Nitrate Solubility

Potassium nitrate (KNO₃) is often chosen in salt bridges due to its exceptional solubility in water. Solubility is essentially how well a compound dissolves in a liquid, forming a solution.

The high solubility of KNO₃ ensures that there are plenty of K⁺ and NO₃⁻ ions available to travel through the salt bridge, maintaining the electrochemical cell's electrical neutrality. This does not only help in balancing the chemical equations but also supports the seamless operation of the cell.

Despite this, the key feature enabling KNO₃'s usage in salt bridges is the similar ion velocities of K⁺ and NO₃⁻. While high solubility is advantageous because it ensures a steady supply of ions, the almost identical speeds of these ions across the bridge are what truly make it ideal for this purpose.

• Highly soluble in water
• Provides ample ions for the cell
• Key factor is similar ion velocities