Problem 158 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})\) At constant temperature, forward reaction is favoured by (a) introducing inert gas at constant volume (b) introducing inert gas at constant pressure (c) introducing chlorine gas at constant volume (d) increasing the volume of the container

Step-by-Step Solution

Answer

The forward reaction is favored by (d) increasing the volume of the container.

Step 1: Understanding the Chemical Equilibrium

The equilibrium reaction given is \( \mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \). The forward reaction involves the decomposition of \( \mathrm{PCl}_{5} \) into \( \mathrm{PCl}_{3} \) and \( \mathrm{Cl}_{2} \). It is important to understand that reactions may shift according to Le Chatelier's Principle, which states that a system at equilibrium will adjust to counteract any changes applied to it.

Step 2: Analyzing Effects of Inert Gas Introduction

Introducing an inert gas at constant volume (option a) does not affect the equilibrium position because it does not change the partial pressures of the reacting gases. Introducing an inert gas at constant pressure (option b) decreases the total volume, increasing partial pressures, potentially favoring the side with fewer moles of gas. Here, the reactant \( \mathrm{PCl}_{5} \) has fewer moles than the products.

Step 3: Effects of Introduced Reactant

Introducing chlorine gas (option c) at constant volume increases the concentration of \( \mathrm{Cl}_{2} \), one of the products. According to Le Chatelier's Principle, the equilibrium will shift to the left to reduce this change, thus favoring the reverse reaction, not the forward.

Step 4: Evaluating Volume Increase Impact

Increasing the volume of the container (option d) decreases the overall pressure. The system will shift to increase pressure by favoring the side with more moles of gas, which in this case is the products' side \( \mathrm{PCl}_{3} \) and \( \mathrm{Cl}_{2} \), thus favoring the forward reaction.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fascinating concept in chemical equilibrium. It helps us predict how a system at equilibrium will respond to external changes. When a change in concentration, temperature, or pressure occurs, the equilibrium will shift to counteract this change. This principle allows chemists to manipulate reactions, making certain outcomes more favorable. For instance, if products are removed or reactants are added, the equilibrium will shift to produce more products. This dynamic adjustment is crucial for many industrial processes where yield needs to be optimized.

Effects of Pressure Changes

Pressure changes chiefly affect reactions involving gases. When we alter the pressure in a gaseous equilibrium, it impacts how the reaction proceeds. In general, increasing the pressure in a system with more gas moles on one side tends to shift the equilibrium towards the side with fewer gas moles. Conversely, decreasing the pressure encourages the shift towards the side with more moles. This means that if you increase the volume of a container, it effectively reduces the pressure, shifting the equilibrium to the side with more gas moles. Understanding these effects allows chemists to control reaction conditions efficiently.

Gaseous Equilibria

Gaseous equilibria involve reactions where gases are reactants and products. These reactions are unique because the number of moles and volume play significant roles in determining the position of equilibrium. When dealing with gaseous equilibria, factors like pressure, volume, and appropriate concentrations must be considered. For the reaction \( \mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \), understanding these aspects is essential. Adjustments to any of these variables can shift the balance, favoring either the formation or the breakdown of \( \mathrm{PCl}_{5} \) depending on the conditions set. This enables precise control over the reaction’s progression and results.