Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction \(\mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g})\) is an example of (a) backward reaction (b) forward reaction (c) irreversible reaction (d) reversible reaction

Step-by-Step Solution

Answer

The reaction is reversible (d).

Step 1: Analyze the Reaction Equation

The given reaction is \( \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3} + \mathrm{Cl}_{2} \). This equation has an arrow pointing in both directions (\( \rightleftharpoons \)), which indicates that the reaction can proceed in both the forward and backward directions.

Step 2: Understand Reaction Types

In chemistry, a forward reaction proceeds in the direction of the products being formed from the reactants, while a backward (or reverse) reaction goes in the direction of the reactants being re-formed from the products. A reversible reaction is indicated by a double arrow \( \rightleftharpoons \), showing that both forward and backward reactions occur.

Step 3: Determine the Type of Reaction

Given the bidirectional arrow in \( \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3} + \mathrm{Cl}_{2} \), the reaction is both forward and backward, meaning it is reversible. It is not irreversible, as that would require an arrow pointing in only one direction (\( \rightarrow \) or \( \leftarrow \)).

Step 4: Conclude the Reaction Type

Based on the analysis, the reaction \( \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3} + \mathrm{Cl}_{2} \) can proceed in both forward and backward directions, confirming it is a reversible reaction.

Key Concepts

Equilibrium Chemistry

In chemistry, equilibrium is a vital concept that describes the state of a chemical reaction where the concentrations of reactants and products remain constant over time. Imagine a seesaw perfectly balanced in the middle - this is similar to equilibrium in a chemical reaction. At equilibrium, the rate of the forward reaction, where reactants convert to products, equals the rate of the backward reaction, where products revert to reactants.
This dynamic balance means the system is not static but continuously active, with molecules constantly transforming, yet their concentrations remain unchanged in the macroscopic view.

• Equilibrium can only be achieved in a closed system where substances cannot escape the reaction mixture.
• Changing conditions like temperature or pressure can shift the position of equilibrium, altering reactant and product concentrations.

Understanding equilibrium chemistry is crucial for mastering more complex concepts in chemistry such as Le Chatelier's principle and kinetics.

Forward Reaction

A forward reaction moves reactants towards forming products. In the reaction \( ext{PCl}_5 ightleftharpoons ext{PCl}_3 + ext{Cl}_2 \, \, \, \, \, \, \, \), the formation of \(\mathrm{PCl}_{3}\mathrm{~and~Cl}_{2}\) from \(\mathrm{PCl}_{5}\) exemplifies a forward reaction.

• Forward reactions are often driven by conditions such as heat, concentration of reactants, or pressure.
• The main focus of a forward reaction is moving towards the completion where the reactants become products.

This action of moving towards product formation is a key focus when examining chemical processes. Observing what happens during the forward reaction offers insights into the efficiency and feasibility of chemical production processes.

Backward Reaction

A backward or reverse reaction is essentially the opposite of a forward reaction. It involves the conversion of products back into reactants. If we look back at our example of \(\mathrm{PCl}_{3}\mathrm{~and~Cl}_{2}\) turning back into \(\mathrm{PCl}_{5}\), this represents the backward reaction of our system.

• The backward reaction is always happening in a reversible reaction setup, working against the forward reaction to maintain equilibrium.
• Conditions that favor the backward reaction can include changes in temperature or pressure, similar to what affects forward reactions, but achieve the opposite outcome.

Understanding backward reactions is vital, especially in industrial applications where maintaining or controlling equilibrium is crucial for product yield.

Chemical Equilibrium

Chemical equilibrium occurs when both the forward and backward reactions happen at equal rates, and no net change in concentrations of reactants and products is observed in the system. Looking at \(\mathrm{PCl}_{5} ightleftharpoons \, \, \, \, \, \, \, \ \mathrm{PCl}_{3} + \, \, \, \, \, \, \, \ \mathrm{Cl}_{2}\), we see that the system has reached equilibrium when the amounts of \(\mathrm{PCl}_{5}\) and \(\mathrm{PCl}_{3}\mathrm{~and~Cl}_{2}\) stabilize even though the reactions continue.

• Achieving chemical equilibrium does not mean the reaction stops; it simply reaches a state of balance.
• External factors like temperature, pressure, and concentration affect the equilibrium and are exploited to shift reactions for desired outcomes.

Grasping the concept of chemical equilibrium is crucial for manipulating reactions effectively in fields such as pharmacology, materials science, and chemical engineering.