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

Question

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

Step-by-Step Solution

Answer

The reaction is a reversible reaction (Answer: d).

Step 1: Understand the Equation

The given reaction is \( \mathrm{PCl}_{5}(\mathrm{~g}) \rightarrow \mathrm{PCl}_{3}(\mathrm{~s}) + \mathrm{Cl}_{2}(\mathrm{~g}) \). It shows phosphorus pentachloride decomposing into phosphorus trichloride and chlorine gas.

Step 2: Identify Characteristics

Consider whether the products can revert back to reactants. A reversible reaction can proceed in both forward and reverse directions, often achieving equilibrium. An irreversible reaction only proceeds in one direction under standard conditions.

Step 3: Recognize Physical States

Review the physical states in the reaction. \( \mathrm{PCl}_{5} \) is gaseous, \( \mathrm{PCl}_{3} \) is solid, and \( \mathrm{Cl}_{2} \) is gaseous. A phase change is apparent from gas to solid, suggesting some reversibility.

Step 4: Determine Reaction Type

In many chemical systems, especially involving phase changes, the possibility exists for the reaction to proceed in both directions under different conditions. Hence, intimate equilibrium suggests this as a reversible reaction.

Key Concepts

Chemical Equilibrium

In a chemical equilibrium, the forward and reverse reactions occur at the same rate. This means the amount of reactants and products remain constant over time. Equilibrium is not static but dynamic, allowing both reactions to continue without an overall change in concentrations.
The concept of equilibrium can be visualized by imagining a seesaw balancing two equally weighted objects. Both sides push up and down, yet maintain a balance.
Chemical equilibrium is crucial in understanding reversible reactions. When studying these reactions, one should also consider the equilibria involved, especially the conditions under which the reaction reverses.

• Equilibrium can be disturbed by changes in conditions like temperature or pressure.
• An increase in products can lead to the reaction favoring the reverse direction to restore equilibrium.

Recognizing when equilibrium is achieved is vital. Concentrations, temperature adjustments, and pressure changes all play roles in shifting the equilibrium position.

Phase Changes in Reactions

Phase changes in reactions involve a transformation from one state of matter to another, such as solid to liquid, or gas to solid. These changes can significantly impact the behavior and properties of the substances involved.
In the reaction \(\text{PCl}_{5}( ext{g}) ightarrow ext{PCl}_{3}( ext{s}) + ext{Cl}_{2}( ext{g})\), we observe a gas transforming into a solid and a gas. Understanding this transition helps predict the conditions under which the reaction can reverse.

• Phase changes can provide a visual indication of a reaction's directionality (e.g., gas to solid might imply cooling).
• These changes can alter energy requirements, like heat absorption or release, to initiate or reverse a reaction.

Considering phase changes is essential when determining whether a reaction might be reversible, as specific conditions can allow reactants to reform from products.

Decomposition Reactions

Decomposition reactions break down one compound into two or more simpler substances. This is a common reaction type typically requiring energy input. The given reaction \(\text{PCl}_{5}( ext{g}) \rightarrow \text{PCl}_{3}( ext{s}) + \text{Cl}_{2}( ext{g})\) is a classic decomposition.
These reactions often involve a single reactant splitting into multiple products, demonstrating a basic form of chemical reaction.

• Energy, in the form of heat, light, or electricity, can be necessary to initiate decomposition.
• The reverse process, synthesis, is the building up of compounds from simpler substances.

Understanding decomposition is crucial for studying reaction mechanisms and predicting product formation. It becomes even more interesting when analyzing reversible reactions since the products might recombine to form the original reactant.