Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reaction will be favoured at low pressure? (a) \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightleftharpoons 2 \mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2}+\mathrm{I}_{2} \rightleftharpoons 2 \mathrm{HI}\) (c) \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) (d) \(\mathrm{N}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{NO}\)

Step-by-Step Solution

Answer

Reaction (c) is favored at low pressure.

Step 1: Identify the Effect of Pressure on Equilibrium

In chemical equilibrium, Le Chatelier's principle states that a system will adjust to minimize the change imposed on it. A reduction in pressure favors the side of a reaction with more moles of gas.

Step 2: Count Moles of Gas in Reaction (a)

For the reaction \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightleftharpoons 2 \mathrm{NH}_{3}\), the left-hand side has 4 moles of gas (1 mole of \(\mathrm{N}_{2}\) and 3 moles of \(\mathrm{H}_{2}\)), while the right-hand side has 2 moles of \(\mathrm{NH}_{3}\).

Step 3: Count Moles of Gas in Reaction (b)

For the reaction \(\mathrm{H}_{2}+\mathrm{I}_{2} \rightleftharpoons 2 \mathrm{HI}\), there are 2 moles of gas on both sides (1 mole of \(\mathrm{H}_{2}\) and 1 mole of \(\mathrm{I}_{2}\) on the reactant side, and 2 moles of \(\mathrm{HI}\) on the product side).

Step 4: Count Moles of Gas in Reaction (c)

For the reaction \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\), there is 1 mole of gas on the left-hand side and 2 moles of gas on the right-hand side.

Step 5: Count Moles of Gas in Reaction (d)

For the reaction \(\mathrm{N}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{NO}\), there are 2 moles of gas on the left-hand side and 2 moles of gas on the right-hand side.

Step 6: Determine Which Reaction is Favored at Low Pressure

At low pressure, reactions that produce more moles of gas will be favored. Thus, reaction (c) \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) is favored as it goes from 1 mole of gas to 2 moles of gas.

Key Concepts

Equilibrium Shift

In chemical reactions, the concept of equilibrium shift is essential for understanding how reactions respond to varying conditions. According to Le Chatelier's principle, when a system at equilibrium is disturbed by an external change, such as a change in concentration, temperature, or pressure, the system adjusts itself to counteract the disturbance and achieve a new equilibrium.

Consider a reaction where gases are involved. If the pressure changes, the equilibrium will shift in such a way as to minimize the effect of that pressure change. This results in the favoring of either the forward or reverse reaction, depending on the number of gas moles on each side of the reaction equation.

• If pressure increases, the equilibrium will shift toward the side with fewer moles of gas. This minimizes the increase in pressure.
• If pressure decreases, the equilibrium shifts toward the side with more moles of gas to counteract the loss of pressure.

Understanding equilibrium shifts also entails recognizing that not all changes will affect every reaction in the same way, as it greatly depends on the specific conditions set by the reaction's stoichiometry and external changes.

Pressure and Gas Moles

The relationship between pressure and the moles of gases in a chemical reaction is pivotal in understanding how equilibrium shifts occur. Generally, for a given reaction, the side with a greater number of gas moles will be more sensitive to changes in pressure.

In gaseous reactions, the total number of gas moles can dictate whether the forward or reverse reaction is favored under varying pressure conditions. For our example,

• Reaction (a): \[\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3}\] The reactants collectively have more gas moles (4) than the products (2).
• Reaction (b):\[\mathrm{H}_{2}+\mathrm{I}_{2} \rightarrow 2 \mathrm{HI}\]Both sides have 2 gas moles, so the equilibrium is unaffected by pressure changes.
• Reaction (c):\[\mathrm{PCl}_{5} \rightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\] Goes from 1 mole to 2 moles of gas, favoring the products at low pressure.
• Reaction (d):\[\mathrm{N}_{2}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}\] There are 2 gas moles on both sides, so pressure changes have no effect.

Identifying which side of the equilibrium equation has more gas moles enables us to predict how the equilibrium will shift when pressure changes.

Reaction Dynamics

Reaction dynamics is the study of the rates of chemical processes and the mechanism by which chemical reactions occur. These dynamics are influenced by many factors, including changes in pressure and concentration as described by Le Chatelier's principle. In cases involving gases, the dynamic balance is especially intricate.

During a gaseous equilibrium, factors such as pressure alterations can lead to changes in the concentration of reactants or products. For instance, lowering the pressure in a system can result in an increase in volume according to the ideal gas law, and thereby influence the concentrations of gases involved:

• Reaction pathways may alter to favor the production of more gas moles.
• Reaction rates may change depending on how concentration changes impact the collision frequency of gas molecules.

Understanding reaction dynamics allows scientists and students alike to predict possible outcomes and understand underlying processes of a chemical system at equilibrium.

Gaseous Equilibrium

Gaseous equilibrium is a specific type of chemical equilibrium pertinent to reactions involving gases. It occurs when the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentration of reactants and products.

The study of gaseous equilibrium revolves around the concentrations of the gases involved and how external factors such as pressure influence these concentrations. Le Chatelier’s principle provides valuable insights:

• In a closed system, the equilibrium state will be characterized by constant pressure and temperature if undisturbed.
• Introducing or removing a reactant or product as a gas will result in a shift in equilibrium to re-establish balance.
• Pressure changes affect reactants and products according to their gaseous moles, shifting equilibrium as needed.

Comprehending gaseous equilibrium is essential for predicting the behavior of chemical reactions in real-world applications, especially those that take place in industrial and environmental settings.