Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following change will shift the reaction in forward direction? \(\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 21(\mathrm{~g})\) Take \(\Delta \mathrm{H}^{\circ}=+150 \mathrm{~kJ}\) (a) increase in concentration of I (b) increase in total pressure (c) decrease in concentration of \(\mathrm{I}_{2}\) (d) increase in temperature

Step-by-Step Solution

Answer

Increasing temperature will shift the reaction forward.

Step 1: Understanding the Reaction

The given reaction is \[ \mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2\mathrm{I}(\mathrm{~g}) \] and we are informed that \( \Delta \mathrm{H}^{\circ} = +150 \mathrm{~kJ} \), which indicates the reaction is endothermic. In endothermic reactions, heat can be considered a reactant.

Step 2: Analyzing Option (a)

Increasing the concentration of iodine \( \mathrm{I} \) shifts the reaction toward the reactants side (backward reaction), as per Le Chatelier's Principle. This would not shift the reaction forward.

Step 3: Analyzing Option (b)

Increasing the total pressure shifts the position of equilibrium towards the side with fewer moles of gas. In this reaction, there are 1 mole of \( \mathrm{I}_{2} \) and 2 moles of I, so this change shifts the equilibrium backwards to favor \( \mathrm{I}_{2} \).

Step 4: Analyzing Option (c)

Decreasing the concentration of \( \mathrm{I}_{2} \) reduces I2's availability, which shifts the reaction backward as the system attempts to replenish \( \mathrm{I}_{2} \) according to Le Chatelier's Principle. Therefore, this also does not shift the reaction forward.

Step 5: Analyzing Option (d)

Increasing the temperature of an endothermic reaction adds heat as a reactant, promoting the formation of products. In this case, it will shift the reaction toward the right (forward) in favor of forming more \( \mathrm{I} \).

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is an important concept in chemistry that helps us understand how a chemical equilibrium responds to changes in concentration, pressure, and temperature. According to this principle, if a dynamic equilibrium is disturbed by external conditions, the position of equilibrium shifts in a direction that counteracts the change. This means that the system adjusts itself to minimize the imposed change and re-establish a new equilibrium state.

For instance:

• If reactants or products are added or removed, the system shifts to favor the side that will consume the added substance or replenish the removed one.
• If temperature is changed, the system shifts in the direction that absorbs excess heat (for endothermic or exothermic reactions).
• If pressure is increased (by reducing volume or adding inert gases), the system will move towards the side with fewer gas molecules.

Understanding these responses can help predict which way a reaction will shift under different conditions.

Endothermic Reactions

Endothermic reactions are a type of chemical reaction where heat is absorbed from the surroundings. In these reactions, the products have higher energy than the reactants, as indicated by a positive value of enthalpy change \( \Delta \mathrm{H} \).

For example, the reaction \( \mathrm{I}_{2} \rightleftharpoons 2\mathrm{I} \) discussed in the exercise is endothermic, with \( \Delta \mathrm{H} = +150 \, \mathrm{kJ} \). Here, the absorption of heat shifts the equilibrium towards the product side, mimicking the addition of a reactant. This is because heat is a reactant in an endothermic reaction.

An easy way to remember this is that endothermic reactions "take in heat," so an increase in temperature tends to favor the forward reaction, generating more products.

Reaction Shifts

Reaction shifts refer to the change in the position of equilibrium due to external influences. This is where the reaction "leans" towards either the reactants or products to balance changes applied to the system. The direction of the shift depends on what is changed:

• An increase in concentration of a reactant pushes the equilibrium towards the products to use up the added reactant.
• A reduction in pressure causes the equilibrium to shift towards the side with more moles of gas, as the system compensates for the pressure drop.
• For endothermic reactions, raising the temperature shifts the equilibrium towards the product side, as we discussed earlier.

Predicting the shift can be quite straightforward but requires understanding of all variables involved in equilibrium.

Equilibrium Changes

Equilibrium changes occur when a dynamic chemical reaction is subjected to external stimuli such as change in pressure, concentration, or temperature. Each of these factors can influence the reaction's balance between the concentration of reactants and products.

Whenever there is a change:

• The system responds to pressure changes by shifting to the side with fewer or more gas molecules, depending on whether pressure is increased or decreased, respectively.
• Concentration change influences the system to adjust such that it opposes this change by either creating more products or reactants.
• Temperature changes for endothermic reactions are particularly significant; higher temperatures lead the reaction to favor product formation.

All these aspects are interconnected and can change the delicate balance of the chemical equilibrium, making understanding these changes crucial for predicting the reaction's behavior.