Consider the reaction: A + 2B$_{(g)} \rightleftharpoons$ 2C + D$_{(g)}$ ($\Delta$H = +ve) What will be the effect of decrease in temperature on the reaction?

Analysis of the Question

This question tests your understanding of Le Chatelier’s Principle, which states that if a dynamic equilibrium is disturbed by changing the conditions (temperature, pressure, or concentration), the position of equilibrium moves to counteract the change.

Concept Explanation

The reaction is given as: A + 2B$_{(g)} \rightleftharpoons$ 2C + D$_{(g)}$ ($\Delta$H = +ve)

• $\Delta$H = +ve: This indicates that the forward reaction is endothermic (it absorbs heat).
• Reverse Reaction: Since the forward reaction is endothermic, the backward (reverse) reaction must be exothermic (it releases heat).

According to Le Chatelier’s Principle, when you decrease the temperature of a system at equilibrium, the system will try to produce more heat to compensate for the loss. It does this by favoring the exothermic direction.

Step-by-Step Solution

1. Identify the nature of the reaction: The forward reaction is endothermic ($\Delta$H > 0). Therefore, the backward reaction is exothermic ($\Delta$H < 0).
2. Apply the temperature change: You are decreasing the temperature.
3. Determine the shift: The system will favor the exothermic direction to generate heat. Since the backward reaction is exothermic, the equilibrium position will shift to the left (towards the reactants).
4. Analyze the rates: As the equilibrium shifts to the left, the rate of the backward reaction increases relative to the forward reaction until a new equilibrium is established.

Evaluating the Options

• A. Rate of backward reaction will increase: This is correct because the system favors the exothermic (backward) direction to counteract the cooling.
• B. The equilibrium constant will shift: While the equilibrium constant ($K$) changes with temperature, "shifting" is a term usually reserved for the position of equilibrium, not the constant itself.
• C. Rates of forward and backward reactions are not affected: This is incorrect; temperature changes always affect the rate of reaction.
• D. Rate of forward reaction will increase: This is incorrect because the forward reaction is endothermic; decreasing the temperature will actually decrease the rate of the forward reaction.

Exam Tip

Remember the "Heat-Balance" rule:

• If $\Delta$H is positive (Endothermic), think of heat as a reactant.
• If $\Delta$H is negative (Exothermic), think of heat as a product.
• Decrease temperature $\rightarrow$ System moves to produce heat.
• Increase temperature $\rightarrow$ System moves to consume heat.