PCL5(g)= PCL3(g) + Cl2(g) In the reaction above, a decrease in pressure will

Analysis

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

Concept: Effect of Pressure on Equilibrium

To determine how pressure affects a gaseous equilibrium, look at the number of moles of gas on both sides of the equation:

• Reactant side: $PCl_5(g)$ = 1 mole of gas.
• Product side: $PCl_3(g) + Cl_2(g)$ = 1 + 1 = 2 moles of gas.

The Rule:

1. Increase in pressure: The equilibrium shifts toward the side with fewer moles of gas to reduce the pressure.
2. Decrease in pressure: The equilibrium shifts toward the side with more moles of gas to increase the pressure.

Step-by-Step Solution

1. Identify the change: The question specifies a decrease in pressure.
2. Apply the rule: According to Le Chatelier’s Principle, a decrease in pressure favors the side with a larger volume (more moles of gas).
3. Analyze the reaction: • The product side has 2 moles of gas. • The reactant side has 1 mole of gas.
4. Determine the shift: Since the product side has more moles, a decrease in pressure will shift the equilibrium to the right (towards the products).
5. Conclusion: Shifting to the right means more $PCl_5$ is consumed to produce more $PCl_3$ and $Cl_2$. Therefore, the yield of $PCl_3$ increases.

Evaluation of Options

• A & D: Pressure changes affect the position of equilibrium, not necessarily the rate (deceleration/acceleration) in the context of equilibrium yield.
• C: This would happen if pressure were increased.
• B: This is correct because the equilibrium shifts to the side with more moles (the products).

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Exam Tip

Always count the moles of gas on both sides of the equation before applying pressure rules. Remember: "High pressure favors the side with fewer moles; Low pressure favors the side with more moles."