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Difficulty: MediumLe Chatelier's Principle
Consider the reversible endothermic gas-phase reaction represented by the thermochemical equation below:
PCl5(g)⇌PCl3(g)+Cl2(g)ΔH=+92.5 kJ mol−1PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \quad \Delta H = +92.5\text{ kJ mol}^{-1}
Which of the following conditions will shift the position of equilibrium to the right to favor the yield of gaseous products?
  1. Increasing the temperature and decreasing the total pressureAnswer
  2. B
    Adding a suitable catalyst to the reaction vessel
  3. C
    Increasing the total pressure while lowering the temperature
  4. D
    Continuous removal of phosphorus pentachloride reactant gas

Answer

Increasing the temperature and decreasing the total pressure will shift the position of equilibrium to the right.
The forward reaction is endothermic (ΔH>0\Delta H > 0), so increasing the temperature drives the reaction forward (to the right) to absorb excess thermal energy. Additionally, the decomposition converts 1 mole of gaseous reactant into 2 moles of gaseous products; decreasing the pressure shifts the equilibrium toward the right side, which contains more gas moles.

Step-by-Step Solution

1
Analyze the temperature effect using the enthalpy sign
Since ΔH=+92.5 kJ mol−1\Delta H = +92.5\text{ kJ mol}^{-1} (endothermic), heat acts as a reactant. Increasing the system temperature shifts the equilibrium to the right to absorb the extra heat.
Le Chatelier's principle states that a system at equilibrium responds to stress by shifting in the direction that minimizes that stress.
2
Analyze the pressure effect by counting gaseous stoichiometric coefficients
The left side has 1 mole of gas (PCl5PCl_5), while the right side has 2 moles of gas (PCl3+Cl2PCl_3 + Cl_2). Decreasing the total pressure causes the equilibrium to shift toward the side with a greater number of gas moles (to the right).
Lowering pressure causes the system to shift toward producing more gas particles to counteract the pressure decrease.
3
Combine both conditions to determine the overall shift
Both an increase in temperature and a decrease in pressure independently shift the equilibrium position to favor product formation.
Both stresses act synergistically to drive the equilibrium to the right.

Key Concept

Le Chatelier's Principle and Equilibrium Shifts
Estimated Time:1m 15s
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Topics

Le Chatelier's Principle

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