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Difficulty: MediumLe Chatelier's Principle

Consider the reversible gas-phase reaction represented by the equation below:

N2O4(g)⇌2NO2(g)ΔH=+57.2 kJ mol−1N_2O_4(g) \rightleftharpoons 2NO_2(g) \quad \Delta H = +57.2\text{ kJ mol}^{-1}

Which of the following conditions will shift the position of equilibrium to favor the yield of nitrogen dioxide (NO2NO_2)?

  1. Increasing the temperature of the reaction vesselAnswer
  2. B
    Adding a platinum catalyst to the equilibrium mixture
  3. C
    Increasing the total pressure on the equilibrium system
  4. D
    Decreasing the volume of the reaction container at constant temperature

Answer

Increasing the temperature of the reaction vessel
Because the forward reaction absorbs heat (endothermic, ΔH>0\Delta H > 0), an increase in temperature shifts the position of dynamic equilibrium toward the right (forward direction) to absorb the excess thermal energy, thereby increasing the equilibrium yield of nitrogen dioxide (NO2NO_2).

Step-by-Step Solution

1
Analyze the enthalpy change (ΔH\Delta H) of the forward reaction
The forward reaction N2O4(g)→2NO2(g)N_2O_4(g) \rightarrow 2NO_2(g) has ΔH=+57.2 kJ mol−1\Delta H = +57.2\text{ kJ mol}^{-1}, indicating an endothermic process.
Temperature perturbations affect equilibrium based on whether heat is absorbed or released.
2
Apply Le Chatelier's principle for a temperature increase
Increasing temperature adds heat energy to the system, causing the system to absorb heat by shifting in the endothermic (forward) direction.
The system opposes the stress of added heat by favoring the reaction direction that consumes heat.
3
Determine the effect on product yield
Shifting to the right increases the equilibrium concentration and yield of NO2(g)NO_2(g).
The forward reaction produces NO2(g)NO_2(g).

Key Concept

Le Chatelier's Principle and Temperature Perturbation
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Topics

Le Chatelier's Principle

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