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Difficulty: MediumLe Chatelier's Principle
Consider the high-temperature reversible reaction occurring in a closed vessel, represented by the thermochemical equation below:
N2(g)+O2(g)⇌2NO(g)ΔH=+180.5 kJ mol−1N_2(g) + O_2(g) \rightleftharpoons 2NO(g) \quad \Delta H = +180.5\text{ kJ mol}^{-1}
Which of the following conditions will shift the equilibrium position to favor the yield of nitrogen(II) oxide?
  1. Increasing the temperature of the systemAnswer
  2. B
    Increasing the total pressure on the reaction vessel
  3. C
    Adding a suitable platinum catalyst
  4. D
    Removing nitrogen gas from the mixture

Answer

Increasing the temperature of the system shifts the equilibrium position to the right, favoring the formation of nitrogen(II) oxide.
The forward synthesis of nitrogen(II) oxide is endothermic, absorbing heat from the surroundings. According to Le Chatelier's principle, when temperature is increased, the system shifts in the direction that absorbs heat (the forward endothermic direction) to relieve the thermal stress, resulting in a higher yield of nitrogen(II) oxide.

Step-by-Step Solution

1
Analyze the enthalpy change of the forward reaction
The positive sign of \(\Delta H = +180.5\text{ kJ mol}^{-1}\) indicates that the forward reaction is endothermic.
According to Le Chatelier's principle, supplying thermal energy by raising the temperature causes an endothermic reaction to shift forward to absorb the excess heat.
2
Analyze the volume/mole change across gaseous species
Reactant gas moles = \(1 + 1 = 2\) moles; Product gas moles = \(2\) moles.
Since the total number of gaseous moles is identical on both sides of the equation, pressure changes produce no net shift in equilibrium position.
3
Evaluate the effect of a catalyst and reactant removal
Adding a catalyst accelerates attainment of equilibrium equally in both directions; removing a reactant causes a leftward shift.
Neither catalyst addition nor removal of reactants increases product yield at equilibrium.

Key Concept

Effect of temperature, pressure, and catalyst on dynamic chemical equilibrium via Le Chatelier's Principle
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Topics

Le Chatelier's Principle

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