For the exothermic reaction 2NO2(g) ⇌ N2O4(g), which change will increase the yield of N2O4 at equilibrium?
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Correct answer
D. Increasing the pressure
Principle or equation
Le Chatelier's principle: a system at equilibrium responds to a stress by shifting to counteract the change. For a reaction with fewer moles of gas on the product side, increasing pressure shifts equilibrium to the side with fewer gas molecules.
Why this answer is correct
The forward reaction has 2 moles of gas (2NO2) producing 1 mole of gas (N2O4). Increasing pressure favors the side with fewer gas molecules, thus increasing N2O4 yield. Increasing temperature favors the endothermic reverse reaction (since forward is exothermic). Decreasing pressure favors the side with more gas molecules (reactants). A catalyst speeds up both directions equally and does not shift equilibrium.
Example
In N2(g) + 3H2(g) ⇌ 2NH3(g), increasing pressure shifts equilibrium to the right because there are 4 moles gas on left and 2 on right.
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