Le Châtelier’s Principle (College Board AP® Chemistry): Exam Questions

Ethanol (C₂H₅OH) is widely used in industrial applications. One method of production involves the following reversible reaction:

C₂H₄ (g) + H₂O (g) ⇌ C₂H₅OH (g)    ΔH = -46 kJ mol^-1

The reaction is typically conducted at pressures between 60 and 70 atm to optimize ethanol yield.

Explain the effect of increasing the total pressure on the equilibrium position of the reaction. Justify your answer in terms of Le Châtelier’s principle.

i) Based on Le Châtelier’s principle, determine whether a high or low temperature should be used to maximize the equilibrium yield of ethanol.

ii) Identify a potential practical issue that may arise from operating at the temperature identified.

A reaction mixture is prepared in a sealed syringe containing dinitrogen tetroxide (N₂O₄​), a colorless gas, and nitrogen dioxide (NO₂​), a dark brown gas, at equilibrium:

N₂O₄ (g) ⇌ 2NO₂ (g)               ΔH∘ = +58 kJ mol^-1

The reaction mixture becomes darker in color when heated. Explain this observation in terms of Le Châtelier’s Principle.

The plunger of the syringe is pressed, reducing the volume of the gas mixture. Predict the initial change in appearance and explain your answer in terms of Le Châtelier’s Principle.