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.

A sealed 5.00 L container is used to study the following gas-phase equilibrium at constant temperature:

2NO₂ (g) ⇌ 2NO (g) + O₂ (g)

NO₂ is a brown gas. NO and O₂ are colorless gases.

The system reaches equilibrium with the following partial pressures:

After equilibrium is established, the system is subjected to changes in volume, concentration, and temperature.

Calculate the value of the reaction quotient, Q, for the system.

If the equilibrium constant K_p = 0.0625, determine whether the system is at equilibrium. Justify your answer.

The volume of the container is suddenly decreased to 2.50 L. Predict the direction in which the system will shift to re-establish equilibrium. Justify your answer.

Immediately after the volume change in part (c), the brown color of the gas mixture briefly intensifies and then fades. What does this suggest about the concentration of NO₂ during and after the shift?

After equilibrium is re-established, a small amount of NO (g) is injected into the container. Predict how the concentrations of all three species will change as the system shifts to a new equilibrium.

i) The forward reaction is exothermic. If the temperature is increased, will the new equilibrium mixture contain more or less NO₂ than before the temperature change? Justify your answer.

ii) Explain how increasing the temperature affects the value of the equilibrium constant, K_p​.

A student investigates the equilibrium established when yellow chromate ions (CrO₄²⁻) react with hydrogen ions in solution:

2CrO₄²⁻ (aq) + 2H⁺ (aq) ⇌ Cr₂O₇²⁻ (aq) + H₂O (l)

The dichromate ion, Cr₂O₇²⁻, is orange.

Hydrochloric acid is added dropwise to the solution. Describe the expected color change and explain the shift in terms of Le Châtelier’s Principle.

Sodium hydroxide solution is added to the system after the addition of HCl. Predict the new color and justify the change based on pH and equilibrium shift.

The student adds solid potassium chromate (K₂CrO₄) to the solution. Explain the effect on the equilibrium.

The student adds Ba(NO₃)₂ to the system. A yellow precipitate of BaCrO₄ (s) forms, and the solution becomes lighter.

Explain, using equilibrium reasoning, why the precipitate forms and why the solution color changes.

After the addition of Ba(NO₃)₂, describe how the system re-establishes equilibrium in terms of the value of K_c​.