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

A chemist investigates the decomposition of dinitrogen tetroxide into nitrogen dioxide:

N₂O₄ (g) ⇌ 2NO₂ (g) ΔH^∘ = +58 kJ/mol

Write the equilibrium constant expression K_c​ for this reaction.

The system is initially at equilibrium. The chemist increases the volume of the reaction vessel while keeping the temperature constant.

Predict whether the equilibrium will shift toward the reactants or the products. Justify your answer using Le Châtelier’s principle.

The reaction is endothermic.

Predict whether the value of K_c​ will increase, decrease, or remain the same if the temperature is increased. Justify your answer.

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​.

The formation of hydrogen iodide from hydrogen gas and iodine gas is represented by the following equilibrium reaction:

H₂ (g) + I₂ (g) ⇌ 2HI (g) ΔH^∘ = +10 kJ/mol

The system is at equilibrium in a sealed container.

Predict the effect on the equilibrium position if the temperature is increased. Justify your answer.

Predict the effect on the value of the equilibrium constant, K_c​, if the temperature is increased. Justify your answer.

The system is cooled, and the concentration of HI (g) is monitored over time. Describe how the concentration of HI (g) changes as the system moves to a new equilibrium.

The volume of the container is increased. Predict the effect on the number of moles of HI (g) at equilibrium. Justify your answer.

A reaction between hydrogen gas and iodine gas is represented by the following equilibrium:

H₂ (g) + I₂ (g) ⇌ 2HI (g) ΔH^∘ = +10 kJ/mol

A mixture is at equilibrium in a sealed container at constant temperature.

Predict the effect on the equilibrium position if additional H₂ (g) is added to the container. Justify your answer.

Predict the effect on the number of moles of I₂ (g) at equilibrium if some HI (g) is removed from the system. Justify your answer.

Explain why adding an inert gas to the container at constant volume does not affect the equilibrium position.

The temperature of the system is increased. Predict the effect on the number of moles of HI (g) at equilibrium. Justify your answer.

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​.

At a given temperature, the following reaction has an equilibrium constant K_c=3.50:

CO (g) + H₂O (g) ⇌ CO₂ (g) + H₂ (g)

A sealed 1.50 L flask initially contains 0.600 mol CO, 0.400 mol H₂O, 0.300 mol CO₂, and 0.200 mol H₂.

Write the equilibrium expression for K_c​.

Calculate Q_c​. Based on your result, determine whether the system at equilibrium, favors the forward reaction or favors the reverse reaction.

The reaction is exothermic. Explain how an increase in temperature would affect K_c​.