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AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumLe Châtelier’s Principle

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

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Le Châtelier’s Principle

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1a1 mark

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

N2O4 (g) ⇌ 2NO2 (g) ΔH∘ = +58 kJ/mol

Write the equilibrium constant expression Kc​ for this reaction.

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1b1 mark

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.

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1c2 marks

The reaction is endothermic.

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

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2a2 marks

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.

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2b2 marks

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.

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3a2 marks

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.

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3b2 marks

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.

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1a1 mark

At a given temperature, the following reaction has an equilibrium constant Kc=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 Kc​.

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1b2 marks

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

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1c1 mark

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

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2a1 mark

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.

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2b1 mark

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

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2c1 mark

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.

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2d1 mark

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

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3a1 mark

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.

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3b1 mark

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.

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3c1 mark

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

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3d1 mark

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

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