Study GuidesExam QuestionsPast Papers
AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumEquilibrium

Equilibrium (College Board AP® Chemistry): Exam Questions

30 mins12 questions

Select a download format for Equilibrium

Scroll for more

Select an answer set to view for
Equilibrium

Medium
Hard
1a1 mark

A student investigates the evaporation and condensation of ethanol in a closed container at a constant temperature. The system reaches equilibrium according to the process:

C2H5OH (l) ⇌ C2H5OH (g)

Define equilibrium in terms of the rates of the forward and reverse processes.

How did you do?

1b1 mark

Sketch a graph showing how the concentration of liquid ethanol and gaseous ethanol change over time as equilibrium is established.

How did you do?

1c2 marks

The student opens the container, allowing ethanol vapor to escape. Predict the effect on the system and explain your answer in terms of equilibrium shifts.

How did you do?

Did this page help you?

2a1 mark

A student places a sample of solid ammonium chloride (NH4Cl) into a sealed glass container and heats it. The following reversible reaction occurs:

NH4Cl (s) ⇌ NH3 (g) + HCl (g)

After some time, the system reaches equilibrium.

Explain how the reaction shown is an example of a reversible reaction.

How did you do?

2b1 mark

Describe the molecular-level processes occurring in this system at dynamic equilibrium.

How did you do?

2c2 marks

The student measures the total pressure in the container over time. Sketch a graph showing how pressure changes as the system approaches equilibrium. Label your axes and indicate the trend observed.

Explain why the pressure changes in this way.

How did you do?

Did this page help you?

3a1 mark

A student investigates the reaction between nitrogen dioxide (NO2​) and dinitrogen tetroxide (N2O4​) in a sealed container at a constant temperature:

2NO2 (g) ⇌ N2O4 (g)

Initially, the container contains only NO2​ gas at a concentration of 0.50 M. The system is allowed to reach equilibrium.

Describe how the concentrations of NO2​ and N2O4​ change as equilibrium is established.

How did you do?

3b2 marks

Sketch a graph showing the rate of the forward and reverse reactions over time as equilibrium is established. Explain the trend observed in the graph.

How did you do?

3c1 mark

After equilibrium is reached, additional NO2​ gas is injected into the container. Predict and explain how the rate of the forward reaction changes immediately after this disturbance.

How did you do?

Did this page help you?

1a1 mark

A chemist investigates the thermal decomposition of phosphorus pentachloride (PCl5​) in a closed reaction vessel:

PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)

The system starts with 0.80 M of PCl5​ and no products. Over time, the reaction proceeds until equilibrium is established. The equilibrium constant for this reaction is Kc = 1.25 at the given temperature.

Explain why this reaction is classified as reversible.

How did you do?

1b1 mark

Sketch a graph showing how the concentrations of PCl5​, PCl3​, and Cl2​ change over time as the system reaches equilibrium. Label your axes and indicate the trend observed.

How did you do?

1c2 marks

At equilibrium, a chemist measures the forward and reverse reaction rates. Justify why the reaction is still occurring at the molecular level, even though macroscopic changes are no longer observed.

How did you do?

Did this page help you?

2a1 mark

A chemist studies the equilibrium system for the decomposition of ammonium chloride in a closed reaction vessel:

NH4Cl (s) ⇌ NH3 (g) + HCl (g)

A sample of solid ammonium chloride is placed in an evacuated reaction vessel at constant temperature.

Explain how both physical and chemical changes are involved in this system as it reaches equilibrium.

How did you do?

2b1 mark

Predict the ratio of NH3​ to HCl in the gas phase at equilibrium. Justify your answer.

How did you do?

2c2 marks

After equilibrium is reached, the chemist measures the total pressure in the vessel. Justify why the pressure remains constant at equilibrium, despite continuous molecular activity.

How did you do?

Did this page help you?

3a1 mark

A chemist investigates a closed system containing a mixture of iodine monochloride (ICl) and chlorine gas (Cl2​) at constant temperature. The following reversible reaction occurs:

ICl (g) + Cl2 (g) ⇌ ICl3 (g)

Initially, the reaction vessel contains only ICl and Cl2​. Describe how the rate of the forward reaction changes as the system approaches equilibrium.

How did you do?

3b1 mark

The chemist measures the total number of gas molecules in the reaction vessel over time. Predict how this number changes as the system approaches equilibrium. Justify your answer.

How did you do?

3c2 marks

Describe an experimental method that could confirm whether the system is in dynamic equilibrium. Justify how the collected data would provide evidence for equilibrium.

How did you do?

Did this page help you?