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AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumEquilibrium

Equilibrium (College Board AP® Chemistry)

Exam Questions

6 mins6 questions
Medium
Hard
11 mark

Which statement below best describes a dynamic equilibrium?

  • The rate of the forward reaction is equal to the reverse reaction, and the concentrations of reactants and products is constant

  • The rate of the forward reaction is equal to the reverse reaction in a closed system, and the concentrations of the reactants and products are equal

  • The rate of the forward reaction is equal to the reverse reaction in a closed system, and the concentrations of the reactants and products is constant

  • The rate of reaction changes in either direction to counteract a change in conditions

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21 mark

When a chemical reaction reaches equilibrium which of the following can be said about the concentrations of the substances present, and the rates of reaction?

  • The concentrations of the reactants and products continue to change and the rates of forward and reverse reactions remain the same.

  • The concentrations of the reactants and products remains constant and the rates of forward and reverse reactions remain the same.

  • The concentrations of the reactants and products continue to change and the rates of forward and reverse reactions are different.

  • The concentrations of the reactants and products remain constant and the rates of forward and reverse reactions are different.

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31 mark
Sample rate versus time graph for a reversible reaction

The graph above shows how rate changes with time for a reversible reaction.

Which of the following is not correct about point A?

  • The rate of the forward and reverse reaction reactions remain constant.

  • Partial pressures remain constant

  • The number of particles of each species remains constant

  • The concentrations of reactants and products are equal

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11 mark

N2O4 (g) ⇌ 2NO2 (g)

A sealed container with a volume of 2.0 L contains 2.0 atm of N2O4 (g) at a constant temperature 350 K. The gas decompose according to the reaction.

The partial pressures of 2NO2 (g) over time are recorded in the table below:

Time elapsed (s)

Partial pressure N2O4 (atm)

Partial pressure NO2 (atm)

0

2.00

0.00

20

1.85

0.30

40

1.70

0.60

60

1.60

0.80

80

1.55

0.90

100

1.55

0.90

Based on the data, what claim can be made about the system?

  • The system is at equilibrium after 60 seconds because the pressure of N2O4 is decreasing.

  • Equilibrium is reached after 80 seconds because the pressures of both gases remain constant.

  • The forward and reverse reactions are still occurring but at unequal rates between 60 and 80 seconds.

  • The system is at equilibrium after 100 seconds because the forward reaction has stopped.

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21 mark

C2H6 (g) ⇌ C2H4 (g) + H2 (g)

The reaction above occurs in a 1.0 L sealed container at 500 K.

Initially, 0.050 M of C2H6 (g) is introduced into the container. The following data show the concentration of H2 (g) over time:

Time elapsed (s)

Concentration of H2 (M)

0

0.00

40

0.012

80

0.019

140

0.022

200

0.025

300

0.027

400

0.027

500

0.027

Based on the data, what claim can be made about the system?

  • Equilibrium was established between 200 and 300 seconds because the concentration of ​ H2 (g) stopped changing.

  • The forward reaction stops after 300 seconds because all of the C2H6 (g) has decomposed.

  • Equilibrium has not been reached because the concentration of ​ H2 (g) ​ is less than 50% of the starting C2H6 (g).

  • It is not possible to determine equilibrium without knowing the concentration of ​C2H6 (g).

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31 mark

PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)

When sample of PCl5 (g) is introduced into a sealed container at 673 K the above reaction occurs.

The system is observed carefully for changes in pressure, color, and concentration.

Which of the following observations would provide the strongest evidence that the system has reached equilibrium?

  • The total pressure in the container increases steadily as PCl5 (g) decomposes.

  • The concentration of Cl2 (g) continues to increase as PCl5 (g) decreases steadily.

  • The color of the system fades to colorless as PCl5 is converted into PCl3 (g) and Cl2 (g)

  • The pressure in the container becomes constant.

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