AP 7.1 & 7.2 Equilibrium and Direction of Reactions

In an experiment, X(g) and Y(g) were combined in a rigid container at constant temperature and allowed to react as shown in the equation above. The table provides the data collected during the experiment. Based on the data, which of the following claims is most likely correct?

• The reaction was about to reach equilibrium 15 minutes after the reactants were combined because the concentrations of X and XY were almost the same.

• The reaction reached equilibrium between 75 minutes and 155 minutes after the reactants were combined because the concentrations of X and XY remained constant.

• The reaction did not reach equilibrium because only 86% of the initial concentration of X was consumed.

• The reaction did not reach equilibrium because initially there was no XY inside the container.

H₂ gas and N₂ gas were placed in a rigid vessel and allowed to reach equilibrium in the presence of a catalyst according to the following equation.

3 H₂(g) + N₂(g) ⇄ 2 NH₃(g)         ΔH^o = -92 kJ/mol_rxn

The diagram below shows how the concentrations of H₂ , N₂ , and NH₃ in this system changed over time.

Which of the following was true for the system between time t₁ and time t₂?

• The concentration of N₂ decreased.

• The temperature of the system decreased.

• The number of effective collisions between H₂ and N₂ was zero.

• The rates of the forward and reverse reactions were equal.

• The rate of formation of NH₃ molecules was equal to the rate of disappearance of H₂ molecules.

A sample of N2O4(g) is placed into an evacuated container at 373K and allowed to undergo the reversible reaction N2O4(g)⇄2NO2(g). The concentration of each species is measured over time, and the data are used to make the graph shown above. Which of the following identifies when equilibrium is first reached and provides a correct explanation?

PCl₅(g)   ⇄    PCl₃(g) + Cl₂(g)

PCl₅(g) decomposes into PCl₃(g) and Cl₂(g) according to the equation above. A pure sample of PCl₅(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl₅(g) is 1.00 atm. The temperature is held constant until the PCl₅(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system.

As the reaction progresses toward equilibrium, the rate of the forward reaction

• increases until it becomes the same as the reverse reaction rate at equilibrium

• stays constant before and after equilibrium is reached

• decreases to become a constant nonzero rate at equilibrium

• decreases to become zero at equilibrium

The figure shows two closed containers. Each contains the same volume of acetone in equilibrium with its vapor at the same temperature. The vapor pressure of the acetone is

• higher in container 1 because the surface area of the liquid is greater

• higher in container 1 because the volume of vapor is greater

• lower in container 1 because the level of the liquid is lower

• the same in both containers because the volume of the liquid is the same

• the same in both containers because the temperature is the same

Which of the following statements is true for the equilibrium vapor pressure of a liquid in a closed system?

• It remains constant when the temperature increases.

• It decreases to half its original value if the volume of the gas phase is doubled.

• It increases to twice its original value if the volume of the liquid phase is doubled.

• It decreases to half its original value if the surface area of the liquid is reduced by one-half.

• It is independent of the volume of the vapor phase.

A cylinder with a moveable piston is completely filled with a small amount (100 millimoles) of liquid water at a pressure of 1.0atm1.0atm and a temperature of 80∘C80∘C. All the air in the cylinder is excluded. The cylinder is placed in a water bath held at 80∘C80∘C. The piston is slowly moved out to expand the volume of the cylinder to 20L20L as the pressure inside the cylinder is monitored. A plot of the pressure versus volume for the system is shown in the figure above. Which of the following statements most closely indicates, with justification, the region of the curve where the equilibrium represented below occurs?

• Region A, because the initial pressure inside the cylinder is equal to the pressure outside the cylinder, so there is no net force on the piston.

• Region B, because the pressure inside the cylinder is equal to the vapor pressure of water at 80∘C when both liquid and gas phases are present.

• Region C, because the water vapor is behaving according to the ideal gas law as expansion occurs.

• Region D, because the pressure inside the cylinder has leveled off.

The particle diagram above illustrates the changes that take place when X(g) and Y(g) are placed inside a rigid container at constant temperature. Which of the following is a characteristic of a system at equilibrium that is best represented by the particle diagram?

• The particle diagram shows that initially the reaction proceeds to the right to form products, which is a characteristic of a system at equilibrium.

• The particle diagram shows that after 200s the rate of the reverse reaction is slower than the rate of the forward reaction, which is a characteristic of a system at equilibrium.

• The particle diagram shows that after 200s there are no observable changes in the amounts of reactants and products, which is a characteristic of a system at equilibrium.

• The particle diagram shows that between 0s and 200s the rates of the forward and reverse reactions are the same, which is a characteristic of a system at equilibrium.

An equimolar mixture of X(g) and Y(g) is placed inside a rigid container at constant temperature. The particle diagram above represents the changes that occur over time. Based on the particle diagram, which of the following best predicts whether or not the system has reached equilibrium by 300s?

• It is not possible to determine that the system has reached equilibrium by 300s because the stoichiometry of the reaction is not known.

• It is not possible to determine that the system has reached equilibrium by 300s because the amounts of X, Y, and XY have continued to change.

• The system has reached equilibrium by 300s because the rate of formation of XY is constant.

• The system has reached equilibrium by 300s because the rates of consumption of X and Y are equal.

A reversible reaction is represented by the equation above. The amounts of reactants and products at time 1 are shown in the particle diagram on the left. The particle diagram on the right shows the amounts of reactants and products at time 2. Based on the diagrams, what can be inferred about the relative rates of the forward and reverse reactions between time 1 and time 2 ?

• Nothing can be inferred because the total number of X and Y atoms is the same in each diagram.

• Nothing can be inferred because the temperature of the system may have been changed.

• The rate of the reverse reaction is greater than the rate of the forward reaction.

• The rate of the forward reaction is greater than the rate of the reverse reaction.