Equilibrium Review

The graph above represents the data collected under certain conditions for the decomposition of N₂O_4(g) according to the chemical equation above. Based on the graph, at approximately which time is equilibrium established?

• At time A, because N₂O_4(g) is expanding to fill the container.

• At time B, because the reaction is reversible and [NO₂] = [N_2­O₄].

• At time D, because there are no observable changes in [NO₂] and [N_2­O₄].

• At time C, because the reaction is about to reach completion and [NO₂] > [N_2­O₄].

A sample of acetone is placed into a container. The container is sealed and attached to a pressure sensor, as shown in the diagram above. The container is allowed to sit on the lab table for a few minutes as the pressure in the container is monitored at regular intervals. At the end of 2.00 minutes, some acetone liquid remains in the container. Which of the following best explains the pressure data presented in the table above?

• The acetone heats up over time, causing more of it to vaporize at an increasing rate.

• The acetone has completely vaporized after 1.50 minutes, so the pressure becomes constant.

• The acetone vaporizes from the liquid at a constant rate, the rate of condensation increases until it becomes equal to the rate of evaporation, and then the pressure stays constant.

• The acetone vaporizes from the liquid at a rate that is fast in the beginning but then slows down until the vaporization process stops completely.

A sample of pure NO_2(g) in a sealed tube at 20⁰C is placed in a temperature bath at 30⁰C. Observations of changes in the color, pressure, and mass of the mixture are recorded as a function of time. Which of the following is an observation that would best support the claim that the reaction represented above has reached equilibrium at 30⁰C?

• The total mass of the system remains constant because the amounts of reactant and product do not change with time at equilibrium.

• The color of the system changes from brown to completely colorless because only the product will be present at equilibrium.

• The total pressure of the system decreases then reaches a constant value because the amounts of reactant and product no longer change at equilibrium.

• The temperature of the system remains constant because the temperature must be constant at equilibrium.

In an experiment involving the reaction shown above, a sample of pure HI was placed inside a rigid container at a certain temperature. The table above provides the initial and equilibrium concentrations for some of the substances in the reaction. Based on the data, which of the following is the value of the equilibrium constant (K_eq) for the reaction, and why?

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SO_2(g) reacts with Cl_2(g) to produce SO₂Cl_2(g), according to the equation above. A mixture of 1.00 atm of SO_2(g) and 1.00 atm of Cl_2(g) is placed in a rigid, evacuated 1.00L container, and the reaction mixture is allowed to reach equilibrium at a certain temperature. The table below shows the initial partial pressure of each gas and the equilibrium partial pressures of SO_2(g) and SO₂Cl_2(g) at the same temperature.

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A student poured 10 mL of CoCl₄^2- into a test tube and added a few drops of concentrated HCl, which resulted in a deep-blue solution. The reaction that occurred is represented by the chemical equation shown above. Then, the student placed the test tube inside a beaker that contained ice and water for about five minutes. Which of the following describes what the student most likely observed next, and why?

• The color of the solution changed from blue to pink, because lowering the temperature increased the collision frequency between CoCl₄^2- and H₂O.

• The color of the solution did not change, because heat is not released from this reaction.

• The color of the solution did not change, because more water was not added to the solution.

• The color of the solution changed from blue to pink, because cooling caused the equilibrium to shift to form the pink-colored Co(H₂O)₆²⁺.

The formation of FeSCN²⁺ in an aqueous solution is represented by the chemical equilibrium shown above. A light red-brown solution is prepared combining 12.50 mL of 0.5 M Fe(NO₃)₃ , 0.5 mL of 0.002 M KSCN, and 37.0 mL of water that had been slightly acidified. If an additional 1.0 mL of 0.5 M Fe(NO₃)₃  is added, which of the following predicts and explains correctly whether the darkness of the color of the solution will change?

• The color of the solution will lighten because most of the volume added is solvent.

• The color of the solution will darken because the equilibrium will favor the formation of more FeSCN²⁺.

• The color of the solution will darken because the equilibrium will favor the dissociation of FeSCN²⁺.

• The color of the solution will not change because the solution already contains Fe³⁺ ions.

A chemist carried out the reaction at 573 K, starting with 0.100 mol of each reactant in a 1.00 L container with variable volume. The reaction mixture quickly reached equilibrium, as indicated in the graph above. After 20 seconds, the chemist reduces the volume of the equilibrium system by half while keeping the temperature constant. Which of the following predictions about the yield of NOCl_(g) is best, and why?

• By halving the volume, the pressure doubles. The system will respond to the increase in pressure by increasing the total number of moles of gas in the system. Thus, the yield will decrease because the reaction will shift toward more reactants.

• By halving the volume, the pressure doubles. The system will respond to the increase in pressure by decreasing the total number of moles of gas in the system. Thus, the yield will increase because the reaction will shift toward more product.

• By halving the volume, the pressure is reduced by half. The system will respond to the decrease in pressure by decreasing the total number of moles of gas in the system. Thus, the yield will increase because the reaction will shift toward more products.

• By halving the volume, the pressure is reduced by half. The system will respond to the decrease in pressure by increasing the total number of moles of gas in the system. Thus, the yield will decrease because the reaction will shift toward more reactants.

The table lists the initial concentrations of each substance in the system represented by the equation above at a given temperature. Which of the following best predicts what will occur as the system approaches equilibrium?

• The rate of the reverse reaction will be less than the rate of the forward reaction, and additional CH₃OH_(g) will be consumed because K_c < Q_c.

• The rate of the reverse reaction will be greater than the rate of the forward reaction, and additional CH₃OH_(g) will be consumed because K_c < Q_c.

• The rate of the forward reaction will be less than the rate of the reverse reaction and, additional CH₃OH_(g) will be produced because K_c < Q_c.

• The rate of the forward reaction will be greater than the rate of the reverse reaction, and additional CH₃OH_(g) will be produced because K_c < Q_c.

The chemical reaction shown above took place inside a rigid container at constant temperature. The table provides the concentrations of reactants and products at some point during the reaction. Based on this information, which of the following explains whether or not the reaction has reached equilibrium, and why?

• The reaction is not at equilibrium because Q_c > K_c; the forward reaction is favored in order to form more XY.

• The reaction is at equilibrium because Q_c = K_c; the amount of X₂, Y₂ and XY will remain the constant.

• The reaction is not at equilibrium because Q_c < K_c; the forward reaction is favored in order to form more XY.

• The reaction is not at equilibrium because Q_c < K_c; the reverse reaction is favored in order to form more X₂ and Y₂.