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- 1. Multiple Choice
X(g) + Y(g) ↔ XY (g)
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.
- 2. Multiple Choice
X(g) + Y(g) ↔ XY(g)
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 200 s 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 200 s 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 0 s and 200 s the rates of the forward and reverse reactions are the same, which is a characteristic of a system at equilibrium.
- 3. Multiple Choice
CO(g) + 2 H2(g) ⇄ CH3OH(g) Δ H < 0
The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5.
Which of the following explains the effect on the equilibrium constant, Kc, when the temperature of the reaction system is increased to 650 K?
Kc will increase because the activation energy of the forward reaction increases more than that of the reverse reaction.
Kc will increase because there are more reactant molecules than product molecules.
Kc will decrease because the reaction is exothermic.
Kc is constant and will not change.
- 4. Multiple Choice
The table above shows data for two reactions carried out in two separate evacuated 1.0-liter rigid containers at constant temperature of 298 K. To each container 0.50 mol of the appropriate reactants was added, and the reaction was allowed to reach equilibrium. Based on this information, which of the following correctly compares the relative concentrations of BrCl and NO present inside their respective containers at equilibrium?
[BrCl]eq = [NO]eq because equimolecular mixtures of the reactants were allowed to reach equilibrium at the same constant temperature.
[BrCl]eq > [NO]eq because Br2 and Cl2 are larger molecules that can collide more frequently to form products.
[BrCl]eq > [NO]eq because the much larger Keq for reaction 1 means that a much higher concentration of products will be present at equilibrium for reaction 1 compared with reaction 2.
[BrCl]eq < [NO]eq because the much larger Keq for reaction 1 means that hardly any products will be present at equilibrium compared with reaction 2.
- 5. Multiple Choice
At night, an equilibrium reaction between two different nitrogen compounds generates N2O5 in the atmosphere, as represented below.
Reaction 1: NO3(g) + NO2(g) ⇄ N2O5(g) K = 2.6×10−11
A mixture of NO3 and a ten-fold excess of NO2 are placed inside a rigid container at constant temperature and allowed to reach equilibrium. Which of the following provides a correct comparison of the equilibrium concentrations of these chemical species, and why?
[N2O5] << [NO3] because a small K value indicates that the consumption of the reactants is favored at equilibrium.
[NO2] << [N2O5] because a small K value indicates that the consumption of the reactants is favored at equilibrium.
[NO2] << [NO3] because a small K value indicates that the formation of the products is not favored at equilibrium.
[N2O5] << [NO2] because a small K value indicates that the formation of the products is not favored at equilibrium.
- 6. Multiple Choice
In the atmosphere, small water droplets are suspended in the air, forming an aerosol. N2O5 (g) can form HNO3 (aq) under these conditions, as shown in reaction 2, represented below.
Reaction 1: NO3 (g) + NO2 (g) ↔ N2O5 (g)
Reaction 2: N2O5 (g) + H2O (l) → 2HNO3 (aq)
Which of the following predicts the effect that the formation of will have on the equilibrium shown in reaction 1, and why?
The equilibrium of reaction 1 will shift toward the formation of more product, because N2O5 (g) is removed when it reacts to form HNO3 (aq).
The equilibrium of reaction 1 will shift toward the formation of more product, because H2O (l) acts as a catalyst for reaction 1.
The equilibrium of reaction 1 will shift toward the formation of more reactants, because the product N2O5 (g) is removed when it reacts to form HNO3 (aq).
The equilibrium of reaction 1 is not affected, because neither H2O (l) nor HNO3 (aq) are gases.
- 7. Multiple Choice
Fe3+(aq) + SCN-(aq) ↔ FeSCN2+(aq)
When colorless solutions containing Fe3+(aq) ions and SCN-(aq) ions are combined, a deep-red complex ion FeSCN2+(aq), quickly forms, as shown in the net ionic equation above. Which of the following explains the observation that adding a few additional crystals of KSCN(s) results in the red color of the solution becoming deeper?
The added KSCN(s) dissolves, disturbing the charge balance in the solution, causing Fe(SCN)3 (s) to precipitate as a red solid.
The added KSCN(s) dissolves, causing the solution to become saturated in SCN- ions, which appear red at high concentrations.
The added KSCN(s) dissolves, causing the reaction system to respond by producing more product to partially consume SCN-(aq) and reduce its concentration.
The added KSCN(s) dissolves, causing the reaction system to respond by forming more Fe3+ ions, which have a deep orange color at high concentrations.
- 8. Multiple Choice
Fe3+(aq) + SCN-(aq) ⇄ Fe(SCN)2+(aq)
For the reaction represented above, the value of the equilibrium constant, Keq, is 240 at 25°C. From this information, correct deductions about the reaction at 25°C include which of the following?
I - The reaction is quite rapid.
II - The product is favored over the reactants at equilibrium.
III - The reaction is endothermic.
I only
II only
I and II only
II and III only
I, II and III
- 9. Multiple Choice
For which of the equilibrium systems represented below will the amount of product(s) at equilibrium increase if the volume of the reaction vessel is increased at a constant temperature?
PCl5(g) ⇄ PCl3(g) + Cl2(g)
2 NO(g) + O2(g) ⇄ 2 NO2(g)
N2(g) + O2(g) ⇄ 2 NO(g)
2 CO(g) ⇄ C(s) + CO2(g)
- 10. Multiple Choice
For which of the following reaction mixtures at equilibrium would reducing the volume of the container at constant temperature cause the equilibrium to shift toward the products?
SrCO3(s) ⇄ SrO(s) + CO2(g)
C(s) + CO2(g) ⇄ 2 CO(g)
CO(g) + H2O(g) ⇄ CO2(g) + H2(g)
2 NO2(g) ⇄ N2O4(g)
- 11. Multiple Choice
H2(g) + I2(g) ⇄ 2HI(g) ΔH > 0
Which of the following changes to the equilibrium system represented above will increase the quantity of HI(g) in the equilibrium mixture?
I. Adding H2(g)
II. Increasing the temperature
III. Decreasing the pressure
I only
III only
I and II only
II and III only
I, II and III
- 12. Multiple Choice
HgO(s) + H2O ⇄ HgI42– + 2 OH–
Consider the equilibrium above. Which of the following changes will increase the concentration of HgI42– ?
Increasing the concentration of OH–
Adding 6 M HNO3
Increasing the mass of HgO present
Adding a catalyst
- 13. Multiple Choice
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) ΔHo = -92 kJ/molrxn
The diagram above shows how the concentrations of H2 , N2 , and NH3 in this system changed over time.
More NH3 gas is added to the system at time t2 while the temperature is held constant. Which of the following will most likely occur?
The value of the equilibrium constant will increase
The value of the equilibrium constant will decrease.
The total pressure in the container will decrease.
The amount of N2 will increase.
The amount of H2 will decrease.
- 14. Multiple Choice
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) ΔHo = -92 kJ/molrxn
The diagram above 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.
- 15. Multiple Choice
N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) ΔH < 0
NH3(g) was synthesized at 200°C in the presence of a powdered Os(s) catalyst, leading to the equilibrium system represented above. Which of the following changes would result in more NH3(g) in the mixture after equilibrium is reestablished?
Replacing the powdered Os(s) with a solid cube of Os(s) of the same total mass
Increasing the temperature of the system to 250°C at constant pressure
Removing some H2(g)
Adding some N2(g)
- 16. Multiple Choice
A sample of N2O4 (g) is placed into an evacuated container at 373 K and allowed to undergo the reversible reaction N2O4 (g) ↔ 2 NO2 (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?
At 14 seconds, because [N2O4] is twice [NO2], which implies that the forward and reverse reaction rates are equal.
At 23 seconds, because [NO2] equals [N2O4], which shows that equal concentrations are present at equilibrium.
At 40 seconds, because [NO2] is twice [N2O4], which matches the stoichiometry of the balanced chemical equation.
At 60 seconds, because [NO2] and [N2O4] remain constant, indicating that the forward and reverse reaction rates are equal.
- 17. Multiple Choice
Reaction A: 4 HCl(g) + O2(g) ⇄ 2 Cl2(g) + 2 H2O(g)
Reaction B: N2O4(g) ⇄ 2 NO2(g)
Reaction C: H2(g) + I2(g) ⇄ 2 HI(g)
Reaction D: 2 NH3(g) ⇄ N2(g) + 3 H2(g)
The reactions represented above are carried out in sealed, rigid containers and allowed to reach equilibrium. If the volume of each container is reduced from 1.0 L to 0.5 L at constant temperature, for which of the reactions will the amount of product(s) be increased?
Reaction A
Reaction B
Reaction C
Reaction D
- 18. Multiple Choice
The equilibrium system represented by the equation above initially contains equal concentrations of Cr2O72−(aq) and CrO42−(aq). Which of the following statements correctly predicts the result of adding a sample of 6.0 M NaOH(aq) to the system, and provides an explanation?
The mixture will become more orange because OH−(aq) will oxidize the Cr in CrO42−(aq).
The mixture will become more yellow because OH−(aq) will reduce the Cr in Cr2O72−(aq).
The mixture will become more yellow because OH−(aq) will shift the equilibrium toward products.
The color of the mixture will not change because OH−(aq) does not appear in the equilibrium expression.
- 19. Multiple Choice
A mixture of NO2(g) and N2O4(g) is placed in a glass tube and allowed to reach equilibrium at 70°C, as represented above.
Which of the following best predicts how the partial pressures of the reacting species will be affected if a small amount of Ar(g) is added to the equilibrium mixture at constant volume?
PNO2 will decrease and PN2O4 will increase.
PNO2 will increase and PN2O4 will decrease.
Both PNO2 and PN2O4 will decrease.
No change will take place.
- 20. Multiple Choice
A mixture of NO2(g) and N2O4(g) is placed in a glass tube and allowed to reach equilibrium at 70°C, as represented above.
Which of the following statements best helps to explain why the contents of the tube containing the equilibrium mixture turned a lighter color when the tube was placed into an ice bath?
The forward reaction is exothermic.
The forward reaction is endothermic.
The ice bath lowered the activation energy.
The ice bath raised the activation energy.
- 21. Multiple Choice
A sealed rigid vessel contains BaO2(s) in equilibrium with BaO(s) and O2(g) as represented by the equation above. Which of the following changes will increase the amount of BaO2(s) in the vessel?
Removing a small amount of O2(g)
Removing a small amount of BaO(s)
Adding He gas to the vessel
Lowering the temperature
- 22. Multiple Choice
Which of the following changes alone would cause a decrease in the value of Keq for the reaction represented above?
Decreasing the temperature
Increasing the temperature
Decreasing the volume of the reaction vessel
Increasing the volume of the reaction vessel
- 23. Multiple Choice
2 SO3(g) ⇄ 2 SO2(g) + O2(g)
After the equilibrium represented above is established, some pure O2(g) is injected into the reaction vessel at constant temperature. After equilibrium is reestablished, which of the following has a lower value compared to its value at the original equilibrium?
Keq for the reaction
The total pressure in the reaction vessel
The amount of SO3(g) in the reaction vessel
The amount of SO2(g) in the reaction vessel
- 24. Multiple Choice
4 HCl(g) + O2(g) ⇄ 2 Cl2(g) + 2 H2O(g)
Equal numbers of moles of HCl and O2 in a closed system are allowed to reach equilibrium as represented by the equation above. Which of the following must be true at equilibrium?
I. [HCI] must be less than [Cl2].
II. [O2] must be greater than [HCl].
III. [Cl2] must equal [H2O].
I only
II only
I and III only
II and III only
I, II, and III
- 25. Multiple Choice
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 above show the initial and equilibrium conditions of the system.
Additional Cl2(g) is injected into the system at equilibrium. Which of the following graphs best shows the rate of the reverse reaction as a function of time? (Assume that the time for injection and mixing of the additional Cl2(g) is negligible.)
- 26. Multiple Choice
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 above 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
- 27. Multiple Choice
CaCO3(s) ⇄ CaO(s) + CO2(g) ΔH° = 178 kJ/molrxn
The reaction system represented above is at equilibrium. Which of the following will decrease the amount of CaO(s) in the system?
Increasing the volume of the reaction vessel at constant temperature
Lowering the temperature of the system
Removing some CO2(g) at constant temperature
Removing some CaCO3(s) at constant temperature
- 28. Multiple Choice
The chemical equations and equilibrium expressions for two reactions at the same temperature are given above. Based on the information, which of the following expressions can be used to calculate the value of K3 for reaction 3 at the same temperature?
K3 = (-K1) + (-K2)
K3 = (-K1) - (-K2)
K3 = K1 x K2
K3 = 1/K1 x 1/K2
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