Le Chatelier's Principle

Fe³⁺(aq) + SCN⁻(aq) ⇄ FeSCN²⁺(aq)

Fe³⁺(aq) is colorless.

SCN⁻(aq) is colorless.

FeSCN²⁺(aq) is red.

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 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.

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?

• K_eq for the reaction

• The total pressure in the reaction vessel

• The amount of SO₃(g) in the reaction vessel

• The amount of O₂(g) in the reaction vessel

• The amount of SO₂(g) in the reaction vessel

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 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.)

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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.

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.

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)

HgO(s) + H₂O ⇄ HgI₄^2– + 2 OH^–

Consider the equilibrium above. Which of the following changes will increase the concentration of HgI₄^2–?

• Increasing the concentration of OH^–

• Adding 6 M HNO₃

• Increasing the mass of HgO present

• Increasing the temperature

• Adding a catalyst

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 K_c for the reaction at 483 K is 14.5.

A mixture of CO(g) and H₂(g) is pumped into a previously evacuated 2.0 L reaction vessel. The total pressure of the reaction system is 1.2 atm at equilibrium. What will be the total equilibrium pressure of the system if the volume of the reaction vessel is reduced to 1.0 L at constant temperature?

• Less than 1.2 atm

• Greater than 1.2 atm but less than 2.4 atm

• 2.4 atm

• Greater than 2.4 atm

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)

• PCl₅(g) ⇄ PCl₃(g) + Cl₂(g)

• 2 NO₂(g) ⇄ N₂O₄(g)

2 BaO₂(s) ⇄ 2 BaO(s) + O₂(g)

ΔH°= 162 kJ/mol_rxn

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(g)

• Adding He gas to the vessel

• Lowering the temperature

2 NO(g) + O₂(g) ⇄ 2 NO₂(g) ΔH < 0

Which of the following changes alone would cause a decrease in the value of K_eq 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

• Adding a catalyst