Le Châtelier’s Principle (College Board AP® Chemistry)

Consider the reaction below:

AgCl (s) + 2NH₃ (aq) ⇌ Ag(NH₃)₂⁺ (aq) + Cl^- (aq)

Explain what happens to the amount of NH₃ (aq) and Cl^- (aq) when the following stresses are applied after equilibrium is established.

1. NH₃ (aq) is added to the system

2. Ag(NH₃)₂⁺ (aq) is removed from the system

3. Cl^- (aq) is added to the system

4. AgCl (s) is removed from the system

Answers:

Answer 1:

• When a reactant is added, the system shifts in the forward reaction direction to decrease the amount of reactants

• In response to the stress, the amount of NH₃ (aq) decreases and the amount of Cl^- (aq) increases

Answer 2:

• When a product is removed, the system shifts in the forward reaction direction to increase the amount of products

• In response to the stress, the amount of NH₃ (aq) decreases and the amount of Cl^- (aq) increases

Answer 3:

• When a product is added, the system shifts in the reverse reaction direction to decrease the amount of products

• In response to the stress, the amount of NH₃ (aq) increases and the amount of Cl^- (aq) decreases

Answer 4:

• AgCl is a solid reactant

• Pure solids to do appear in an equilibrium expression

• So once equilibrium is established, removing AgCl(s) will have no effect on the amounts of NH₃(aq) or Cl^-(aq)

PCl₃ (g) + Cl₂ (g) ⇌ PCl₅ (g)

K_P = 0.952 at 250°C

 A mixture of PCl₃(g), Cl₂(g), and PCl₅(g) is placed in a sealed, rigid container at 250°C. After equilibrium is established, the pressure inside the container is doubled while the temperature is held constant at 250°C. Which of the following will most likely occur?

(A) The partial pressure of PCl₅ will increase.

(B) The value of K_P will decrease.

(C) The amount of Cl₂ will increase.

(D) The partial pressure of PCl₃ will remain constant.

Answer:

• Changing the pressure of a system at equilibrium does not affect the value of K_P

• When the pressure is increased, the system shifts in the reaction direction that produces the fewest number of moles of gaseous particles

• From the balanced chemical equation, there are 2 moles of gaseous reactants and one mole of gaseous products

• So, the system will shift in the direction of the products.

  • The amounts of PCl₃ and Cl₂ will decrease

  • The amount of PCl₅ will increase

• According to the ideal gas law, pressure and the number of moles of a gas are directly proportional

• Therefore, doubling the pressure of the system will result in an increase in the partial pressure of PCl₅

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

The equilibrium constant, K_c, for the reaction above at two different temperatures is shown in the table below.

Is the forward reaction endothermic or exothermic?

Answer:

• When the temperature is increased, the system shifts in the endothermic reaction direction

• From the table, as the temperature increased the K_c value decreased

• In general terms

• The smaller the K_c value, the smaller the concentration of the products and the larger the concentration of the reactants

• This indicates that as the temperature increased, the system shifted toward the reactants

• Therefore, the reverse reaction must be endothermic (+ΔH)

• For a reversible reaction, the enthalpy of the forward reaction has the same magnitude but the opposite sign of the reverse reaction

• So, the forward reaction is exothermic (-ΔH)

Some text books refer to the relative amounts of products and reactants in an equilibrium mixture as the equilibrium position.

• If a stress causes an increase in the amount of products, we say the equilibrium position shifts to the right.

• If a stress causes an increase in the amount of reactants, we say the equilibrium position shifts to the left.

Sr(OH)₂ (s) ⇌ Sr²⁺ (aq) + 2OH^- (aq)                    K_c(298K) = 3.2 x 10^-4

A sample of Sr(OH)₂ (s) is dissolved in water at 298 K to produce a saturated solution according to the equation above. After equilibrium is established, 2 drops of 2.0 M SrCl₂ (aq) is added to the saturated solution. Which of the following best predicts what takes place?

(A)   The pH decreases and K_c decreases.

(B)   The pH remains constant and K_c increases.

(C)   The pH increases and K_c remains constant.

(D)   The pH decreases and K_c remains constant.

Answer:

• The addition of SrCl₂(aq) increases the amount of Sr²⁺ and Cl^- ions in the solution

  • Cl^- ions are spectator ions that do not react with the Sr²⁺ or OH^- ions present

• The addition of a product or a reactant does not affect the K_c value

• When a product is added, the system shifts in the reverse reaction direction to decrease the amount of products

• In response to the stress, the amount of Sr²⁺(aq) and OH^-(aq) in the system decreases

• For aqueous systems at 298 K

[H⁺][OH^-] = 1 x 10^-14

• So, as the concentration of OH^- ions decreases the concentration of H⁺ ions increases

• The equation for pH is

pH = -log[H⁺]

• The greater the concentration of H⁺ ions the smaller the pH value

• Therefore, upon addition of SrCl₂(aq) the pH decreases and K_c remains constant.

CoCl₄^2- (aq) + 6H₂O (l) ⇌ Co(H₂O)₆²⁺ (aq) + 4Cl^-(aq)                ΔH < 0

 (blue)                                  (pink)               (colorless)                    

The equation above describes the equilibrium between two different cobalt(II) ions. A 100.0 mL purple solution containing an equilibrium mixture of the two ions is produced in a 250 mL flask. Predict the color of the solution when the following stresses are applied to the equilibrium mixture.

1. The temperature is increased

2. The temperature is decreased

3. 10 mL of water is added

4. The solution is transferred to a 150 mL flask

Answers: 

Answer 1:

• The forward reaction is exothermic (ΔH < 0)

• The enthalpy of the reverse reaction has the same magnitude but the opposite sign as the forward reaction

• Therefore the reverse reaction is endothermic (ΔH > 0)

• When the temperature is increased, the system shifts in the endothermic reaction direction

• Therefore, the concentration of blue CoCl₄^2- increases and the concentration of pink Co(H₂O)₆²⁺ decreases

• The overall color of the solution will be blue

Answer 2:

• When the temperature is decreased, the system shifts in the exothermic reaction direction

• The forward reaction is exothermic (ΔH < 0)

• Therefore, the concentration of blue CoCl₄^2- decreases and the concentration of pink Co(H₂O)₆²⁺ increases

• The overall color of the solution will be pink

Answer 3:

• Diluting an aqueous equilibrium system with water, decreases the concentration of all aqueous species

• In response, the system shifts in the reaction direction that produces the greatest number of moles of aqueous particles

• There is 1 mole of aqueous reactant particles and 5 moles of aqueous product particles

• Therefore, the system shifts to decrease the concentration of blue CoCl₄^2- and increase the concentration of pink Co(H₂O)₆²⁺

• The overall color of the solution will be pink

Answer 4:

• Changing the volume only affects gas-phase equilibrium systems

• So, the system will not shift and the color of the solution will remain purple