Dynamic Equilibrium and Le Chatelier’s Principles

Dynamic Equilibrium Demonstration

  • Demonstration requires observation and note-taking of dynamic equilibrium conditions.

Equilibrium and Le Chatelier’s Principle

  • Connection to Learner Profile: Thinkers can link observable color changes of a chemical reaction to changes in equilibrium position.
  • Learning Objectives:
    • Describe the characteristics of a dynamic equilibrium.
    • Predict the effects of changing temperature, concentration, or pressure on the equilibrium position.
    • Explain the effects of these changes on the position of equilibrium.
  • Learning Goal: To understand how Le Chatelier’s Principle explains qualitative observations.
  • Key Terms:
    • Dynamic Equilibrium: A state where the rates of forward and reverse reactions are equal and concentrations of reactants and products remain constant.
    • Closed System: A system where no matter enters or leaves the environment, allowing the reaction to reach equilibrium.
    • Le Chatelier’s Principle: If a dynamic equilibrium is disturbed, the system adjusts to counteract the change and restore equilibrium.

Characteristics of a Dynamic Equilibrium

  • A reversible reaction reaches equilibrium under the following conditions:
    • The rates of the forward and reverse reactions are equal and non-zero.
    • Concentrations of reactants and products remain constant, though they are not necessarily equal.
    • No observable change in macroscopic properties such as color or density occurs.

Progress Check: True or False?

  • Evaluate the following statements:
    • Statement: A system reaches equilibrium when the concentration of reactants equals that of products.
    • True/False: False.
    • Statement: When reactants are mixed together, the rate of the forward reaction increases until equilibrium is established.
    • True/False: True.
    • Statement: A catalyst increases the rate of both forward and reverse reactions.
    • True/False: True.
    • Statement: In an equilibrium system where the forward reaction is endothermic, the reverse reaction is exothermic.
    • True/False: True.
    • Statement: At equilibrium, the rate of the forward and reverse reaction is zero.
    • True/False: False.
  • Extension: Rewrite erroneous statements to correct their truth value.

Investigating a Reaction

  • Reaction Under Investigation:
    • Reaction: [Co(H2O)6]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl4]²⁻(aq) + 6H2O(l) (Forward reaction is endothermic).
    • Tasks:
    1. Notice the initial color of the solution provided.
    2. Add water dropwise to the solution.
    3. Add acid dropwise to the solution.
    4. Place the boiling tube in a beaker of boiling water.
    5. Place the boiling tube in ice water.
  • Note: Describe the color changes noted in each step, and explain why these changes occur in reference to the chemical equation above.

Observations in Chemical Reaction

  • Changing Concentration:
    • Addition of chloride ions shifts equilibrium to the right, resulting in a color change to blue.
    • Addition of water shifts equilibrium to the left, resulting in a color change to pink.
  • Changing Temperature:
    • Placing the boiling tube in boiling water shifts equilibrium to the right, observable color change is blue.
    • Placing the boiling tube in ice water shifts equilibrium to the left, observable color change is pink.

Le Chatelier’s Principle

  • Factors Affecting Equilibrium:
    • Concentration,
    • Temperature,
    • Pressure (for reactions involving gases).
  • Principle Statement: If a dynamic equilibrium is disturbed by changes in conditions, the system will shift to counteract the disturbance and reestablish equilibrium.

Predictions based on Le Chatelier’s Principle

  • Given Reaction:
    • 2A(g) + B(g) ⇌ C(g) + D(g) ∆H = exothermic.
  • Predictions:
    • a. Increasing concentration of C results in equilibrium shifting left (favoring reactants).
    • b. Removing some A results in equilibrium shifting left (favoring reactants).
    • c. Increasing the system temperature results in equilibrium shifting left (favoring the endothermic reaction).
    • d. Decreasing the system temperature results in equilibrium shifting right (favoring the exothermic reaction).
    • e. Increasing pressure results in equilibrium shifting right (favoring the reaction producing fewer moles of gas).
    • f. Increasing the volume results in equilibrium shifting right (favoring the reaction producing more moles of gas).

Progress Checkpoint for N2O4 and NO2

  • Equilibrium Reaction:
    • N2O4(g) ⇌ 2 NO2(g) ∆H = +
  • Questions:
    • a. What happens with decreased temperature?
    • Answer: Equilibrium shifts left, favoring the exothermic reaction, resulting in lightening of color due to increased N2O4 production.
    • b. What happens with removal of NO2?
    • Answer: Equilibrium shifts right, favoring the formation of NO2, resulting in darkening color.
    • c. What happens with decreased pressure?
    • Answer: Removes pressure; equilibrium shifts to the side with more moles (right), resulting in darkening color.

Learning Checkpoint 1

  • Reaction:
    • 2SO2(g) + O2(g) ⇌ 2SO3(g) ∆Hο = –200 kJ.
  • Question: What temperature and pressure conditions yield the greatest amount of SO3?
  • Options:
    • A. Low temperature, low pressure
    • B. Low temperature, high pressure
    • C. High temperature, high pressure
    • D. High temperature, low pressure

Learning Checkpoint 2

  • Reaction:
    • [Co(H2O)6]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl4]²⁻(aq) + 6H2O(l).
  • Question: Which changes would shift equilibrium to the right?
  • Options:
    • I. Addition of 0.01 M HCl
    • II. Addition of concentrated HCl
    • III. Evaporation of water.
  • Answer Choices:
    • A. I and II only
    • B. I and III only
    • C. II and III only
    • D. I, II and III only

Additional Chemistry Notes

  • Le Chatelier's Principle Part 1
  • Le Chatelier's Principle Part 2
  • Exothermic Reaction Example: ∆H = -75k.