Chapter 5: Extent of Chemical Reactions Notes

Key Concepts of Chapter 5: Extent of Chemical Reactions

5A Equilibrium Reactions

  • Reversible vs Irreversible Reactions:
    • Irreversible Reaction: Can only proceed in one direction; products cannot revert to reactants (e.g., baking a cake).
    • Reversible Reaction: Can proceed forwards and backwards; both reactants and products are present at equilibrium.
  • Dynamic Equilibrium: When the rates of the forward reaction (reactants → products) and the reverse reaction (products → reactants) are equal, resulting in constant concentrations.
    • Represented in graphs: Concentration-time and rate-time graphs show how concentrations change until equilibrium is reached.
  • Key Terms:
    • Concentration-time graph: Plots concentration vs. time for equilibrium reactions.
    • Yield: Mass of product obtained; compared to the theoretical yield (calculated from stoichiometry).
    • Dynamic Equilibrium: State in reversible reactions where the concentration of reactants and products remains constant, despite ongoing reactions.

5B Calculating Equilibrium Constants

  • Equilibrium Constant (K): Ratio of concentrations of products to reactants at equilibrium, defined by
    K = \frac{[C]^c[D]^d}{[A]^a[B]^b} ,
    where A, B are reactants, C, D are products, and a, b, c, d are their stoichiometric coefficients.
  • Factors affecting K:
    • K varies with temperature; if the reaction is endothermic, increasing temperature increases K, while if exothermic, increasing temperature decreases K.
  • Reaction Quotient (Q): Used to predict the direction a reaction must shift to reach equilibrium.
    • If Q < K , forward reaction is favored.
    • If Q > K , reverse reaction is favored.

5C Changes to Equilibrium

  • Le Chatelier’s Principle: If a system at equilibrium experiences a change, the system adjusts to counteract that change and re-establish equilibrium.
    1. Adding Reactants/Products:
    • Adding reactants shifts equilibrium towards products (right).
    • Adding products shifts equilibrium towards reactants (left).
    1. Removing Reactants/Products:
    • Removing reactants shifts equilibrium towards reactants (left).
    • Removing products shifts equilibrium towards products (right).
    1. Changes in Temperature:
    • Increasing temperature shifts equilibrium toward endothermic direction (absorbs heat).
    • Decreasing temperature shifts equilibrium toward exothermic direction (releases heat).
    1. Changes in Pressure:
    • Increased pressure favors the side of the reaction with fewer moles of gas.
    • Decreased pressure favors the side with more moles of gas.

5D Compromise Conditions

  • Compromise conditions are used to balance factors such as cost and reaction rate.
  • Industrial processes (e.g., Haber-Bosch):
    • Optimal yield of ammonia is achieved at high pressure and low temperature for equilibrium but faster rates at higher temperatures often used.
  • Catalysts: Utilized to increase reaction rates without affecting yield, making processes more energy-efficient.

Summary of Key Knowledge

  • Dynamic Nature of Equilibria: Concentrations of products and reactants remain unchanged at equilibrium but reactions are continuously occurring.
  • Equilibrium Expressions: Calculating the equilibrium constant requires understanding the stoichiometry and concentrations at equilibrium.
  • Shifts in Equilibrium: Understanding how different factors affect equilibrium allows for predictions about the direction of reactions, critical for optimizing industrial processes.
  • Green Chemistry: Emphasizes sustainable practices and reducing carbon footprints in chemical production.

Important Questions for Review

  • Describe the impact of temperature and pressure changes on equilibrium systems using Le Chatelier’s principle.
  • Calculate equilibrium constants and utilize the reaction quotient effectively.
  • Explore how catalysts and compromise conditions improve industrial processes, particularly in fertilizer production.