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Understanding Chemical Equilibrium Principles

May 30, 2025

Lecture Notes: Chemical Equilibrium

Introduction to Equilibrium

  • Equilibrium in Chemistry: Occurs when the rate of the forward reaction equals the rate of the reverse reaction.
  • Dynamic Equilibrium: The reactions continue to occur, but the concentrations of reactants and products remain constant.

Conceptual Illustration

  • Example: Two cities (A and B) with cars moving between them.
    • Cars moving at equal rates from A to B and B to A result in no net change in the number of cars in each city.
  • Concentration Profile: Graph with concentration vs. time
    • Concentration of reactants and products become constant over time at equilibrium.

Reaction Rates

  • Rate of Forward and Reverse Reactions:
    • Forward reaction rate: (k_1[A])
    • Reverse reaction rate: (k_{-1}[B])
    • At equilibrium, these rates are equal._

Equilibrium Constant (K)

  • Definition: Ratio of the concentration of products to reactants at equilibrium.
    • (K = \frac{k_1}{k_{-1}})
  • Expressions:
    • For concentration: (K_c)
    • For partial pressure: (K_p)_

Calculating Equilibrium Constants

  • General Reaction: (aA + bB \rightleftharpoons cC + dD)
  • Equilibrium Expression:
    • (K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b})
    • (K_p = \frac{P_C^c P_D^d}{P_A^a P_B^b})

Example Problems

  1. Writing Equilibrium Expressions:

    • Use stoichiometric coefficients as exponents.
    • (K_c): concentration of products/reactants.
    • (K_p): pressure of products/reactants.
  2. Calculating Equilibrium Constants:

    • For given chemical reactions, balance the equation, determine concentrations or pressures at equilibrium, and solve for (K_c) or (K_p).
  3. Converting Between (K_c) and (K_p):

    • Use the equation (K_p = K_c(RT)^\Delta n)
    • (\Delta n) is the change in moles of gas.
  4. Adjusting Equilibrium Constants:

    • Doubling a Reaction: (K' = K^2)
    • Halving a Reaction: (K' = K^{1/2})
    • Reversing a Reaction: (K' = \frac{1}{K})
  5. ICE Tables:

    • Initial, Change, Equilibrium tables to solve for unknown concentrations or pressures.

Advanced Calculations

  • Complex Reactions:
    • Balance equations and solve using given data and ICE tables to find equilibrium concentrations and constants.
  • Partial Pressure Calculations:
    • Convert between concentration and pressure using the ideal gas law.

Key Points

  • Remember the distinction between (K_c) and (K_p).
  • Consider changes in reaction stoichiometry and how it affects equilibrium constants.
  • Use ICE tables for systematic solving of equilibrium problems.
  • Understand the relationship between forward and reverse rate constants and equilibrium constants.