Overview

Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of all substances. This topic explains how to write equilibrium expressions, calculate the equilibrium constant, and predict shifts in equilibrium using Le Châtelier’s Principle.

Key Concepts and Structures

• Dynamic Equilibrium: The reaction continues in both directions, but concentrations remain constant over time.
• Equilibrium Expression (K): For a general reaction aA + bB ⇌ cC + dD, the expression is
  K = [C]^c[D]^d / [A]^a[B]^b
• Reaction Quotient (Q): Calculated the same as K, but with initial concentrations. Comparing Q to K predicts the direction the reaction will shift.
• Le Châtelier’s Principle: If a system at equilibrium is disturbed, it will shift to counteract the change. Affects changes in:
  • Concentration: Adding reactants shifts toward products.
  • Pressure: Increasing pressure shifts toward fewer gas molecules.
  • Temperature: Adding heat favors the endothermic direction.
• Kc vs. Kp: Kc is based on concentration (mol/L); Kp is based on pressure (atm). Kp = Kc(RT)^Δn for gas reactions.
• Effect of Catalysts: Speeds up the rate to reach equilibrium but does not affect equilibrium position or value of K.
• Solubility Product Constant (Ksp): Applies to slightly soluble salts. Expresses the equilibrium between dissolved and undissolved forms.

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