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Reaction Quotient and Equilibrium Constant

The reaction quotient (Q) and the equilibrium constant (K) are essential tools to predict the direction of a chemical reaction.

Key Concepts​

  • Reaction Quotient (Q): Calculated the same way as K but for any point in the reaction
    • Q = [products]^coeff / [reactants]^coeff
  • Comparing Q and K:
    • Q < K → reaction proceeds forward to form more products
    • Q > K → reaction proceeds reverse to form more reactants
    • Q = K → reaction is at equilibrium

Example​

For A + B ⇌ C + D, K = 50:

  • At a given moment, if Q = 10 → reaction shifts forward
  • If Q = 100 → reaction shifts reverse

Importance​

  • Predicts reaction direction under non-equilibrium conditions
  • Helps control reactions in laboratory and industrial processes
  • Provides a foundation for understanding Le Châtelier’s Principle

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