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Reaction Quotient and Le Châtelier’s Principle

The reaction quotient (Q) helps predict how a system at a given moment will respond, while Le Châtelier’s Principle describes how a system at equilibrium reacts to changes.

Key Concepts​

  • Reaction Quotient (Q): Q = [products]^coeff / [reactants]^coeff for any point in the reaction
  • Comparing Q and K:
    • Q < K → reaction shifts forward toward products
    • Q > K → reaction shifts reverse toward reactants
    • Q = K → system is at equilibrium
  • Le Châtelier’s Principle: A system at equilibrium will shift to counteract a disturbance
    • Change in concentration, pressure, or temperature will affect Q and the reaction direction

Example​

For N₂ + 3 H₂ ⇌ 2 NH₃, ΔH < 0:

  • Adding H₂ → Q < K → reaction shifts toward products
  • Increasing temperature → system shifts toward reactants

Importance​

  • Predicts reaction shifts under non-equilibrium conditions
  • Integrates quantitative (Q vs. K) and qualitative (Le Châtelier) approaches
  • Essential for industrial and laboratory optimization

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