Reaction Quotient and Equilibrium Constant

Reaction Quotient and Equilibrium Constant

Reaction Quotient (Q)

  • The Reaction Quotient (Q) is a measure of the relative amounts of products and reactants present during a reaction at a given point in time.
  • It is calculated similarly to the equilibrium constant, but the concentrations used in calculation can be from any point in the reaction, not just at equilibrium.

  • For a reaction represented by the general form aA + bB ⇌ cC + dD, the reaction quotient is given by the formula:

    Q = ([C]^c [D]^d)/([A]^a [B]^b)

Equilibrium Constant (Kc and Kp)

  • The Equilibrium Constant (K) is a measure of the ratio of concentrations of products to reactants at equilibrium.
  • It quantifies the direction in which a chemical reaction will proceed to reach equilibrium, given the concentrations of the reactants and products.
  • For reactions involving gases, the equilibrium constant may be expressed in terms of either concentrations (Kc) or pressure (Kp).

  • The equilibrium constant for the general reaction aA + bB ⇌ cC + dD is given by:

    Kc = ([C]^c [D]^d)/([A]^a [B]^b)

    or

    Kp = (P_C^c P_D^d)/(P_A^a P_B^b), where P denotes the partial pressures of the gases.

Relationship Between Q and K

  • If Q = K, the system is at equilibrium. The forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.
  • If Q > K, there are too many products and not enough reactants. The reaction will shift to the left (reverse direction) to reach equilibrium.
  • If Q < K, there are too many reactants and not enough products. The reaction will shift to the right (forward direction) to reach equilibrium.

Le Chatelier’s Principle

  • In line with the relationship between Q and K, changes to a system at equilibrium obey Le Chatelier’s Principle.
  • According to this principle, if a stress is applied to a system at equilibrium, the system will adjust itself to relieve that stress and return to equilibrium.
  • Stresses to the system can include changes in concentration, pressure, volume, or temperature.

Significance

  • The understanding of the reaction quotient and equilibrium constant provides deep insights into how a reaction will proceed and how changes can impact that reaction.
  • It is crucial for predicting the direction of a reaction and for adjusting conditions to favour a particular outcome, which is particularly useful in industrial chemistry for controlling yield in chemical synthesis.