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Enduring Understanding 6.A: Reversible Reactions

  • Chemical reactions are reversible at a molecular level - they can proceed in either direction.
  • When a reaction and its reverse are happening at the same rate, such that the concentration of reactants and products are not changing over time, this is called dynamic chemical equilibrium.
  • Chemical equilibrium can be expressed mathematically:
  • When the reaction is in equilibrium, the concentrations of products and reactants can be used to calculate the equilibrium constant, Kc. The c denotes concentration; an equilibrium constant that uses pressure is Kp.
  • Example : The reaction:
    • SO2Cl2 (g) ⇆ SO2 (g) + Cl2 (g)
  • At equilibrium in a rigid container, the concentration of SO2 and Cl2 is 0.085 mol/L and the concentration of SO2Cl2 is 0.915 mol/L. The Kc can be calculated:
    • Kc = [SO2] [Cl2] / [SO2Cl2] = (0.085)(0.085)/(0.915) = 0.0079
  • The reaction quotient is similar, but for situations in which the reaction mixture is NOT at equilibrium.
  • Example : If 0.50 moles of SO2Cl2 and 0.20 moles of SO2 and Cl2 are combined in a 1L container, Q can be calculated:
    • Qc = [SO2] [Cl2] / [SO2Cl2] = (0.20) (0.20) / (0.50) = 0.080
  • Q and K can be compared, to determine which direction a chemical reaction will go in.
  • If K > Q, the reaction will proceed in the forward direction.
  • If K < Q, the reaction will proceed in the reverse direction.
  • When K = Q, the reaction is at equilibrium. The rate of the forward and reverse reactions are equal.
  • Note that only reactants that can change concentration or pressure contribute to the calculation of a K or a Q.
  • Solid and liquid reagents are not included in equilibrium constant calculations.
  • Example: The reaction quotient for the reaction:
    • 2 ClO (s) ⇆ Cl2(g) + O2(g)
  • Is given by:
    • Q = [Cl2] [O2]
  • The solid reagent ClO, cannot change in concentration, so it is not included in the equation.
  • Similarly, the Kp for the reaction P4 (g) + 6F2 (g) ⇆ 4PF3 (l) is given by 1/[PP4] [PF2]6. The liquid product does not have a pressure, so cannot contribute to the pressure equilibrium constant.



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