7.3 Reaction Quotient and Equilibrium Constant

Represent the reaction quotient Qc or Qp, for a reversible reaction, and the corresponding equilibrium expressions Kc = Qc or Kp = Qp. Equilibrium UNIT 7

• The reaction quotient Qc describes the relative concentrations of reaction species at any time. For gas phase reactions, the reaction quotient may instead be written in terms of partial pressures as Qp. The reaction quotient tends toward the equilibrium constant such that at equilibrium Kc = Qc and Kp = Qp. As examples, for the reaction a A + b B  c C + d D the law of mass action indicates that the equilibrium expression for (Kc, Qc) is EQN: [] [] [] [] =KCD AB c cd ab and that for (Kp, Qp) is EQN: =KPP PP () () () () p cd ab CD AB
  • Exclusion: Conversion between K c and K p will not be assessed on the AP Exam. Students should be aware of the conceptual differences and pay attention to whether Kc or K p is used in an exam question. Equilibrium calculations on systems where a dissolved species is in equilibrium with that species in the gas phase will not be assessed on the AP Exam.
• The reaction quotient does not include substances whose concentrations (or partial pressures) are independent of the amount, such as for solids and pure liquids.