Kc

Kc is the symbol commonly used in chemistry for the equilibrium constant expressed in terms of concentrations. For a general reaction aA + bB ⇌ cC + dD, Kc is defined as Kc = [C]^c [D]^d / [A]^a [B]^b, where [X] denotes the molar concentration of species X. If activities are used instead of concentrations, Kc becomes dimensionless; in practice, the value is often treated with units reflecting the net change in moles of gas, especially for reactions in the gas phase.

In gas-phase chemistry, the related constant Kp uses partial pressures: Kp = p(C)^c p(D)^d / p(A)^a p(B)^b. These two constants are connected by Kp = Kc (RT)^{Δn}, where Δn = (c + d) − (a + b) is the change in the number of moles of gas.

Kc is temperature dependent. As temperature changes, the equilibrium composition shifts according to the enthalpy change of the reaction. The van’t Hoff equation describes this dependence: d ln K / dT = ΔH° / (R T^2). The standard Gibbs free energy change relates to the equilibrium constant by ΔG° = −RT ln K, with K being the appropriate dimensionless equilibrium constant.

Kc can be determined from equilibrium concentrations or inferred from the kinetics of the forward and reverse reactions. For a reversible elementary step, Kc equals the ratio of the forward to reverse rate constants (Kc = k_forward/k_reverse). The value of Kc provides insight into the position of equilibrium and helps predict how changes in concentration, temperature, or pressure will affect the system.