Kp

Kp is the symbol commonly used to denote the equilibrium constant expressed in terms of the partial pressures of gaseous species in a chemical reaction. For a general gaseous reaction aA + bB ⇌ cC + dD, Kp = (PC)^c (PD)^d / (PA)^a (PB)^b, where P_i are the partial pressures of the species i. When standard states are defined at 1 bar (or 1 atm) and activities are approximated by partial pressures, Kp is typically treated as dimensionless, with reported values reflecting the chosen convention.

Kp is related to the reaction’s concentration-based equilibrium constant, Kc, by the equation Kp = Kc (RT)^{Δn}, where Δn = (c + d) − (a + b) and R is the ideal gas constant. This relation connects changes in mole number of gases to how the equilibrium shifts with temperature, since R and T appear in the expression.

Interpreting Kp, larger values (Kp > 1) indicate a tendency toward products at equilibrium, while smaller values (Kp < 1) indicate a tendency toward reactants. Temperature strongly influences Kp via the enthalpy change, described by the van’t Hoff relation: ln(Kp) changes with temperature according to ΔH°/(RT^2). If a reaction involves pure solids or liquids, their activities are unity and do not appear in the Kp expression.

In other fields, Kp may denote other concepts such as partition coefficients or permeability constants, depending on the context. A common example is the equilibrium for N2O4 ⇌ 2 NO2, where Kp = (P_NO2)^2 / P_N2O4.