Equilibrium Constant (Kp)
Definition
The equilibrium constant written with gas partial pressures instead of concentrations: used when all the players are gases.
Symbol: ·
Technical definition
Kp is the equilibrium constant expressed in partial pressures for gas-phase reactions. It relates to Kc through Kp = Kc(RT)^delta-n, where delta-n is the change in moles of gas, and the two are equal only when delta-n is zero.
Example
For N2(g) + 3H2(g) forming 2NH3(g), delta-n = -2, so Kp = Kc(RT)^-2. The two constants differ unless you account for the mole change.
Common confusion
Kp and Kc are not interchangeable numbers. They have different units and values whenever the moles of gas change; only their ratio is fixed by temperature.