The equilibrium expression for Kp in regards to a reaction is the measurement of the equilibrium partial pressures of all the variables within the reaction formula. Assuming there is no change in temperature, the value of Kp will always remain the same and is not affected by the values of the variables. Gases used in the expression are modified using partial pressures rather than concentrations.
Kp is defined as a term describing the partial pressures of gases and can be used for both homogeneous and heterogeneous reactions that involve gases. In the case of a homogeneous equilibrium of gases, everything present within the mixture must be at the same phase. A further limitation in the usage of Kp in a homogeneous equilibrium is that every variable involved is gas.
Heterogeneous equilibrium formulas will have variables that are not in the same state, such as solids that come into contact with gases. When writing out the equilibrium expression using Kp, however, no terms for the solid should be used.
Another equilibrium constant of gaseous mixtures aside from Kp is Kc. Kp only applies to a closed system and represents the partial pressures of gases within it. Kc, on the other hand, can only be defined by the molar concentrations.