what is the difference between Kp and Kc?

K_p is used for reactions involving gases, and the equilibrium constant is expressed in terms of the partial pressures of the gases. Whereas, K_c is used as a general constant, and the concentrations (in moles/liter) are used in calculating its value.

K_c is the ratio of products’ concentration over reactants’ concentration and K_p is the ratio of products’ partial pressures and reactants’ partial pressures.

Kp = Kc X (R T)^( ?n)