The equilibrium expression for kp for the reaction below is n2 (g) o2 (g) 2no (g)

For the reaction;
N2(g) + O2(g) = 2NO(g)

Kp = pNO²/ pN₂pO₂; (No units)
where;
pNO is the partial pressure of NO;
pN₂ is the partial pressure of nitrogen
pO₂ is the partial pressure of Oxygen
The equilibrium constant  Kp is deduced from the balanced chemical equation for a reversible reaction, NOT experimental data as is the case for rate expressions in kinetics. 
Kp changes with temperature considerably changing the position of an equilibrium, and, at a constant temperature, and therefore constant K, the position of an equilibrium can change significantly depending on relative concentrations/pressures of 'reactants' and 'products'.