For the reaction  N2O4g⇌2NO2g at 298 K and at 500 K;  Kp values are 0.98 and 1700 respectively .Kp for the same reaction at 400 K is

Dissociation reaction is endothermic. For endothermic reactions Kp increases with increase in temperature.The value at 400K is greater than at 298K and lesser than at 500K