I think I already know the answer to this, but I can't find an easy reliable source online. Does KP predict reaction direction in the same way as KC? i.e. If KP is greater than 1, the reaction favors the products, and if it is less than 1 it favors the reactants? Or is it just that if KP is greater than 1, the partial pressure of the products is greater, and if it is less, the partial pressure of the reactants is greater? I can imagine there are cases where the pressure is not in the same direction as the concentration, particularly in heterogeneous reactions.