[ay1]

Please consider the reaction

CaCO₃(s)  CaO(s) + CO₂(g)

The equilibrium constant K_p at a specific temperature should be equal to a certain value of P_CO2 regardless of initial amounts of reactants of products.

However, if I have some amount of CaO in a closed container and add CO₂ such that its partial pressure is less than K_p for that temperature, will the system reach equilibrium? What if it is instead equal?

I believe that it shouldn't reach equilibrium since the partial pressure of CO₂ wouldn't be able to reach K_p but I'm not sure because I can't imagine how the reaction will proceed if equilibrium isn't reached. To completion?

Thank you

[Hunter2]

I think nothing will happen. If p less as kp it cannot react equilibrium. Its only possible if its higher as kp, so CO2 would react to carbonate to lower the pressure to kp value.

[Borek]

Note: assumption that activities of all solids equals exactly one is only an approximation. Quite a good one, but once you try exotic cases and exotic conditions it can fail.

But think about another case: you have an AgNO₃ solution and you add some HCl, but not enough to have the [Ag⁺][Cl^-] product reach the Ksp value. Does the AgCl appear? Is there an dissolution equilibrium present? Do you see the analogy?