Does KP predict reaction direction?

[Mike Dacre]

I think I already know the answer to this, but I can't find an easy reliable source online.

Does K_P predict reaction direction in the same way as K_C? i.e. If K_P 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 K_P 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.

 

[Borek]

K_p doesn't say anything about volume

Do you know how to convert between K_c and K_p, knowing Δn?

 

[Mike Dacre]

You are right, I meant pressure.

And yes, I know the relationship: $$K_P = K_C RT^{\Delta n}$$ What I am curious about is if it is possible for there to be a situation where K_C is greater than 1 but K_P is less than 1. Hypothetically if Δn is negative then from the math alone K_P can be less than 1 even if K_C is greater than 1. Since K_C indicates reaction direction, my question is if there is every a case in which they can give opposite predictions? i.e. can you use the value of K_P to predict reaction direction?

 

[Borek]

my question is if there is every a case in which they can give opposite predictions

Reaction doesn't bother about our predictions, it goes the way it is meant to. If our predictions are correct, they have to point in the same direction.

 

[Mike Dacre]

That confuses me a little. Are you saying that an equilibrium constant greater than 1 does not always favor the products?

 

[Borek]

No, I am saying that Kc and Kp can't predict two different things for the same reaction. Behavior of the system doesn't depend on the way we describe it.

Think about it this way. Let's say you have a ball on the inclined table. It is obvious the ball will roll down and drop to the floor. If you have two ways of describing the ball, and one of them says the ball will fall down, and the other says ball will go up, there is something wrong with one of them. Every correct model must predict the same behavior - as there is only one way the ball can go. Same with any reaction mixture.

 

[Mike Dacre]

Thank you, that makes sense to me. The reason for my confusion was that I was not sure that they were necessarily describing exactly the same thing.

In your example, one could be describing if the ball was going to fall, and another could be describing how big the ball was.

In hindsight, I realize I was thinking about what K_P is incorrectly. My understanding now is that K_P is telling you about the partial pressures while K_C is telling you about the concentrations, and so if the temperature is constant, then if the products are at a higher concentration than the reactants, the partial pressure of the products must also be higher than that of the reactants. The reason I got confused is because I was thinking that in non-ideal gasses it would be possible for some gas to have a lower partial pressure than the reactant that formed it, even if its concentration were higher than that reactant. Does that make sense?

In hindsight I realize this is foolish, because differences in the behavior of gaseous molecules (e.g. how much they attract each other) should not have much effect when compared with the difference in relative abundance of those molecules, because very few reactions result in an exact 50:50 ratio between products and reactants.

 

[Borek]

The reason I got confused is because I was thinking that in non-ideal gasses it would be possible for some gas to have a lower partial pressure than the reactant that formed it, even if its concentration were higher than that reactant. Does that make sense?

Non ideality is also present in the Kc - while we routinely use concentrations, in fact we should use activities. This is especially visible when dealing with ionic solutions. There is a parameter called ionic strength of the solution, which tels us how far from ideality (ie equivalence between concentration and activity) we are.

Compare http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients.