Ap Chem

posted by on .

Why is it possible for Kp to be less than Kc? If this is the case then when Kc>1 it favors products, but if Kp <1 than how can that reaction favor products? Or does that rule only apply for Kc

• Ap Chem - ,

It might help to explain your predicament if you post a complete problem. As is we are dealing with nebulous essentials. First, however, remember that Kc applies to ALL reactions while Kp applies to gaseous reactions (in Kp expression) only.

• Ap Chem - ,

Consider the following reaction at 1000°C:CO(g) + 3H2(g) CH4(g)+ H2O(g)
At equilibrium, the following concentrations are measured: [CO] = 0.0613 M, [H2] = 0.1839 M, [CH4] = 0.0387, [H2O] = 0.0387 M. Calculate the value of Kc for this reaction. Calculate the value of Kp.
Ans.: Kc = 3.93, Kp = 3.60 x 10-4

• Ap Chem - ,

I don't know that Kp is always less than Kc although I didn't try any numbers. Since Kp = Kc(RT)delta n the variation of Kp and Kc depends upon the value of delta n. That is, if delta n = 0, then Kp=Kc. If delta n is +, Kp>Kc and if delta n is -, Kp<Kc (this is the case with the reaction in your post).
As for different rules, I must admit I hadn't thought about that and it is an interesting question. I'm glad you asked. Here is what I think. Whether a reaction shifts to the left or to the right ALWAYS depends upon Q and K. That is, if Q<K, the reaction shifts forward, if Q>K, the reaction shifts to the reverse, and if Q=K there is no change. That is ALWAYS true and it doesn't change with Kp or Kc. A quote from a freshman chemistry text I have says this. "If Kc is MUCH larger than 1 the products predominate and if Kc is MUCH less than 1 the reactants predominate." I think the secret to that statement being true resides in how we define the word much. I was taught in grad school the same think you quoted in your post; i.e, k>1 and equilibrium was to the right, etc. But I believe that is for the standard reaction of 1:1 reactants and products. If we have A + B ==> C + D, and we write the expression for K and substitute equal values for concns, then K = 1 and we say that is equilibrium. BUT, if we change the equation to 2A + 3B ==> C + D. I don't think a K of 1 is the equilibrium state. For example, for
PCl3 + Cl2 ==> PCl5, K = 1.9. The equilibrium values are PCl5 = 0.25M, PCl3 = 0.16M and Cl2 = 0.82M. It is true that K>1 and PCl5 happens to be greater than either PCl3 or Cl2 but they are close enough for the K>1 for the products to predominate. In another example of 2A + B ==> C suppose we say K = 1 at some temperature. If we start with 1 mole A, 1 mole B, and 1 mole C, we can calculate the equilibrium concns. I did that (a cubic equation is involved) but I obtained equilibrium concn of (A) = 0.544M, (B) = 0.772M and (C) = 1.228 M. Note that with K = 1 and the rule "we grew up with", we would be tempted to say the system was at equilibrium because K = 1; however, we see it isn't and the product is GREATER than the reactants at equilibrium. Of course I rigged that by making it 2A. The point I am struggling to make is to go back to the MUCH greater than 1 and MUCH less than 1 statements and suggest that all of this depends to a large extent on how we define much greater and much smaller. Finally, I think the Kc = 1, Kc>1 and Kc<1 works as long as we don't get carried away with our conclusions AND that it works well in many many circumstances but we don't bet out life on it. I think Kp is another story because while concns are based on 1M for the standard state (and most concns are usually in the range of 0.1- 3M)[which gives K = 1 = equilibrium conditions], Kp would be based on 1 atm for partial pressures and those fly all over the map.
I suppose I could have said I think your last statement is true but I don't think that would have been sufficient.

I hope another tutor reads this and makes some comment. I don't think I've just dreamed up an answer for the sake of having an answer. I hope this is worth something to you now or in the future.

• Ap Chem - ,

thanks and i understand. i appreciate your write up!