posted by Jane 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
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.
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
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.
thanks and i understand. i appreciate your write up!