Chem II
posted by Ken on .
Kp < Kc at 25 degrees C for which of the reactions below?
N2 + 3H2 <> 2NH3
2NO <> N2 + O2
C + O2 <> CO2
2NOCl <> 2NO + Cl2
I don't even know where to start. I really want to understand this, not just an answer

You look at the equation for converting Kp to Kc (or Kc to Kp). One of them is as follows:
Kp = Kc(RT)^{delta n}
Therefore, the ONLY difference between Kp and Kc is the (RT)^delta n part. So if delta n is zero; i.e., the same number of mols on each side, then delta n = 0, any number to the zero power is 1 and that would mean the RT term would be 1*Kc and Kp = Kc. So what does that mean? You look for an equation where the number of mols on one side is the same as the number of mols on the other. For example, the first equation has 4 mols on the left and 2 on the right so that one isn't the answer. You can do the others. 
That is what I thought also, but my teacher says no. He said the correct answer is the first equation, and I don't understand why. Can you explain further?

Yes, I can explain. Both of us made the same mistake and I wouldn't have noticed it if you hadn't reposted. Reread the question. It is NOT (as I first read it) Kp = Kc (Look at my first response and you can see I am always talking about equal). The question reads Kp < Kc. So you want delta n to be negative value so that Kp = Kc(RT)^2 = Kc/(RT)^2 = Kc/larger term = smaller Kp. #1 is the only equation with a larger number of mols on the left than the right except for #3. I assume #3 doesn't count because C is a solid So you have 1 mol gas on the right and 1 mol on the left.