Reaction equilibrium Part II - Reverse reaction weirdness

Jimmy King

Had a "take it as many times as you need to" multiple choice online quiz for class. There's one problem which I was able to get right, being multiple choice and unlimited tries, but my brain is not doing the math on this one and I can't get anything useful out of google.

If Kc equals 0.11 at 25 deg. C for the reaction: N2O4 (g) ↔ 2 NO2 (g),
what is Kc for the reaction: NO2 (g) ↔ ½ N2O4 (g)

The answer, it seems, is 3. Now, I know the reverse reaction Kc is 1/Kc. 1/.11 = 9.09, so that means this is 1/3Kc. Why?

KafkaAU

It's because the concentration is halved.

[N2O4] = k[NO2]^2
[NO2]^2 = 1/k[N2O4]
[NO2] = SQRT(1/k[N2O4])

Jimmy King

Ah, got it. Thanks. Hopefully a few more like that crop up in homework before the test. I'm normally fine with with (high A's in calculus classes, etc) but my brain just did not see that and I'm not certain I'll pick up on it if it comes up again if I don't see it until test time. Hopefully I'll remember though, it totally makes sense now.

Jimmy King

Just popping back in to say thanks, although really thanks should be given on more than just this thread. You guys here in H&A got me through the most recent test with 112 out of 100. Fuck yeah. Now there's about 30% of the course grade left outstanding (CRAZY with only a month or so left) and I'm sitting at 97% in the class, so I could just stop doing anything and pass... not that I'm going to, but I could, which means I can totally relax now.