Posted by K on .
Given the equation
2H^2S(g) <> 2H^2(g) + S^2(g)
With a Kp = 1.2x10^2 at 1338 Kelvin
Find the value of Kc for the reaction
H^2(g) + 1/2S^2(g) <> H^2S(g) at 1338 Kelvin

Chemistry 
DrBob222,
I would first convert Kp to Kc using Kp = Kc(RT)^delta n, then see your next post to adjust for the equation. It appears to me to be reversed and 1/2; therefore, K'c = 1/(sqrt Kc).

Chemistry 
K,
So instead of looking at each compound in the equation you would only look at the product side of it and compare it to the other equation and only either square,square root etc. that value?

Chemistry 
DrBob222,
I'm not sure I follow your thinking.
Kp = Kc(RT)^dn
Kc = Kp(RT)^dn
Kc = 1.2E2(0.08205*1338)^(32)
Kc = 1.2E2(o.08205*1338) = ??
Then K'c = (1/sqrt Kc) 
Chemistry 
K,
Yeah I did all the previous things to find Kc and I got kc = 0.012. But you know how for to get the other kc you said I would have to do 1/sqrt Kc, I know that I would do that because the product is H2S and I would do the inverse because it is reversed and I would sqrt because it is halved. I get all of that but does that mean that we don't have to do anything to the reactants side of 1/2S2 and H2? Wouldn't we have to do the inverse of each of those since they are reversed as well and do the sqrt of those too as well since both are halved from the original equation?

Chemistry 
DrBob222,
First, let me correct an error I made in typing. I omitted the divisor sign.
Kp = Kc(RT)^dn
Kc = Kp(RT)^dn
This should be
Kc = Kp/(RT)^dn
Kc = 1.2E2(0.08205*1338)^(32)
This should be
Kc = 1.2E2/(0.08205*1338)^32
Kc = 1.2E2(o.08205*1338) = ??
This should be
Kc = 1.2E2/(0.08205*1338) = ?? for which I obtained 1.1E4.
Then K'c = (1/sqrt Kc)
Then 1/(sqrt 1.093E4) =?? for which I obtained 95.6 which rounds to 96 to two s.f.
Your confusion arises over K and your concept of what this procedure does.
K for the original equation is
Kc = (H2)^2(S2)/(H2S)^2 = 1.093E4
When you reverse it, the equation becomes 2H2 + S2 ==> 2H2S and
Kc now is (H2S)^2/(H2^2)(S2) = 1/1.093E4 = 9149.69 (You can see that the new K is the reciprocal of the old K from the way two K expressions are written.) And I am operating on K; for example, I don't take the reciprocal once for H2S, once for S and once for H2 and that is because the new K is just the reciprocal of the old K. Now when we go to 1/2, note we are taking 1/2 of H2S, 1/2 of S2 and 1/2 of 2H2 so the new K now is (H2S)/(H2)(S)^1/2. By taking the sqrt of K (the entire expression), we actually take the sqrt of (H2S)^2[it was (H2S)^2 and becomes (H2S)]; we take the sqrt of (H2)^2 [it was (H2)^2 and becomes (H2)]; we take the sqrt (S) [it was (S) and becomes sqrt S]/ I hope this helps. It's important that you understand what we've done. ;).

Chemistry 
K,
Oh okay, thank you so much for your whole explanation. I finally understand the whole concept behind it and what we are suppose to do. Thanks so much again :)