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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: HorseFurHire on February 13, 2011, 07:41:13 PM

Title: Calculating Kc for different reactions
Post by: HorseFurHire on February 13, 2011, 07:41:13 PM

Hey all, I'm really stumped. I'm taking the second half of a first year chem course and since I took a break from between classes, which resulted in me forgetting what I knew before. The problem is this:

Determine K_c for the reaction:

1/2N₂(g) + O₂(g) + 1/2 (g) (equilibrium sign) NOBR (g)

from the following information (at 298 K):

2 NO (g) (equilibrium sign) N₂ (g) + O₂ (g)

K_c = 2.1 x 10³⁰

NO (g) + 1/2 Br₂ (g) (equilibrium sign) NOBr (g)

K_c = 1.4

(Those K_c values at the end are paired with the reactions)

My problem comes into play with not knowing what formula to use to calculate this... Help?

Title: Re: Calculating Kc for different reactions
Post by: opti384 on February 13, 2011, 08:04:25 PM

Try to get the first equation by adding and subtracting the ones that you know the K_c.

Title: Re: Calculating Kc for different reactions
Post by: HorseFurHire on February 13, 2011, 08:23:40 PM

Quote from: opti384 on February 13, 2011, 08:04:25 PM

Try to get the first equation by adding and subtracting the ones that you know the K_c.

So minus the second one from the first one?

Title: Re: Calculating Kc for different reactions
Post by: opti384 on February 13, 2011, 08:32:03 PM

Actually it doesn't matter to subtracting which from which because you can reverse the equation if necessary.

Just try to make this equation

Quote

1/2N2(g) + O2(g) + 1/2 (g) (equilibrium sign) NOBR (g)

The thing is that do you know what happens to the K_c when you add and subtract equations?