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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: kimi85 on June 28, 2007, 09:47:48 PM

Title: Calculating equilibrium constants and heat of dissociation
Post by: kimi85 on June 28, 2007, 09:47:48 PM

At constant pressure of 1 atm, N₂O₄ is 50% dissociated at 60 degree C, and 79% at 100 degree Celsius.  Calculate the equilibrium constantat these temperatures for the reaction N₂O₄ = 2NO₂.

This is what I did but I didn't get the right answer:

I find the delta G using the values found at the table. I multiplied each to the number of moles during the dissociation, then product minus the reactant
so, my delta G at 60C is 2365 J.  Then I used delta G = -RTlnK
I got 0.4256, but the correct answer is 1.33

For the delta H, I used delta G = delta H - T deltaS
but I my answer is still wrong.  The correct answer is 41.5 kJ.

Does anybody know what's wrong with my solution?
Thank you very much!

Title: Re: Calculating equilibrium constants and heat of dissociation
Post by: Borek on June 29, 2007, 02:47:12 AM

If you know how much gas is dissociated simple stoichiometry allows calculation of equilibrium concentrations, put them into reaction quotient and you are done. No need for delta G tables.

Title: Re: Calculating equilibrium constants and heat of dissociation
Post by: kimi85 on June 29, 2007, 03:26:59 AM

Thank you sir. ;D

I'll try it.

Title: Re: Calculating equilibrium constants and heat of dissociation
Post by: Yggdrasil on June 29, 2007, 05:58:51 PM

The information in the tables is usually for either 0^oC or room temperature.  You are asked to find the value at 60^oC, which will be different from the value at other temperatures.

Title: Re: Calculating equilibrium constants and heat of dissociation
Post by: kimi85 on June 30, 2007, 06:32:33 AM

Thank you for that information. :)