# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: oque on May 01, 2019, 03:54:51 PM

Title: NO2 problem
Post by: oque on May 01, 2019, 03:54:51 PM
Nitrogen dioxide NO2 is one of a number of oxides of nitrogen found in our atmosphere. It
can dimerize to give N2O4 (g):
2NO2 (g) ::equil:: N 2 O 4 (g)

At 298 K, the ∆G of formation for N2O4 (g) is 98.28 kJ, whereas for NO2 (g) is 51.84
kJ. Starting with one mole of N2O4 (g) at 1.0 atm and 298 K, calculate what fraction
will be decomposed if the total pressure is kept constant at 1.0 atm and the
temperature is maintained at 298 K.

Supposedly the answer is 0.166, but I haven't been able to find the way to solve it correctly, my result was 0.39.

I calculated the constant K with ΔG= -RTlnK. The ΔG for the whole reaction is (according to me) 5.4kJ
The constant I calculated is 0.99782. Then:

(NO2)2/(N2O4)

(2x)2/(1-x)= K

Solve the cuadratic ecuation and it's 0.39,  Where did I go wrong?
Title: Re: NO2 problem
Post by: Borek on May 02, 2019, 02:54:51 AM
The ΔG for the whole reaction is (according to me) 5.4kJ
The constant I calculated is 0.99782.

Check this part.
Title: Re: NO2 problem
Post by: oque on May 02, 2019, 04:58:07 PM
I calculated again and its 0.113. I realized that ΔG it's in kJ, not in J in order to use R.
But again, now I solved the quadratic equation and its 0.15468 not close enough to the right answer.
Title: Re: NO2 problem
Post by: mjc123 on May 03, 2019, 04:45:13 AM
I agree with you, but you get the given answer if K = 0.133. Perhaps the book made a transcription error in doing the quadratic equation.
Title: Re: NO2 problem
Post by: AWK on May 03, 2019, 06:17:25 AM
The problem comes from ICHO24 and its solution is given
https://www.alquimicos.com/oq_reg/internacionales/icho_24.pdf
Title: Re: NO2 problem
Post by: mjc123 on May 03, 2019, 07:55:23 AM
Ah. Total pressure is 1 atm, for mole fraction must divide by (1+x).