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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: dtomasiewicz on March 02, 2009, 11:18:24 PM

Title: Finding Bond Dissociation Enthalpy
Post by: dtomasiewicz on March 02, 2009, 11:18:24 PM
Alright so I have a chemistry exam coming up in a couple days and I'm reviewing old exams as practice. I've come across a question that I believe *should* be easy but I just can't find the right answer to.

Given that the standard molar enthalpy of formation for nitrogen dioxide gas, NO2(g), and for dinitrogen tetraoxide gas, N2O4(g), is 33.85 kJ*mol-1 and 9.66 kJ*mol-1, respectively, the nitrogen-to-nitrogen, N-N, bond dissociation enthalpy in dinitrogen tetraoxide, O2N-NO2 (the dimer of NO2), is

a) 9.7 kj*mol-1
b) 24.2 kj*mol-1
c) 33.8 kj*mol-1
d) 58.0 kj*mol-1
e) 77.4 kj*mol-1

Thanks in advance! Glad I came across this forum, lots of useful info here.
Title: Re: Finding Bond Dissociation Enthalpy
Post by: Astrokel on March 02, 2009, 11:33:32 PM
Hey dtomasiewicz, you have to show some attempts first.
Title: Re: Finding Bond Dissociation Enthalpy
Post by: dtomasiewicz on March 03, 2009, 12:04:38 AM
Quote from: Astrokel on March 02, 2009, 11:33:32 PM
Hey dtomasiewicz, you have to show some attempts first.
Sorry, my bad.

The only result I can understand is

b) 24.2 kJ*mol-1

As that would be 33.85 - 9.66. The answer is actually d, which would be 2(33.85) - 9.66, but I don't understand where the 2 comes from. Is it because we are looking at the dimer?
Title: Re: Finding Bond Dissociation Enthalpy
Post by: Astrokel on March 03, 2009, 12:44:22 AM

              33.85
1/2N2 + O2  :rarrow: NO2

              9.66
N2 + 2O2   :rarrow: O2N-NO2  ---- (2)

                                                      9.66
(2) can also be expressed as: N2 + 2O2   :rarrow:   2NO2 + BE of N-N

If you apply hess' law it would work to be 2(33.85) - 9.66. There is a 2 because of 2 NO2 molecules as shown above.

When dealing with enthalpy change problem, it is good to write out chemical equations as you are able to see it clearly.