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Homework Help: Thermodynamics homework

  1. Jan 6, 2005 #1
    Hello all


    (a) N2(g) + O2(g) --> 2NO(g) ∆H = + 180. 7 kJ

    (b) 2NO(g) +O2(g) --> 2NO2(g) ∆H = -113.1 kJ

    (c) 2N20(g) --> 2N2(g) --> O2(g) ∆H = -163.2 kJ

    Desired Reaction: N2O (g) + NO2(g) --> 3NO (g)

    Can someone please tell me where to start? How do I apply Hess's Law? I know I have to work backwards. However, how do I do this in a systematic way?

    Any help is appreciated

    Last edited: Jan 6, 2005
  2. jcsd
  3. Jan 6, 2005 #2
    please rewrite the equations. Just look at B), for instance, it's not even balanced... and C makes no sense since how can you get O2 from 2 N2...
  4. Jan 6, 2005 #3


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    A good place would be to state the equations correctly:
    [tex] N_{2}+O_{2}\rightarrow 2NO [/tex] [tex] (\Delta H)_{1}=+180.7 kJ [/tex]

    [tex] 2NO+O_{2}\rightarrow 2NO_{2} [/tex] [tex] (\Delta H)_{2}=-113.1 kJ[/tex]

    [tex] 2NO_{2}\rightarrow N_{2}+2O_{2} [/tex] [tex] (\Delta H)_{3}=-67.6 kJ [/tex]

    Now,which is the quantity u wanna compute and what's the reaction u wish to get??

  5. Jan 6, 2005 #4
    I want to compute ∆H for

    N2O (g) + NO2(g) --> 3NO (g)
  6. Jan 6, 2005 #5
    I think C should be 2 N2O(g) -> 2N2(g) + O2(g)...

    Balance the reactions so that you will get rid of the unwanted reactants or products.... For instance, N2 + O2 -> 2NO

    you you might want to multiply the reaction by 2 since N2 is not wanted and there's an N2 in reaction C of the product. Im assuming you know they will cancel out when added together...
    Last edited: Jan 6, 2005
  7. Jan 7, 2005 #6
    yes but is there an actual method to solve the problem? Or do you have to just guess and check?
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