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Homework Help: Arrhenius Equation - Chemical Kinetics

  1. Aug 24, 2008 #1
    Hello everyone,

    I am having trouble with determining where I erred in the following exercise. If someone could point out my mistake, I would appreciate the help.

    Thank you!


    1. The decomposition of N2O5 has an activation energy of 103 kJ/mol and a frequency factor of [tex] 4.3E13 s^-1 [/tex]. What is the rate constant for this decomposition at 20°C?


    I simply substituted the given values into the Arrhenius Equation (omitting units to save time in typing):

    [tex] k = Ae^{\frac{-E_a}{RT}}[/tex]

    [tex] k = 4.3E13 \times e^{\cfrac{-103}{8.314 \times 293 K}}[/tex]

    [tex] k = 4.1E13 1/s[/tex]


    However, the given answer is:
    1.9E-5 1/s
  2. jcsd
  3. Aug 24, 2008 #2
    Are you *sure* you're using the right units, since you omitted them in your original post? What units are you using for the gas constant? Activation energy? Temperature?

    I would go back and check them again.

    P.S. - I think these sorts of questions belong in the Homework Help section (even if it's not technically a homework question, it very strongly resembles one, at least IMO.)
  4. Aug 24, 2008 #3
    Thanks for your reply, Mike H.

    I apologise; I didn't realise that this was the correct forum. If someone could please move this to the Homework Help section, that would be appreciated.

    I have rewritten my work with units:

    [tex] k = 4.3E13 \frac{1}{s} \times e^{\cfrac{-103 \frac{kJ}{mol}}{8314 \frac{kJ}{K mol} \times 293 K}}[/tex]

    [tex] k = 4.3E13 \frac{1}{s} [/tex]

    I get a different answer, but one that is still incorrect.
  5. Aug 24, 2008 #4


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    R is 8.314J K-1 mol-1 = 0.008314 kJ K-1 mol-1.

    [edit: thanks GCT]
    Last edited: Aug 24, 2008
  6. Aug 24, 2008 #5


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    Science Advisor
    Homework Helper

    You're one decimal place off.
  7. Aug 24, 2008 #6
    Thank you very much for your help!
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