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Homework Help: What is the pre‐exponential coefficient for this reaction?

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data
    I have two problems with this:
    1) I am not sure if I am using the right constant.
    2) I have no idea if my values are realistic.

    The actual question:

    The rate of a reaction is 25.00 units per time at 20.00°C.
    The activation energy barrier for this reaction is 1.50 eV.
    a) What is the pre‐exponential coefficient for this reaction?
    b) What is the rate of reaction at 40.00 °C?
    c) What is the rate of reaction at 60.00 °C?
    d) What is the rate of reaction at 80.00 °C?
    e) What is the rate of reaction at 100.00 °C?

    2. Relevant equations
    Boltzmann’s constant k = 8.6173 E ‐5 eV/K

    Ra = C*exp[-Q/(RT)]

    Ra is rate of reaction
    C is constant
    Q is activation energy
    R is gas constant
    T is absolute temperature

    3. The attempt at a solution
    I believe I should use just the Boltzmann Constant for R. Earlier in the homework assignment we did things in eV/atom, and when I converted R from eV/K*mol to eV/K*atom I ended up with the Boltzmann Constant. But if I am wrong please tell me.

    And as for the questions these are the numbers I'm getting, I don't know if they are realistic or not, and the fact most of them are in the vague "units per time" doesn't help alleviate any confusion.

    a) The preexponential constant is 1.534 E27 units per time
    b) At 313.15 K the reaction rate is 1109 units per time
    c) At 333.15 K I get 3.120 E4 units per time
    d) At 353.15 K I get 6.016 E5 units per time
    e) At 373.15 K I get 8.445 E6 units per time

    Are these numbers realistic? I don't have grasp on that at all. It is my first time working in Electron Volts. Thanks in advance to anyone who can help.
    Last edited: Sep 30, 2012
  2. jcsd
  3. Oct 1, 2012 #2


    User Avatar

    Staff: Mentor

    Easy to check on the web (or to calculate on your own) - eV is just a 1.602176565(35)×10−19 J. As you are given energy per atom, multiplying it by Avogadro's constant will give you energy per mole.
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