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Homework Help: Order of this reaction?

  1. Nov 4, 2009 #1
    1. The problem statement, all variables and given/known data
    In a reaction between ozone and an organic species, the observed rate was found to depend only on the concentration of ozone at 298 K. Using the data below, determine the order of this reaction and the rate constant (include units).
    Time (s) |Ozone conc (molecules/cm3)
    0 | 4.311×1015
    50 | 2.890×1015
    100 | 2.036×1015
    150 | 1.435×1015
    200 | 0.995×1015
    250 | 0.689×1015

    2. Relevant equations

    3. The attempt at a solution
    Is it first order? I plotted the graph of ln[O3] vs. time on excel and that was a straight line.
    Also to find k, do I use ln[A]t = ln[A]o - kt? If I use this equation, which value do I use for [A]t?--because the value of k differs slightly for different values.
    Last edited: Nov 5, 2009
  2. jcsd
  3. Nov 10, 2009 #2
    When contemplating whether the reaction is first order or not, I would advise taking a graphical approach. Try to imagine what a graph would look like if the line had an equation of a first order reaction versus a second order reaction. Perhaps you may even want to try looking at the graphic behaviour of higher order reactions although the first two may suffice.

    Now, when you are determining the value of [A]t, you have to understand that any point on the line works. "HOWEVER," you must use a point on the graph that you have created. Don't forget that when you are creating a line of best fit, you are assuming only the points on that line are relevant, not the points away from the line that helped form it.
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