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Homework Help: AP Chemistry Chemical Kinetics Problem

  1. Dec 9, 2014 #1
    1. The problem statement, all variables and given/known data
    The rate of the reaction was studied at a certain temperature.
    O(g) + NO2(g) --> NO(g) + O2(g)

    In the first set of experiments, NO2 was in large excess, at a concentration of 1.0 * 10^ 13 molecules/cm3 with the following data collected.

    time (s) [O] (atoms/cm3)
    0 .......................2400

    Part (a). Find the reaction order with respect to O. (Explain with graphs)

    Part (b) Find the rate constant with respect to O.

    2. Relevant equations
    Rate = K[A]
    First Order Reaction Integrated Rate Law: ln[A] = -kt + ln[A]0

    3. The attempt at a solution
    I want to know how to transform the graph/data set into a linear line to find the reaction order (I think it is first order). I think I ln[O] and plot it against time (s) to achieve a linear graph for a first order graph. How do I do that? Do I need to convert atoms/cm3 to mol/L? I tried doing that and it is not looking right. Help would be appreciated!
  2. jcsd
  3. Dec 9, 2014 #2
    You don't have to convert to mol/L. The first three data points are OK, but the 4th data point should be 300 atoms/cc (there must be a typo, or maybe you're supposed to think that the measurement is in the noise at 300 atoms/cc). You can plot the first three points, and leave out the 4th, or you can plot all four, with 300 as the last point. If you plot the first 3 points, they will lie on a straight line on a semi-log plot. If you have Excel, just change the vertical scale to a log scale (which Excel will automatically do for you). You can get the first order rate constant from the slope. Excel will even do a curve fit for you, and provide you with the slope, so that you can know the rate constant immediately.

  4. Dec 9, 2014 #3
    Thanks you! I got the equation to be y = -69.315x + 7.7832 without plotting the last point. This means the rate constant is -69.3 respect to [O]. It is negative because the concentration is going down as time increases. Is that correct?
  5. Dec 9, 2014 #4
    In your equation, there is a negative sign in front of k. So the rate constant should be +69.3/sec.

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