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Homework Help: Chemical Kinetics: determining order of reaction

  1. Jan 25, 2014 #1
    1. The problem statement, all variables and given/known data

    Good day! I have a question from my chem HW that is stumping me. Although the answer may be simple, it has eluded me for quite a bit. It regards rate laws and constants of reactions.

    Question is as stated: This reaction was monitored as a function of time: AB --> A + B

    A plot of 1/[AB] versus time yields a straight line
    with slope= - 0.055 M-1s-1

    What is the value of the rate constant (k) at this temp?

    2. Relevant equations

    SINCE it stated 1/[AB] vs. time, its implying an order 2 reaction since order 2 reaction
    has the integrated rate law:
    1/[AB]t = kt + 1/[AB]0

    y axis = 1/[AB].......

    3. The attempt at a solution

    When a order 2 reaction is plotted against time, k = slope. When the question stated it was talking about a plot of 1/[AB] its referring to a order 2 reaction, and since order 2 reactions (when plotted against time) have a y axis = 1/[AB], im deducing the reaction is order 2.

    I'm confused since the given slope is negative, and every graph of an order 2 reaction vs. time in my book has a straight line with a positive slope.

    Like I said, its an easy problem, with an easy solution, but i'm not seeing it.
  2. jcsd
  3. Jan 25, 2014 #2
    If [AB] is decreasing with time, then 1/[AB] must be increasing with time. That's why the slope is positive.

  4. Jan 25, 2014 #3
    Thanks for the reply, but 1/[AB] has a negative slope not positive in this problem. That's why i'm confused. The graph would indicate that 1/[AB] is decreasing with time which means [AB] is increasing which makes no sense.
  5. Jan 25, 2014 #4
    Oh. I didn't notice that. It must just be a mistake in the problem statement.

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