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Chemical Kinetics: Rate Laws, Orders

  1. Jun 25, 2010 #1
    1. The problem statement, all variables and given/known data
    I understand how to determine the order of a rxn based on a series of experiments one divides one into another to obtain a ratio and then determines the power to which one is raised to determine the order.

    Typical data collected would be:
    Ex1 [A] .2 mol = Initial rate 4.8 mol/L*s
    Ex2 [A] .4 mol = Initial rate 9.6 mol/L*s

    In this case one would divide Ex2/Ex1 and obtain 2 = 2^m ; m=1 a 1st order rxn.

    In our book every example has ex2/ex1 or ex4/ex3 but then in the problems in the back it takes what ever pair of experiments give a round number. Often this is obvious, often not and one would have to run a series of ex/ex to obtain an order..

    2. Relevant equations

    A series I just started working on looked like ex2/ex1 would yield a nice even first order rxn but ended up being a .666 = .444^m; m=.5. The answer key chose a different pair of ex. and gives a m=2 2nd order rxn....

    3. The attempt at a solution

    I have no attempt as my question is with a series of experiments giving different answers how does one know which answer to go with? Are only whole numbers desired? Why wouldn't any set of 2 experiments yield the same ratio or at least very close, with all things being equal?


  2. jcsd
  3. Jun 26, 2010 #2


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    Staff: Mentor

    I am not sure if I follow. Partially because it is not clear what you mean. What ex is - example? experiment? You can't divide example by example, you can divide reaction speeds.

    Please show details of the question you mentioned where answer key selects different pair of "ex".

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