Rate Law

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How to determine the rate law from a table of trials - Real Chemistry

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Key Questions

  • One way is to use the method of initial rates.

    A rate law shows how a change in concentration affects the rate.

    The equation for a component A is

    #"rate" = k["A"]^m#, where #m# is the order of the reaction.

    Zero Order

    #"rate" = k["A"]^0 = k#

    The rate does not depend on the concentration. Whatever you do to the concentration, the rate will not change.

    First Order

    #"rate" = k["A"]^1 = k["A"]#

    The rate is directly proportional to the concentration.

    If you double the concentration, you double the rate.
    If you triple the concentration, you triple the rate.
    If you halve the concentration, you halve the rate, and so on.

    Second Order

    #"rate" = k["A"]^2#

    The rate is proportional to the square of the concentration.

    If you double the concentration, you multiply the rate by four.
    If you triple the concentration, you multiply the rate by nine.
    If you halve the concentration, you divide the rate by four, and so on.

    Since concentration changes during an experiment, we must measure the initial rate of the reaction, before the concentration has had a chance to decrease.

    We set up an experiment and measure the rate. Then we do another experiment in which we change only the concentration of component A. Let's say we double the concentration of A.

    If the rate did not change, the reaction was zero order in A.
    If the rate doubles, the reaction is first order in A.
    If the rate quadruples, the reaction is second order in A.

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