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Math Help - What is a "first-order reaction?"

  1. #1
    s3a
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    What is a "first-order reaction?"

    Can someone tell me what a "first-order reaction" is in easy to understand terms please?
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  2. #2
    o_O
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    Say that we're given a chemical reaction: A + B \to C + D

    And the rate of the reaction is given by: r = k[A]^m [B]^n

    We say that the order of the reaction with respect to A is m and the order with respect to B is n. The total order of the reaction is given by m + n.

    One thing to note that m and n are determined experimentally. There's no way to predict the order without actually carrying out the experiment itself.

    Now for specifics. A first-order reaction means that the rate of the reaction depends only on one of the reactants with order 1, i.e. r = k[A]^1

    A common form is its integrated form: \ln [A]_0 = -kt + \ln [A] where [A]_0 is the initial concentration of our reactant.
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  3. #3
    s3a
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    You went a little ahead of my course by embedding ln into chemistry but I just wanted to ask the following word problem question:

    "Consider the decomposition of N2)5 in carbon tetrachloride (CCl4) at 45C.
    2N205(soln)-->4NO2(g)+O2(g)

    The reaction is first-order in N205, with the specific rate constant 6.08x10^(-4) per second. Calculate the reaction rate under these conditions.

    a) [N2O5] = 0.2 mol/L"

    Thanks!
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  4. #4
    o_O
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    Since this is a first-order reaction, we know the rate is given by: r = k[\text{N}_2\text{O}_5]

    We know the specific rate constant ( k = 6.08 \times 10^{-4} \ s^{-1}) and we know [\text{N}_2\text{O}_5].

    So simply plug it all in.
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