What is the rate of reaction when [A] 0.20 M?

A reaction in which A —–> products was monitored as a function of time and the results are shown below.

Time (s) —- [A] (M)

0 —- 1.000

25 —- 0.914

50 —– 0.829

75 —- 0.744

100 —– 0.659

125 —– 0.573

150 —– 0.488

175 —– 0.403

200 —– 0.318

Okay, well I know the order of the reactions is 0.

  1. Determine the value of the rate constant.

  2. What is the rate of reaction when [A] 0.20 M?

1 Answer

  • 1.

  • Zero order integrated rate law is:

    [A] = [A]₀ – k∙t

    [A]₀ denotes initial concentration.

    So if you plot [A] versus t you get a straight line with the slope -k.

    The result should be something about:

    k = 3.4×10⁻³Ms⁻¹

    Alternatively you can make regression analysis.

    Least square fit of a line of the form

    y = k∙x

    to a set of data pairs (x,y)

    leads to

    k = ∑(y∙x) / ∑( x² )

    (∑ denotes summation over all data points)

    For this problem

    x = t

    y = [A]₀ – [A]

    =>

    k = 3.41×10⁻³Ms⁻¹

    2.

    The rate of a zero order reaction is dependent from the concentration level. As long as some reactant is present it will react away at the same, constant rate.:

    rate = -d[A]/dt = k = 3.41×10⁻³Ms⁻¹

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