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الموضوع: Pseudo First Order Reaction

  1. #1
    عضو ذهبي الصورة الرمزية اسراء
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    Aug 2011
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    افتراضي Pseudo First Order Reaction

    Pseudo First Order Reaction


    A and B react to produce P:



    If the initial concentration of the reactant A is much larger than the concentration of B, the concentration of A will not change appreciably during the course of the reaction The concentration of the reactant in excess will remain almost constant. Thus the rate's dependence on B can be isolated and the rate law can be written



    Equation (1) represents the differential form of the rate law. Integration of this equation and evaluation of the integration constant C produces the corresponding integrated law.

    Substituting [ B ] = c into equation (1) yields



    Integrating equation (2) gives:

    The constant of integration C can be evaluated by using boundary conditions. At t = 0, the concentration of B is co.

    Therefore

    Accordingly, equation (3) can be rewritten as follows:

    If the decrease in concentration of B is followed by photometric measurement the Beer' Law must be taken into account.

    Combining equation (5) and Beer' Law

    A = Absorbance, e = Molar absorbtivity with units of L · mol -1 · cm -1
    c = Concentration of the absorbing species in solution, expressed in mol · L -1, d = Path length through the sample
    Io = Intensity of the initial light beam, I = Intensity of the transmitted light


    gives the relationship between k' and lnA:




    According to equation (7), a plot of lnA versus time should lead to a straight line whose slope is the pseudo-first order rate constant k'. The value of k' can then be divided by the known, constant concentration of the excess compound to obtain the true constant second order k:



    The pseudo-first order rate constant k' can be also determined from the half-life t 1 / 2.




    Measuring a second order reaction rate (see below) can be problematic: the concentrations of the two reactants must be followed simultaneously, which is more difficult; or measure one of them and calculate the other as a difference, which is less precise. A common solution for that problem is the pseudo first order approximation. As a general rule, a minimum of a 20-fold stoichiometric excess is necessary. A 50-fold or 100-fold stoichiometric excess is preferable.

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  2. #2
    مراقب عام الصورة الرمزية Eiman
    تاريخ التسجيل
    Jul 2011
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    الاردن - ماركا
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    افتراضي رد: Pseudo First Order Reaction

    مشكوووووورة اسراء
    من جد وجد ....... ومن سار على الدرب وصل

  3. #3
    عضو ذهبي الصورة الرمزية تامر يوسف
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    Mar 2012
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    افتراضي رد: Pseudo First Order Reaction

    Thank you Esraa

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  4. #4
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
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    Jul 2011
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    افتراضي رد: Pseudo First Order Reaction

    شكرا اسراء

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  5. #5
    عضو مميز الصورة الرمزية قمر بلحاج
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    Jul 2012
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    افتراضي رد: Pseudo First Order Reaction

    جزاكم الله خيرا

    عضو في نادي ماركا الأكاديمي


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