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` what is the difference between first order kinetics and pseudo first order kinetics. Is adsorption kinetics is different. Because Iam working in Dye removal studies by algal biomass, which order kinetics should I follow.`

6 years ago

Dear

Rashmi V,The main difference between a First Order Reaction & a Pseudo First Order reaction is that the Pseudo first order eaction is a reaction that should actually happen by some higher order(higher than 1) but occurs by first order beacuse the concentration of one reactant used is very high & another reactant used is very low ...

The concentration of the reactant which is very low can be negleted & thus the concentration of the reactant which has higher concentration is alone taken !

This makes the reaction follow a first order kinetics rather than some higher order.This is Pseudo First order reaction !

Hope you understood now..

All the Best & Good Luck !

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6 years ago

Pseudo First Order Reaction

AandBreact to produceP:

If the initial concentration of the reactantAis much larger than the concentration ofB, the concentration ofAwill 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 onBcan 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 constantCproduces the corresponding integrated law.

Substituting[ B ] = cinto equation(1)yields

Integrating equation(2)gives: The constant of integrationCcan be evaluated by using boundary conditions. Att = 0, the concentration ofBisc._{o}

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

If the decrease in concentration ofBis followed by photometric measurement theBeer' Lawmust be taken into account.

Combining equation(5)andBeer' 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

I_{o}= Intensity of the initial light beam, I = Intensity of the transmitted light

gives the relationship betweenk'andlnA:

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

Thepseudo-first order rate constant k'can be also determined from thehalf-life t._{1 / 2}

Measuring asecond 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 thepseudo 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.

6 years ago

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