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.

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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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  c_o.

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
I_o =  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.