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.