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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 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.
This makes the reaction follow a first order kinetics rather than some higher order.This is Pseudo First order reaction!
Hope this helps.
Thanks and regards,
Kushagra
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