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Physical Chemistry

Hot goIITian

Joined:	9 Mar 2010
Post:	142

9 Mar 2010 17:25:28 IST

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plz explain the concept of pseudo unimolecular reactions......thanks.

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plz explain the concept of pseudo unimolecular reactions......thanks.

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manish gupta

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Joined: 5 Mar 2010

Posts: 51

10 Mar 2010 11:03:16 IST

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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

produces the corresponding integrated law.

Substituting

[ B ] = c

into equation

(1)

yields:

Integrating equation

(2)

gives:

The constant of integration

can be evaluated by using boundary conditions. At

t = 0

, the concentration of

c_o

Therefore

Accordingly is obtained:

If the decrease in concentration of

is followed by photometric measurement the

Beer' Law

must be taken into account.

Combining equation

(4)

and

Beer' Law

A = absorbance, e = molar absorbtivity with units of L · mol ^-1 · cm ^-1
c = concentration of the compound in solution, expressed in mol · L ^-1
P_o = radiant power for radiation entering; P = radiant power for radiation leaving

gives the relationship between

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

. The value of

can then be divided by the known, constant concentration of the excess compound to obtain the true constant second order

The

pseudo-first order rate constant k'

can be also determined from the

half-life t _{1 / 2}

shivanshi asthana

Hot goIITian

Joined: 9 Mar 2010

Posts: 142

17 Mar 2010 14:24:50 IST

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thanks so much sir....

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