IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry - The hydrolysis of Ester is called PSendo first order reaction.

m.das

Why it is called so?

polymath

its psuedo first order with respect to water bcoz although its actually second order , but since water concentration remains almost unchanged we write a new rate constant assumin water concentration is constant. Not only this all solvolysis have psuedo order with respect to the solvent.

nivedh_89

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 c₀.

Therefore

Accordingly is obtained:

If the decrease in concentration of B 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
P₀= radiant power for radiation entering; P= radiant power for radiation leaving

gives the relationship between k' and lnA:

One needs only monitor the relative concentration of B as a function of time to obtain 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.

Ask & Discuss Questions with Community & Experts