# in psuedo first order reacn of acid hydrolysis of ester does rate depend on ph?

SAGAR SINGH - IIT DELHI
878 Points
13 years ago

Dear neha,

This is a second-order reaction in which one of the reactants is present in such great amounts that its effect is not seen. For example, the hydrolysis of an ester in water. If the water is present in great amounts, any change will be too small, relatively, to be noticed. The reaction then appears dependent on concentration of ester alone. To see the dependence on water, you'd have to dissolve the ester in acetone with a few % water.

For example we have a reaction

A + B ==> Product

the rate of reaction is .....

Rate= dP/dt .... which is also the same as ....

Rate = K.[A].[B]

The total order of reaction will equal to the sum of the orders of the reactants A + B = 1+1 = 2 ... so the above reaction is a second order reaction.

Now If we converted one of those reactants to be in excess concentration in the system, then we can get a pseudo first order reaction

example
ConcA = 0.1M
excess ConcB = 1M

after the reaction [A] = 0 and [B] = 0.99M

so reactant B wont contribute to the total order as it is in excess and viewed as a constant, this is known as pseudo first order.

so the new rate = K'.[A]

and the total order is = to the sum of the orders which is = to 1, as [A] is the only reactant contributing to the rate.

K' = K.[B]

but member even though [B] is assumed as a constant any large changes in [B] will effect K' and therefore effect the rate.

All the best.

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