Question

Explain why molecularity and order of a reaction values are different for the following reaction:

\[\ce{CH3COOC2H5 + H2O -> CH3COOH + C2H5OH}\]

Answer 1

The given reaction is

\[\ce{CH3COOC2H5 + H2O -> CH3COOH + C2H5OH}\]

This is the hydrolysis of ethyl acetate, an ester, when water is present.

The quantity of reactant molecules that collide during a basic reaction step is referred to as molecularity. As an example, we have:

One ester molecule (CH₃COOC₂H₅)

One water molecule (H₂O)

Thus, the total number of molecules involved in the transition state is 2. Therefore, the molecularity of this reaction is:

Molecularity = 2

The total of the powers of the concentration terms in the rate law expression determines the sequence of a reaction. The ester’s rate law of hydrolysis can be written as

Rate = k(CH₃COOC₂H₅)¹(H₂O)¹

However, due to an excess of water, its concentration remains rather stable during the process. Therefore, we may regard it as a constant and redefine the rate rule as:

Rate = k'(CH₃COOC₂H₅)¹

Where k' = k(H₂O) is a new rate constant.

From the modified rate law, we see that the order of the reaction is determined solely by the ester concentration:
Order = 1

The molecularity is 2 because two molecules (the ester and water) are involved in the transition state. The order is 1 because the rate of the reaction depends only on the concentration of the ester, as water is in excess. Thus, the molecularity (2) and order (1) of the reaction are different due to the presence of excess water, which affects the rate law but not the molecularity.