Pseudo Unimolecular Reaction

 What is a Pseudo Unimolecular Reaction?

A pseudo unimolecular reaction is a chemical reaction that is actually a second-order reaction(or higher order) but behaves like a first-order reaction under specific experimental conditions.

 Condition:

This behavior occurs when one of the reactants involved in the reaction is present in a **very large excess** compared to the other reactant(s).

**Why it Behaves as First-Order:**

1. Since one reactant is present in a huge excess, its concentration changes very little during the course of the reaction.

2. Because its concentration remains virtually **constant**, it can be mathematically absorbed into the rate constant ($k$).

3. This makes the overall reaction rate appear to depend only on the concentration of the *other*, limiting reactant, thus giving it the appearance of a first-order reaction.

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 Example: Acid Hydrolysis of an Ester

The best-known example is the **acid-catalyzed hydrolysis of an ester** (like Ethyl Acetate):

{Ethyl Acetate} + {Water} ——> {Acid Catalyst (H}^+) {Acetic Acid} + {Ethanol}

**Explanation of the Example:**

1. **Actual Rate Law (Second-Order):** Theoretically, the rate of this reaction depends on the concentration of *both* the ester and the water:

    $$\text{Rate} = k [\text{CH}_3\text{COOC}_2\text{H}_5] [\text{H}_2\text{O}]$$

2. **Pseudo First-Order Behavior:** In the experiment, **water ($\text{H}_2\text{O}$) is used in large excess**.

    * Because there is so much water, its concentration barely decreases as the reaction proceeds.

    * Therefore, $[\text{H}_2\text{O}]$ is treated as a constant.

3. **Observed Rate Law (First-Order):** The constant concentration of water is incorporated into a new, effective rate constant, $k'$ (the pseudo rate constant):

    $$\text{Rate} = k' [\text{CH}_3\text{COOC}_2\text{H}_5]$$

    $$\text{where } k' = k [\text{H}_2\text{O}]_{\text{constant}}$$

Because the rate now only depends on the concentration of the ester raised to the power of one, the reaction is observed to follow **first-order kinetics**.