Question

Explain molecularity and order of a reaction with the help of an example.

Hint:

Molecularity counts colliding species in one elementary step (whole number, theoretical); order is the sum of exponents in the experimental rate law (can be fractional or zero). Compare using ester hydrolysis (pseudo first order).

Answer 1

Step 1: Molecularity. Molecularity is the number of reacting species (atoms, ions or molecules) that must collide simultaneously in a single elementary step to bring about the reaction. It is a theoretical idea, is always a whole number (1, 2 or 3), can never be zero or fractional, and is defined only for an elementary (one-step) reaction.
• Unimolecular (molecularity 1): decomposition of ammonium nitrite, NH₄NO₂ → N₂ + 2H₂O.
• Bimolecular (molecularity 2): 2HI → H₂ + I₂.

Step 2: Order of reaction. Order is the sum of the powers to which the concentration terms are raised in the experimentally determined rate law. It is found by experiment, not from the balanced equation, and may be zero, a whole number, or even a fraction.
For a reaction with rate law
\[ \text{rate} = k[A]^x[B]^y \]
the order = \(x + y\).

Step 3: Worked example. For the elementary reaction
\[ NO_2 + CO \rightarrow NO + CO_2 \]
two molecules collide in one step, so its molecularity = 2, and the rate law rate = k[NO₂][CO] gives order = 1 + 1 = 2. Here order and molecularity match because the reaction is a single step.

Step 4: Where they differ. For the acidic hydrolysis of an ester,
\[ CH_3COOC_2H_5 + H_2O \xrightarrow{H^+} CH_3COOH + C_2H_5OH \]
two species (ester and water) react, so molecularity = 2, but because water is in large excess its concentration is effectively constant, the observed rate = k'[ester], and the order = 1. Such reactions are called pseudo first order. This shows molecularity (theoretical, from the mechanism) and order (experimental) need not be equal.