Molecularity of an Elementary Step
What Is Molecularity?
Molecularity
The molecularity of an elementary step refers to the number of reacting particles (atoms, ions, or molecules) that must collide simultaneously to drive a chemical change.
It’s a theoretical concept that applies only to individual steps in a reaction mechanism, not the overall reaction.
Here’s how molecularity is classified:
Unimolecular Steps
A single particle undergoes a chemical change without requiring a collision with another particle.
Example- The decomposition of ozone:
$$O_3 \rightarrow O_2 + O$$ - In this step, one ozone molecule spontaneously breaks down into oxygen gas and an oxygen atom.
Unimolecular steps typically involve internal rearrangements or bond breaking within a single molecule, making them relatively common in reaction mechanisms.
Bimolecular Steps
Two particles collide to produce products.
Example- The reaction between nitrogen dioxide and carbon monoxide:
$$NO_2 + CO \rightarrow NO + CO_2$$ - Here, one molecule of $NO_2$ collides with one molecule of $CO$, resulting in nitrogen monoxide and carbon dioxide.
Think of a bimolecular step as a handshake: it requires two participants to come into contact for the interaction to occur.
Termolecular Steps
Three particles collide simultaneously to form products.
Example- The reaction of two nitric oxide molecules with oxygen gas:
$$2NO + O_2 \rightarrow 2NO_2$$ - This step involves three particles interacting at the same time.
- Many students mistakenly equate molecularity with stoichiometry.
- Remember, molecularity refers to the number of particles involved in a single elementary step, whereas stoichiometry reflects the overall balanced equation for the reaction.
Termolecular steps are exceedingly rare due to the improbability of three particles colliding simultaneously with the correct orientation and energy.
