R2.2.8 Molecularity (Higher Level Only)

Molecularity of an Elementary Step

What Is Molecularity?

Definition

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:

1. Unimolecular Steps

Definition: A single particle undergoes a chemical change without requiring a collision with another particle.

Example

The decomposition of ozone:
$O_{3} \to O_{2} + O$
In this step, one ozone molecule spontaneously breaks down into oxygen gas and an oxygen atom.

Tip

Unimolecular steps typically involve internal rearrangements or bond breaking within a single molecule, making them relatively common in reaction mechanisms.

2. Bimolecular Steps

Definition: Two particles collide to produce products.

Example

The reaction between nitrogen dioxide and carbon monoxide:
$N O_{2} + C O \to N O + C O_{2}$
Here, one molecule of $N O_{2}$ collides with one molecule of $C O$ , resulting in nitrogen monoxide and carbon dioxide.

Example

Think of a bimolecular step as a handshake—it requires two participants to come into contact for the interaction to occur.

3. Termolecular Steps

Definition: Three particles collide simultaneously to form products.

Example

The reaction of two nitric oxide molecules with oxygen gas:
$2 N O + O_{2} \to 2 N O_{2}$
This step involves three particles interacting at the same time.

Common Mistake

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.

Note

Termolecular steps are exceedingly rare due to the improbability of three particles colliding simultaneously with the correct orientation and energy.

Why Does Molecularity Matter?

Molecularity is closely tied to the rate law of an elementary step.
Since elementary steps occur in a single collision event, the rate law can be directly deduced from the molecularity:
- A unimolecular step has a rate law proportional to the concentration of the single reactant:
  $Rate = k [A]$
- A bimolecular step has a rate law proportional to the product of the concentrations of the two reactants:
  $Rate = k [A] [B]$
- A termolecular step, if it occurs, would have a rate law proportional to the product of the concentrations of all three reactants:
  $Rate = k [A] [B] [C]$

Tip

The slowest step in a reaction mechanism, known as the rate-determining step, often dictates the overall reaction rate. Understanding its molecularity is key to predicting the reaction rate law.

The Practical Significance of Molecularity

1. Predicting Reaction Mechanisms

Molecularity provides clues about the steps in a reaction mechanism. For example:

A first-order rate law suggests a unimolecular rate-determining step.
A second-order rate law suggests a bimolecular rate-determining step.

2. Understanding Reaction Rates

Molecularity explains why certain reactions are faster or slower:

Unimolecular reactions depend on the stability of the reactant molecule and are not limited by collisions.
Bimolecular reactions require collisions, so their rate depends on the concentrations of both reactants.
Termolecular reactions are slow because the likelihood of three-body collisions is very low.

Analogy

Think of molecularity like organizing a group activity. A one-person activity (unimolecular) is simple and quick to arrange. A two-person activity (bimolecular) requires coordination but is manageable. A three-person activity (termolecular) becomes significantly harder to organize due to the increased complexity.

Examples of Molecularity in Real Reactions

Unimolecular Reaction
- Reaction: The decomposition of dinitrogen pentoxide:
  $2 N_{2} O_{5} \to 4 N O_{2} + O_{2}$
- Elementary Step: $N_{2} O_{5} \to N O_{2} + N O_{3}$ (unimolecular)
Bimolecular Reaction
- Reaction: The reaction of hydrogen and iodine:
  $H_{2} + I_{2} \to 2 H I$
- Elementary Step: $H_{2} + I_{2} \to 2 H I$ (bimolecular)
Termolecular Reaction
- Reaction: The formation of ozone in the atmosphere:
  $O + O_{2} + M \to O_{3} + M$
- Elementary Step: $O + O_{2} + M \to O_{3} + M$ (termolecular, where $M$ is a third body that stabilizes the energy transfer).

Reflection

Self review

Can you identify the molecularity of the following elementary steps?

$C l_{2} \to 2 C l$
$N O + O_{3} \to N O_{2} + O_{2}$
$2 N O + O_{2} \to 2 N O_{2}$

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R2.2.8 Molecularity (Higher Level Only)

Molecularity of an Elementary Step

What Is Molecularity?

1. Unimolecular Steps

2. Bimolecular Steps

3. Termolecular Steps

Why Does Molecularity Matter?

The Practical Significance of Molecularity

1. Predicting Reaction Mechanisms

2. Understanding Reaction Rates

Examples of Molecularity in Real Reactions

Reflection

You've read 2/2 free chapters this week.

Introduction to Molecularity