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.
- 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.
- 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.
- 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.
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:
$$\text{Rate} = k[A]$$ - A bimolecular step has a rate law proportional to the product of the concentrations of the two reactants:
$$\text{Rate} = k[A][B]$$ - A termolecular step, if it occurs, would have a rate law proportional to the product of the concentrations of all three reactants:
$$\text{Rate} = k[A][B][C]$$
- A unimolecular step has a rate law proportional to the concentration of the single reactant:
- 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
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.
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.
- Unimolecular Reaction
- Reaction: The decomposition of dinitrogen pentoxide:
$$2N_2O_5 \rightarrow 4NO_2 + O_2$$ - Elementary Step: $N_2O_5 \rightarrow NO_2 + NO_3$ (unimolecular)
- Reaction: The decomposition of dinitrogen pentoxide:
- Bimolecular Reaction
- Reaction: The reaction of hydrogen and iodine:
$$H_2 + I_2 \rightarrow 2HI$$ - Elementary Step: $H_2 + I_2 \rightarrow 2HI$ (bimolecular)
- Reaction: The reaction of hydrogen and iodine:
- Termolecular Reaction
- Reaction: The formation of ozone in the atmosphere:
$$O + O_2 + M \rightarrow O_3 + M$$ - Elementary Step: $O + O_2 + M \rightarrow O_3 + M$ (termolecular, where $M$ is a third body that stabilizes the energy transfer).
- Reaction: The formation of ozone in the atmosphere:
Can you identify the molecularity of the following elementary steps?
- $Cl_2 \rightarrow 2Cl$
- $NO + O_3 \rightarrow NO_2 + O_2$
- $2NO + O_2 \rightarrow 2NO_2$
