Molecularity of Reaction
Molecularity of Reaction
Understanding Molecularity
Molecularity of a reaction refers to the number of molecules, atoms, or ions that must collide simultaneously with proper orientation to bring about a chemical reaction. It is a theoretical concept that applies to elementary reactions, which are reactions that occur in a single step. Molecularity is always a whole number and cannot be zero. It is determined by the reaction mechanism and cannot be altered by changing the reaction conditions.
Types of Molecularity
There are three main types of molecularity:
Unimolecular Reactions: These involve a single molecule undergoing a rearrangement or decomposition to produce the reaction products. The molecularity of such reactions is one.
Bimolecular Reactions: These involve the collision between two reactant species. The molecularity of such reactions is two.
Termolecular Reactions: These are relatively rare and involve the simultaneous collision of three reactant species. The molecularity of such reactions is three.
Higher molecularity reactions (four or more reactant species) are extremely rare because the probability of such simultaneous collisions is very low.
Differences and Important Points
| Aspect | Molecularity | Order of Reaction |
|---|---|---|
| Definition | The number of reactant particles that collide simultaneously in an elementary reaction step. | The sum of the powers of the concentration terms in the rate law expression. |
| Determination | Determined by the reaction mechanism and is a theoretical concept. | Determined experimentally from the rate law. |
| Values | Must be a whole number (1, 2, or 3). Cannot be zero or a fraction. | Can be zero, a whole number, or a fraction. |
| Dependency | Does not depend on the concentration of reactants or reaction conditions. | Depends on the concentration of reactants and can change with different reaction conditions. |
| Applicability | Only applicable to elementary reactions. | Applicable to both elementary and complex reactions. |
| Example | A unimolecular reaction: $A \rightarrow products$ (Molecularity = 1) | First-order reaction: Rate = $k[A]$ (Order = 1) |
| A bimolecular reaction: $A + B \rightarrow products$ (Molecularity = 2) | Second-order reaction: Rate = $k[A][B]$ (Order = 2) | |
| A termolecular reaction: $2A + B \rightarrow products$ (Molecularity = 3) | Third-order reaction: Rate = $k[A]^2[B]$ (Order = 3) |
Formulas and Examples
Unimolecular Reaction Example
A common example of a unimolecular reaction is the isomerization of cyclopropane to propene:
$$ \text{Cyclopropane} \rightarrow \text{Propene} $$
The molecularity of this reaction is 1 because it involves the rearrangement of a single molecule.
Bimolecular Reaction Example
An example of a bimolecular reaction is the reaction between hydrogen and iodine to form hydrogen iodide:
$$ H_2 + I_2 \rightarrow 2HI $$
The molecularity of this reaction is 2 because it involves the collision between one molecule of hydrogen and one molecule of iodine.
Termolecular Reaction Example
An example of a termolecular reaction, although rare, is the reaction between two nitric oxide molecules and one oxygen molecule to form two nitrogen dioxide molecules:
$$ 2NO + O_2 \rightarrow 2NO_2 $$
The molecularity of this reaction is 3 because it involves the simultaneous collision of two nitric oxide molecules with one oxygen molecule.
Conclusion
Molecularity is a fundamental concept in chemical kinetics that describes the number of reactant species involved in an elementary reaction step. It is a theoretical value that helps chemists understand the mechanism of a reaction. Unlike the order of a reaction, which can be determined experimentally and may vary with reaction conditions, molecularity is fixed for a given elementary reaction and provides insight into the reaction pathway.