In chemical kinetics, molecularity and order are two important concepts used to describe reaction mechanisms. While they are often confused, they are not always the same. However, in certain cases, molecularity and order can be identical.
This topic explains when molecularity and order are the same, their definitions, differences, and examples of reactions where they coincide.
What Is Molecularity?
Definition of Molecularity
Molecularity refers to the number of reactant molecules involved in an elementary reaction step. It is always a whole number because it represents actual molecules colliding.
Types of Molecularity
-
Unimolecular Reaction (Molecularity = 1)
- Involves a single reactant molecule breaking down or rearranging.
- Example: Decomposition of ozone (O₃ → O₂ + O).
-
Bimolecular Reaction (Molecularity = 2)
- Involves two reactant molecules colliding to form products.
- Example: Formation of hydrogen iodide (H₂ + I₂ → 2HI).
-
Termolecular Reaction (Molecularity = 3)
- Involves three reactant molecules colliding simultaneously.
- Example: 2NO + O₂ → 2NO₂ (rare due to low probability of three molecules colliding at the same time).
What Is Order of Reaction?
Definition of Order of Reaction
The order of a reaction is the sum of the powers of concentration terms in the rate law equation. It is determined experimentally and can be a whole number, fraction, or even zero.
General Form of Rate Law
For a reaction:
A + B → Products
The rate law is:
text{Rate} = k [A]^m [B]^n
Where:
- m and n are reaction orders with respect to A and B.
- The overall order is m + n .
Types of Reaction Order
-
Zero-Order Reaction
- Rate is independent of reactant concentration.
- Example: Decomposition of ammonia on a platinum surface.
-
First-Order Reaction
- Rate depends on one reactant’s concentration.
- Example: Radioactive decay.
-
Second-Order Reaction
- Rate depends on two reactants’ concentrations or the square of one reactant.
- Example: Saponification of esters.
When Are Molecularity and Order the Same?
Molecularity and order are the same when:
- The reaction is elementary (occurs in a single step).
- There are no complex reaction mechanisms or intermediates.
- The rate law follows directly from the stoichiometry of the reactants.
Examples of Reactions Where Molecularity = Order
1. Unimolecular Reaction (First-Order Reaction)
Example: Decomposition of nitrogen pentoxide
text{N₂O₅} → text{NO₂} + text{O₂}
- Molecularity = 1 (one molecule of N₂O₅ decomposes).
- Rate law: Rate = k[N₂O₅]
- Order = 1 → Molecularity = Order
2. Bimolecular Reaction (Second-Order Reaction)
Example: Formation of hydrogen iodide
H₂ + I₂ → 2HI
- Molecularity = 2 (two molecules collide).
- Rate law: Rate = k[H₂][I₂]
- Order = 2 → Molecularity = Order
3. Termolecular Reaction (Third-Order Reaction)
Example: Formation of NO₂
$2NO + O₂ → 2NO₂$
- Molecularity = 3 (three molecules must collide).
- Rate law: Rate = k[NO]²[O₂]
- Order = 3 → Molecularity = Order
When Molecularity and Order Are Different
Molecularity and order differ when:
- The reaction occurs in multiple steps (complex reaction).
- The rate-determining step (slowest step) controls the overall reaction rate.
- There are catalysts or intermediate species.
Example of Different Molecularity and Order
Decomposition of Hydrogen Peroxide
$2H₂O₂ → 2H₂O + O₂$
- Experimental rate law: Rate = k[H₂O₂]
- Order = 1, but Molecularity = 2 (two molecules of H₂O₂ are involved in stoichiometry).
- The reaction proceeds via a two-step mechanism, with iodide ions as a catalyst.
Key Differences Between Molecularity and Order
| Feature | Molecularity | Order of Reaction |
|---|---|---|
| Definition | Number of molecules involved in an elementary reaction step. | Sum of concentration exponents in the rate law. |
| Value Type | Always a whole number (1, 2, or 3). | Can be whole, fractional, or zero. |
| Determined By | Theoretical (based on reaction mechanism). | Experimental observation. |
| Applies To | Only elementary reactions. | Both elementary and complex reactions. |
Molecularity and order are identical only in elementary reactions where the rate law follows directly from the reaction mechanism. However, in complex reactions, molecularity and order can be different due to multi-step mechanisms and rate-determining steps. Understanding these concepts is crucial for predicting reaction behavior and kinetics.