In chemical kinetics, reaction order and molecularity are two key concepts that describe how reactions occur. Although they both deal with the number of reacting species, they are fundamentally different. However, in certain cases, they can be the same.
This content explores when reaction order and molecularity are equal, their definitions, differences, and real-world examples of such reactions.
Understanding Reaction Order
Definition of Reaction Order
The reaction order refers to the sum of the exponents of the reactant concentrations in the rate equation. It indicates how the concentration of reactants affects the reaction rate.
Mathematically, for a reaction:
The rate law is expressed as:
where:
- m and n are the reaction orders for reactants A and B.
- The overall reaction order is m + n .
Types of Reaction Orders
-
Zero-Order Reactions ( text{Rate} = k )
- The reaction rate is independent of reactant concentration.
-
First-Order Reactions ( text{Rate} = k[A] )
- The rate is directly proportional to the concentration of one reactant.
-
Second-Order Reactions ( text{Rate} = k[A]^2 or text{Rate} = k[A][B] )
- The rate depends on the square of one reactant or the product of two reactant concentrations.
-
Higher-Order Reactions ( text{Rate} = k[A]^m[B]^n , where m + n geq 3 )
- These are rare because multi-body collisions are unlikely.
Understanding Molecularity
Definition of Molecularity
Molecularity is the number of reactant molecules involved in an elementary reaction (a single-step reaction). It is always a whole number (1, 2, or 3).
Types of Molecularity
-
Unimolecular Reactions
- A single reactant molecule undergoes decomposition or rearrangement.
- Example:
text{N}_2text{O}_5 rightarrow text{NO}_2 + text{O}_2
-
Bimolecular Reactions
- Two reactant molecules collide to form products.
- Example:
text{NO} + text{O}_3 rightarrow text{NO}_2 + text{O}_2
-
Termolecular Reactions (Rare)
- Three molecules collide simultaneously to form products.
- Example:
2text{NO} + text{O}_2 rightarrow 2text{NO}_2
Note: Higher molecularity ( >3 ) is extremely rare due to the low probability of simultaneous collisions.
When Are Reaction Order and Molecularity the Same?
Reaction Order = Molecularity in Elementary Reactions
In an elementary reaction, the reaction occurs in one step, and the rate law is directly derived from the balanced equation.
For an elementary reaction:
- The reaction order is equal to the stoichiometric coefficients of the reactants in the balanced equation.
- The molecularity is also the number of reactant molecules involved.
Thus, reaction order = molecularity in elementary reactions.
Examples Where Reaction Order Equals Molecularity
-
Unimolecular Reaction (First-Order Reaction)
text{A} rightarrow text{Products}- Rate law: text{Rate} = k[A]
- Reaction order = 1
- Molecularity = 1
- Example: Decomposition of N2O5
text{N}_2text{O}_5 rightarrow 2text{NO}_2 + text{O}_2
-
Bimolecular Reaction (Second-Order Reaction)
text{A} + text{B} rightarrow text{Products}- Rate law: text{Rate} = k[A][B]
- Reaction order = 2 (1+1)
- Molecularity = 2
- Example: Reaction between NO and O3
text{NO} + text{O}_3 rightarrow text{NO}_2 + text{O}_2
-
Termolecular Reaction (Third-Order Reaction)
A + B + C rightarrow text{Products}- Rate law: text{Rate} = k[A][B][C]
- Reaction order = 3
- Molecularity = 3
- Example: Formation of NO2
2text{NO} + text{O}_2 rightarrow 2text{NO}_2
When Reaction Order and Molecularity Are Different
In complex (non-elementary) reactions, the reaction occurs in multiple steps, and the overall rate law is determined experimentally.
Example of a Non-Elementary Reaction
Consider the reaction:
- The molecularity (based on reactants) is 3 (termolecular).
- However, the rate law (determined experimentally) may be Rate = k[H2][O2], meaning the reaction order is 2.
Thus, for non-elementary reactions, reaction order ? molecularity.
Key Differences Between Reaction Order and Molecularity
| Feature | Reaction Order | Molecularity |
|---|---|---|
| Definition | Sum of exponents in rate law | Number of molecules colliding in a reaction step |
| Determined by | Experimentally from rate law | From balanced elementary reaction |
| Can be a fraction? | Yes | No, always a whole number |
| Can change with conditions? | Yes | No, fixed for a given reaction |
| Applies to | Overall reaction | Individual elementary steps |
Reaction order and molecularity are equal only for elementary reactions, where the rate law follows the stoichiometry of the reaction.
- In elementary reactions: Reaction order = Molecularity.
- In complex reactions: The overall reaction order is determined by the rate-determining step, which may differ from molecularity.
Understanding this distinction is crucial in chemical kinetics, as it helps in predicting reaction behavior and designing efficient chemical processes.
