In chemical kinetics, two important terms often come up: reaction order and molecularity. While they may seem similar, they have different meanings.
✔ Reaction order refers to how the rate of a reaction depends on the concentration of reactants.
✔ Molecularity is the number of reactant molecules involved in an elementary step.
For most reactions, order and molecularity are not necessarily the same. However, there is a special case where they do match: elementary reactions.
This topic explores:
✔ What are order and molecularity?
✔ How do they differ?
✔ When are order and molecularity the same?
✔ Examples of such reactions
Understanding Reaction Order and Molecularity
1. What Is Reaction Order?
The order of a reaction tells us how the rate is affected by reactant concentrations. It is determined experimentally and is not always related to the balanced equation.
For a reaction:
The rate law is:
✔ m and n are the reaction orders for A and B.
✔ The overall reaction order is m + n.
Reaction order can be:
✔ Zero-order – Rate does not depend on reactant concentration.
✔ First-order – Rate is directly proportional to one reactant.
✔ Second-order – Rate depends on the square of one reactant or two different reactants.
2. What Is Molecularity?
Molecularity is the number of molecules colliding in an elementary step of a reaction.
✔ Unimolecular reaction – Involves one reactant molecule (e.g., decomposition of N₂O₅).
✔ Bimolecular reaction – Involves two reactant molecules (e.g., reaction of NO and O₂).
✔ Termolecular reaction – Involves three reactant molecules (rare).
Molecularity is always a whole number (1, 2, or 3) because it represents physical collisions.
3. Key Differences Between Order and Molecularity
| Feature | Reaction Order | Molecularity |
|---|---|---|
| Definition | Experimentally determined exponent in rate law | Number of molecules involved in an elementary step |
| Can it be fractional? | Yes | No (always a whole number) |
| Can it be zero? | Yes | No (minimum is 1) |
| How is it determined? | Experimentally | Based on reaction mechanism |
| Does it apply to overall reaction? | Yes | Only applies to elementary reactions |
When Are Order and Molecularity the Same?
Order and molecularity are the same only for elementary reactions.
✔ An elementary reaction is a reaction that occurs in a single step, without intermediates.
✔ In these cases, the rate law directly follows the balanced equation.
For example, in a bimolecular elementary reaction:
The rate law is:
✔ Here, the order is 1 + 1 = 2.
✔ The molecularity is also 2 (bimolecular reaction).
✔ Thus, order and molecularity are the same.
Conditions Where Order and Molecularity Match
1️⃣ The reaction must be elementary (single-step mechanism).
2️⃣ No intermediates or complex steps are involved.
3️⃣ The rate law must follow the stoichiometric equation.
Examples of Reactions Where Order = Molecularity
1. Unimolecular Reaction (First-Order Reaction)
✔ Molecularity = 1 (one reactant molecule).
✔ Order = 1 (rate law is text{Rate} = k[N₂O₅] ).
✔ : Order and molecularity are the same.
2. Bimolecular Reaction (Second-Order Reaction)
✔ Molecularity = 2 (two reactant molecules collide).
✔ Order = 2 (rate law is text{Rate} = k[NO]^2[O₂] ).
✔ : Order and molecularity are the same.
3. Termolecular Reaction (Third-Order Reaction, Rare Case)
✔ Molecularity = 3 (three molecules collide).
✔ Order = 3 (rate law is text{Rate} = k[NO]^2[Cl₂] ).
✔ : Order and molecularity are the same.
When Order and Molecularity Are NOT the Same
If a reaction occurs in multiple steps, its rate law does not follow the overall reaction equation. Instead, the slowest step (rate-determining step) controls the rate.
For example, in:
✔ The reaction occurs via multiple steps.
✔ The experimentally determined order is 1.5, which does not match its molecularity.
Frequently Asked Questions (FAQs)
1. Can molecularity ever be a fraction?
✔ No, molecularity is always a whole number (1, 2, or 3).
2. Why do complex reactions not have the same order and molecularity?
✔ Complex reactions occur in multiple steps, and their rate-determining step controls the reaction order.
3. What is the simplest reaction where order = molecularity?
✔ The decomposition of N₂O₅ ( N₂O₅ → NO₂ + O₂ ), where both order and molecularity are 1.
4. Why are termolecular reactions rare?
✔ Three molecules colliding simultaneously is unlikely, making such reactions less common.
5. How do I determine if a reaction is elementary?
✔ If the rate law matches the balanced equation, it is likely elementary. Otherwise, it follows a complex mechanism.
While reaction order and molecularity are different concepts, they match only for elementary reactions. In these reactions:
✔ Order is determined by reactant concentration in the rate law.
✔ Molecularity is the number of molecules colliding in a step.
✔ For unimolecular, bimolecular, and termolecular elementary reactions, order = molecularity.
Understanding this helps in chemical kinetics, reaction mechanisms, and industrial applications. Identifying whether a reaction is elementary or complex is crucial for predicting reaction behavior in real-world applications!