What is Molecularity? - Definition & Examples

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we discuss the difference between molecularity and order of a chemical reaction. We will look at several examples using the concept of elementary steps, where the two different quantities may or may not be equivalent.

Introduction to Molecularity

If you have ever tried to find a friend on a crowded street at lunchtime, you know that the experience can be frustrating. When you finally bump into your friend, there's a fair chance that you will spend at least a few seconds chatting. Maybe you even decide to go to lunch together.

Atoms and molecules are constantly bumping into each other during a chemical reaction, too. But, it's easier logistically for a single molecule to fall apart than for two molecules to bump into each other, though, just like it's easier for you to make a beeline toward a restaurant than to search for your friend first. Here's another way of saying it: the rate at which you can start eating is either dependent on just you, or on both you and your friend.

Let's go back to our molecules bumping into each other and introduce some terminology to describe chemical reaction rates and the number of molecules involved in the reaction.

Molecularity: Definition and Examples

For a simple chemical reaction that occurs in one step, the molecularity tells us how many molecules affect the rate of that reaction.

The Simplest Case: A Unimolecular Reaction

If it's just you going out for lunch, the speed at which you are able to find a restaurant depends on just one person: you.

Similarly, a single-step chemical reaction is said to have a molecularity of 1 if just one molecule transforms into products. We call this a unimolecular reaction. An example is the decomposition of N2 O4.

N2 O4 (g) → 2NO2 (g)

Reactions like this one are called first order because the reaction rate is proportional to the N2 O4, raised to the first power.

Getting More Complicated: Bimolecular Reactions

What about a more complicated example? Like getting lunch with a friend? Clearly, this situation is more difficult, especially because you must first find your friend, then decide on the venue together. If you meet your friend but you each refuse the other's lunch option, you may go your separate ways. But what if you instead bump into a different friend and convince him or her to go to lunch with you? Then, as you finish lunch, your initial friend texts you to set up coffee and dessert!

Similarly, chemicals don't always react when they bump into each other. In many reactions, multiple steps are required to get from reactants to products. Consider the decomposition of NO2 to form NO and O2.

2NO2 (g) → 2NO (g) + O2 (g)

This chemical reaction actually occurs in two steps.

Step 1:

2NO2 (g) → NO (g) + NO3 (g)

Step 2:

NO3 (g) → NO (g) + O2 (g)

These two steps are called elementary steps because they tell us exactly what happens as reactants transform into products in individual steps:

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