Question

We have used the terms order of a reaction and molecularity of an elementary process (that is, unimolecular, bimolecular). What is the relationship, if any, between these two terms?

Short answer

In elementary reactions, the order and molecularity are the same, indicating the number of molecules participating in the reaction. However, for complex reactions that involve more than a single step, the order of the reaction does not necessarily match the sum of the molecularities of the elementary steps, as it is determined by the slowest (rate-determining) step.

Step-by-step solution

Clarify the terms

Before explaining the relationship, let's define the terms. The order of a reaction is the sum of the powers of the concentrations of the reactants in the rate equation of the reaction and it indicates how the reaction rate is affected by the concentrations of the reactants. On the other hand, molecularity of a reaction is defined as the number of molecules, atoms, or ions that must collide simultaneously to result in a chemical reaction. The terms for molecularity are unimolecular (one molecule), bimolecular (two molecules), and termolecular (three molecules).

Identify the correlation

In elementary reactions, which are reactions that occur in a single step, the order of a reaction matches its molecularity. For example, a unimolecular reaction is also first order, a bimolecular reaction is second order and so on.

Notice the exception

However, this correlation does not hold true for complex reactions which involve more than one step (known as reaction mechanism). In complex reactions, the overall order of the reaction does not necessarily equate to the sum of the molecularities of the elementary steps, since the order of the reaction is determined by the slowest (rate-determining) step, not the sum of all steps.

Key concepts

Understanding Molecularity

When we talk about molecularity in reactions, we're focusing on how many molecules come together in a single step of a chemical reaction. This concept is straightforward but crucial:

• Unimolecular: Involves a single molecule. This can lead to changes within the molecule itself, such as rearrangement or decomposition.
• Bimolecular: Involves the collision of two molecules, a common scenario in reactions.
• Termolecular: Involves three molecules colliding. This is less common due to the low probability of three particles colliding simultaneously.

Molecularity is all about what happens at the molecular level during these interactions. It's important to remember: molecularity is only applicable to a single, elementary reaction step. It doesn’t apply to the entire reaction if it involves several steps.

Identifying Elementary Reactions

Elementary reactions are the building blocks of more complex reactions. They occur in a single step, and here's what makes them special:

• In an elementary reaction, the molecularity directly corresponds to the reaction order. This means if two molecules are involved (bimolecular), it's a second-order reaction.
• These reactions happen as one event without intermediates, making them simple to analyze.

Understanding elementary reactions helps us see the fundamental events of a chemical process. They serve as a crucial foundation for studying complex reactions, where the reaction order is not so easily determined.

Exploring Reaction Mechanisms

Complex reactions often involve multiple elementary steps, and this is where reaction mechanisms come into play. Here's what happens:

• The reaction mechanism outlines all the steps in detail, describing how reactants transform into products.
• In these reactions, the overall order does not necessarily equal the sum of the steps' molecularities. Instead, the slowest step, known as the rate-determining step, dictates the reaction order.
• This is why you might see complex reactions where the order is different from what you'd expect based on the individual steps.

Reaction mechanisms provide a deeper understanding by showing us not just what happens, but how it happens. This insight allows chemists to predict and control reactions more effectively.