### Study Notes: Reaction Mechanisms & Rate Law
Think of a chemical reaction not as a single teleportation from A to B, but as a journey with several layovers. This "journey" is the **reaction mechanism**—the granular, step-by-step path reactants take to become products.
#### 1. The Building Blocks: Elementary Steps
Most reactions don't happen in one big bang. They’re broken down into **elementary steps**, which are the actual individual molecular collisions taking place. We categorize these by their **molecularity**—basically, how many "players" are hitting each other at once:
*
**Unimolecular:** One lone molecule does its thing (A → P).
* **Bimolecular:** Two particles collide (A+A → P or A+B → P). This is the "bread and butter" of chemistry.
* **Termolecular:** Three particles hitting at the exact same time. This is incredibly rare—imagine three people trying to high-five at the same moment without missing.
#### 2. The Bottleneck: Rate-Determining Step (RDS)
This is the most critical concept for understanding speed. The **Rate-Determining Step** is the slowest step in the entire mechanism.
* **Why it matters:** It limits the speed of the whole process. Think of a traffic jam at a bridge; it doesn't matter how fast you drive before or after the bridge, the bridge determines when you get home.
*
**Energy Profile:** On a graph, the RDS is the step with the **highest peak** (the greatest activation energy barrier).
#### 3. Hidden Players: Intermediates vs. Catalysts
Both show up in the mechanism but are missing from the final balanced equation. However, they play very different roles:
* **Intermediates:** These are "middle-men." They are **produced** in one step and then **consumed** in a later one. They don't last.
* **Catalysts:** These are "helpers." They are present at the start, participate in the mechanism to lower the activation energy, and are **regenerated** at the end. They aren't "used up".
#### 4. The Rules of Engagement: Rate Law
The **rate law** tells us how concentration affects speed: .
*
**Crucial Distinction:** You cannot just look at a balanced equation and guess the rate law. The exponents (orders) must be found through experiments.
*
**Consistency:** For a mechanism to be "valid," it *must* match the experimental rate law. If your proposed mechanism says the speed depends on "A," but the lab data says it depends on "A and B," your mechanism is wrong.
#### 5. How do we actually prove this?
We can't always "see" these steps, so we use clues:
*
**Spectroscopy:** Catching a glimpse of an intermediate.
*
**Energy measurements:** Mapping out the peaks and valleys of the reaction.
*
**Rate Law Data:** The ultimate "litmus test" for any proposed pathway.
**What this really means:** Chemistry is less like a magic trick and more like an assembly line. By finding the slowest worker (RDS) and the temporary parts (intermediates), we can actually control how fast things happen.
