Topic 5.7 Notes – Introduction to Reaction Mechanisms

1. What a Reaction Mechanism Is

An overall balanced equation tells you the starting materials and final products. A reaction mechanism tells you the sequence of elementary steps that get you there.

Most reactions do not happen in one collision. They happen in multiple small steps.

Elementary steps

An elementary step is:

• A single molecular event
• Written exactly as it occurs (what particles collide or rearrange)
• Associated with its own rate constant and activation energy

When you add all elementary steps together:

• Species that appear on both sides cancel
• The result must match the overall balanced equation

If the steps don’t add up to the net equation, the mechanism cannot be correct.

2. Components of a Reaction Mechanism

When you’re given a mechanism, look for four possible types of species.

Reactants

• Present at the beginning
• Appear in the first step
• Appear in the overall balanced equation

Products

• Formed in the final step
• Appear in the overall balanced equation

Intermediates

An intermediate is:

• Produced in one step
• Consumed in a later step
• Present only while the reaction is happening

How to identify one:

• It appears as a product in one elementary step
• It appears as a reactant later
• It does not appear in the overall equation

Intermediates cancel when steps are added together.

Experimental detection of an intermediate supports a proposed mechanism. On AP, you won’t design detection methods, but you should understand that finding an intermediate strengthens evidence for a mechanism.

Catalysts

A catalyst:

• Is consumed in an early step
• Is regenerated in a later step
• Does not appear in the overall equation

It speeds up the reaction by providing an alternative pathway.

Here’s the clean comparison you should be able to recognize instantly:

Students often mix these up. The direction they appear in the steps is the giveaway.

3. How Elementary Steps Combine to Form the Overall Reaction

When checking a mechanism, do this mentally every time:

1. Add all elementary steps together.
2. Cancel species that appear on both sides.
3. Confirm the result matches the given overall equation exactly.

For example, if you had:

$$\ce{A + B -> C}$$ $$\ce{C + D -> E}$$

Adding gives:

$$\ce{A + B + C + D -> C + E}$$

Cancel C:

$$\ce{A + B + D -> E}$$

That must match the stated net reaction.

On tests, they often give you a mechanism and quietly make one coefficient wrong. If it doesn’t reproduce the overall reaction, it’s invalid.

4. Rate-Determining Step and the Overall Rate Law

In multi-step reactions, one step is slower.

The Rate-Determining Step (RDS)

The rate-determining step:

• Is the slowest elementary step
• Has the largest activation energy
• Controls the overall rate

The overall reaction can only proceed as fast as this slowest step.

Finding the Rate Law from a Mechanism

If the slow step is an elementary step, you can use its coefficients directly as exponents.

Example slow step:

$$\ce{X + 2Y -> Z}$$

Rate law:

$$\text{Rate} = k[X][Y]^{2}$$

This only works for elementary steps, never for overall equations.

A common mistake is using coefficients from the overall balanced equation. That only works if the reaction is truly one elementary step, which most are not.

When the Slow Step Contains an Intermediate

You cannot include intermediates in a final rate law.

If the slow step includes an intermediate:

• Use a fast equilibrium step to write an expression for that intermediate
• Substitute it into the rate expression
• Eliminate the intermediate

On AP, you are expected to understand the logic, but not perform heavy algebra.

5. Big-Picture Connections

• Mechanisms explain why a reaction has a particular rate law.
• Different mechanisms can give the same overall equation but different rate laws.
• Detecting an intermediate supports one mechanism over another.
• Catalysts change the pathway, not the final products.

When you see a mechanism question, immediately look for:

• Species that cancel
• The slow step
• Whether the derived rate law contains only reactants (and possibly catalysts)

That logic shows up more than long calculations.