AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/5/5-7)
Unit 5 · Topic 5.7
5.7 Introduction to Reaction Mechanisms
A reaction mechanism is a series of elementary steps that add up to the overall reaction. An intermediate is made in one step and used up in a later one, while a catalyst is used up in one step and made again later; neither appears in the overall equation.
Key terms
- reaction mechanism
- elementary step
- intermediate
- catalyst
What a mechanism is
A balanced overall equation tells you what you start and end with, but not how the atoms get there. A mechanism is a proposed sequence of elementary steps (topic 5.4) that explains the path. Each step is a single collision or a single particle breaking apart.
For a mechanism to be acceptable, its steps must add up to the overall balanced equation. Later topics add a second test: it must also agree with the experimental rate law.
The parts of a mechanism
An intermediate exists only while the reaction is happening. It's made and then consumed, so it cancels when you add the steps. A catalyst is there at the start, is used up, and is made again, so it also cancels.
| Species | First appears as | Later | In overall equation? |
|---|---|---|---|
| Reactant | reactant in a step | (used up) | yes, on the left |
| Product | product in a step | (stays) | yes, on the right |
| Intermediate | product of an earlier step | used up in a later step | no |
| Catalyst | reactant in an earlier step | regenerated in a later step | no |
Adding steps
To check a mechanism, write all the steps, add everything on the left and everything on the right, and cancel any species that appears on both sides. If a step needs to happen twice, multiply it before adding. What's left should be the overall equation exactly.
What makes a mechanism plausible
Use these tests. A mechanism that fails any one of them can be ruled out. One that passes all of them is a reasonable model, though another mechanism might pass them too.
- Its elementary steps add up exactly to the overall balanced equation.
- Each step involves one or two particles, or very rarely three.
- The rate law predicted by the mechanism agrees with the rate law measured in the lab (topics 5.8 and 5.9).
- Any intermediates it proposes are reasonable species and, ideally, have been detected.
Evidence for mechanisms
Chemists can never watch a mechanism directly, so a mechanism is a model supported by evidence. One strong form of evidence is detecting an intermediate, for example by spotting its color or its spectrum while the reaction runs. If a proposed mechanism predicts an intermediate and it's found, that supports the mechanism over alternatives that don't include it.
You need to understand why detecting an intermediate is useful evidence, but you won't be asked to collect or analyze the data used to detect one.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Finding the overall reaction and the intermediate
A proposed mechanism is: Step 1: NO₂ + NO₂ → NO₃ + NO. Step 2: NO₃ + CO → NO₂ + CO₂. Find the overall reaction and identify any intermediates.
Show the solutionHide the solution
- Step 1: Add the steps: 2NO₂ + NO₃ + CO → NO₃ + NO + NO₂ + CO₂.
- Step 2: Cancel NO₃ (both sides) and one NO₂ (both sides).
- Step 3: Overall: NO₂ + CO → NO + CO₂.
- Step 4: NO₃ is made in step 1 and used in step 2, so it's an intermediate.
Answer: Overall: NO₂ + CO → NO + CO₂; intermediate: NO₃
- Example 2
Telling a catalyst from an intermediate (classic trap)
In the upper atmosphere, ozone can be destroyed by this mechanism: Step 1: Cl + O₃ → ClO + O₂. Step 2: ClO + O → Cl + O₂. Identify the overall reaction, the catalyst and the intermediate.
Show the solutionHide the solution
- Step 1: Add: Cl + O₃ + ClO + O → ClO + O₂ + Cl + O₂. Cancel Cl and ClO.
- Step 2: Overall: O₃ + O → 2O₂.
- Step 3: Cl is a reactant in step 1 and is regenerated in step 2: catalyst.
- Step 4: ClO is produced in step 1 and consumed in step 2: intermediate.
- Step 5: Both cancel out, so look at the order: which one shows up first as a reactant (catalyst) and which first as a product (intermediate).
Answer: Overall O₃ + O → 2O₂; Cl is the catalyst; ClO is the intermediate.
Common mistakes
- Mixing up intermediates and catalysts. Intermediates are made first, then used; catalysts are used first, then remade.
- Leaving intermediates in the overall equation.
- Forgetting to multiply a step that must happen twice before adding.
- Thinking a mechanism is proven because its steps add up correctly. It also has to match the rate law, and it can only be supported, never proven.
On the exam
- Expect to identify intermediates and catalysts from a given mechanism, and to show that the steps add up to the overall equation.
- Questions may ask what evidence would support one mechanism over another. Detecting an intermediate that only one mechanism predicts is a strong answer.
Connected topics
Videos
Check yourself
4 questions on 5.7 Introduction to Reaction Mechanisms. Pick an answer to see if you got it, and why.
The following mechanism is proposed for a reaction between NO₂ and CO at low temperature.
Step 1 (slow): NO₂(g) + NO₂(g) → NO₃(g) + NO(g)
Step 2 (fast): NO₃(g) + CO(g) → NO₂(g) + CO₂(g)
Proposed reaction mechanism
What is the overall balanced equation for the reaction?
Which species is an intermediate in this mechanism?
Hydrogen peroxide decomposes slowly on its own: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). Adding a little KI makes it decompose quickly. A proposed mechanism is shown below.
Step 1 (slow): H₂O₂(aq) + I⁻(aq) → H₂O(l) + IO⁻(aq)
Step 2 (fast): H₂O₂(aq) + IO⁻(aq) → H₂O(l) + O₂(g) + I⁻(aq)
Proposed reaction mechanism
Which of the following correctly identifies the roles of I⁻ and IO⁻ in this mechanism?
The overall reaction 2NO₂Cl(g) → 2NO₂(g) + Cl₂(g) is thought to occur by the following mechanism.
Step 1 (slow): NO₂Cl(g) → NO₂(g) + Cl(g)
Step 2 (fast): NO₂Cl(g) + Cl(g) → NO₂(g) + Cl₂(g)
Proposed reaction mechanism
Which species is an intermediate in this mechanism?
0 of 4 answered