Introduction to Reaction Mechanisms
Sum a balanced mechanism and distinguish intermediate, catalyst and transition state.
AP Chemistry Topic 5.7 · CED printed p. 101 / PDF p. 108. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations and stoichiometric ratios
- Molarity and concentration versus amount
- Bond rearrangement and energy conservation
- Slopes, logarithms and reciprocal graphs
A mechanism is a proposed sequence of elementary events that connects starting reactants to products. Write each step as a real atom-and-charge-balanced event. Label any species consumed first and later regenerated as a catalyst, and any species formed during the sequence and then consumed as an intermediate. Those species can cancel from the net equation, but they have different roles.
Add all elementary steps algebraically. Cancel a species that appears in equal amounts on both sides; the remaining equation must match the observed overall reaction, including coefficients. A mechanism that does not sum correctly is inconsistent before any rate analysis begins. A correct sum is necessary, but many different sequences can share it.
An intermediate is produced in one step and consumed in another, so it may be present transiently even though absent from the net equation. It occupies a local-energy minimum between transition states. Do not label every canceled species an intermediate: a catalyst begins available and is restored. A transition state is not a chemical species that normally accumulates between steps.
If independent supplied evidence detects a proposed intermediate, that supports a mechanism containing it. If another proposed route does not predict that species, the observation can help discriminate between routes. Detection alone does not prove that every other step is correct or that no other mechanism exists. AP practice uses given evidence; designing or collecting intermediate-detection data is explicitly excluded.
Let's work through it
Proposed steps: NO2 + NO2 → NO3 + NO; then NO3 + CO → NO2 + CO2.
- NO3 is produced then consumed; it is an intermediate.
- Adding cancels NO3 and one NO2, leaving NO2 + CO → NO + CO2.
- The canceled NO2 is also a net reactant because two were initially consumed and one returned; cancellation alone does not define a catalyst. Supplied detection of NO3 would support this route, while still leaving its rate predictions to be tested.
Mechanism M predicts intermediate I; mechanism N predicts no I. A supplied composition record detects transient I before product accumulation.
- The observation is consistent with M’s prediction and provides support over N’s stated form.
- It does not establish every step of M or rule out every other route containing I.
- Use the supplied record; collecting intermediate-detection data is excluded from AP assessment.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Does a correct sum prove a mechanism?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Does a correct sum prove a mechanism?
Worked answer and reason
No. It must also agree with kinetic and other evidence; another sequence may give the same net equation.
The net equation constrains a mechanism but does not specify it.
Retrieval 2
How does a catalyst differ from an intermediate?
Worked answer and reason
A catalyst is initially available, consumed and restored; an intermediate is made within the route and later consumed.
The order of production/consumption identifies their roles.
Catch the tempting mistake
- Every species canceled is a catalyst.
- Check whether it is formed then consumed or initially consumed then regenerated, and whether any net amount remains.
- An undetected intermediate cannot exist.
- It may be short-lived or below a detection limit; supplied positive/negative evidence has limits.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 14 · ap synthesis · symbolic
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Scientific calculator
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For the teacher
- Require both a mechanism sum and species-role ledger.
- Use supplied detection findings rather than asking students to devise intermediate-detection experiments.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.7.A
- Identify the components of a reaction mechanism.
- 5.7.A.1
- A reaction mechanism consists of a series of elementary reactions, or steps, that occur in sequence. The components may include reactants, intermediates, products, and catalysts.
- 5.7.A.2
- The elementary steps when combined should align with the overall balanced equation of a chemical reaction.
- 5.7.A.3
- A reaction intermediate is produced by some elementary steps and consumed by others, such that it is present only while a reaction is occurring.
- 5.7.A.4
- Experimental detection of a reaction intermediate is a common way to build evidence in support of one reaction mechanism over an alternative mechanism.
- Collection of data pertaining to detection of a reaction intermediate will not be assessed on the AP Exam.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Reaction Mechanisms — Scientific check: intermediates, elementary molecularity, slow steps and rapid-equilibrium substitution.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Stoichiometry
Stoichiometry uses a balanced chemical equation to relate amounts of reactants and products. Equation coefficients provide mole ratios rather than direct mass ratios. For example, forming water from hydrogen and oxygen consumes two moles of hydrogen molecules per mole of oxygen molecules.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Catalyst
A catalyst participates in a reaction pathway and is regenerated overall. It can increase the reaction rate by providing a different pathway without changing the reaction's equilibrium constant at fixed temperature. For example, a catalyst can help a mixture reach equilibrium sooner without changing the final equilibrium composition.
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Reaction intermediate
A reaction intermediate is produced in one mechanism step and consumed in another. It cancels from the overall equation and differs from a catalyst, which is present before it is consumed and then regenerated. For example, a short-lived species formed in step one and used in step two can be an intermediate.
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Elementary reaction
An elementary reaction is one step in a proposed mechanism rather than the sum of multiple steps. For a specified elementary step, the rate-law concentration factors follow its reacting-particle event. For example, a bimolecular elementary collision can have a rate proportional to the concentrations of its two reactants.
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Transition state
A transition state is the high-energy configuration at a barrier along a reaction pathway. It is not a stable intermediate that accumulates as a separately observable species in the basic model. For example, the top of an elementary step's energy-profile peak represents its transition state.