How do elementary steps combine into one reaction?
You will be able to: Sum a supplied mechanism and cancel species without losing atom balance.
How do elementary steps combine into one reaction?
A delivery can travel through an intermediate stop even though the final receipt shows only its origin and destination. A mechanism records the steps that an overall reaction equation leaves out.
A useful starting point: Is the energy barrier the same as the reaction energy change? →
Words and symbols before equations
- Mechanism
- Proposed sequence of elementary reactions.
- Intermediate
- Species formed in one step and consumed in a later step.
- Overall equation
- Net result after combining and canceling matching species.
What this picture assumes
Supplied proposed NO₂/CO mechanism. Elementary steps conserve atoms. A correct sum is necessary but not proof of a unique actual mechanism.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Step 1: 2NO₂ → NO₃ + NO. Step 2: NO₃ + CO → NO₂ + CO₂. NO₃ forms, then is consumed.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For the supplied steps 2NO₂ → NO₃+NO and NO₃+CO → NO₂+CO₂, add every reactant and product before canceling.
NO₃ is formed then consumed, so it cancels. One NO₂ also appears on each side and cancels, leaving NO₂+CO → NO+CO₂.
Cancellation removes bookkeeping terms, not actual events. The intermediate can exist during the reaction even though it is absent from the net equation.
A correct sum is necessary but not sufficient to validate a mechanism. Its predicted rate law and other evidence must also agree with observations; more than one mechanism can fit limited data.
A worked example, step by step
Add the proposed steps Cl+O₃ → ClO+O₂ and ClO+O → Cl+O₂. Give the overall equation.
- List the left terms Cl+O₃+ClO+O and right terms ClO+O₂+Cl+O₂.
- Cancel one Cl and one ClO from opposite sides.
- The net result is O₃+O → 2O₂.
- Check four O atoms before and after. The canceled species still participate in the individual steps.
A balanced sum does not prove a proposed mechanism is the unique actual pathway.
Does canceling an intermediate mean it was never present?
Compare with an explanation
No. It forms and is consumed during the sequence, so it has no net coefficient.
Predict. Change one thing. Explain.
Switch between separate steps and their uncanceled sum, then the net equation. Track exactly one canceled NO₂ and the intermediate NO₃.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Step 1: 2NO₂ → NO₃ + NO. Step 2: NO₃ + CO → NO₂ + CO₂. NO₃ forms, then is consumed.
Supplied proposed NO₂/CO mechanism. Elementary steps conserve atoms. A correct sum is necessary but not proof of a unique actual mechanism.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using concentration–time slopes, rate-law dependence, encounter geometry or the stated mechanism. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionCombine A+B → I and I+B → AB₂, where I contains one A and one B. Give the net equation, identify I, check atom counts and state one additional test of the mechanism.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Net equation A+2B → AB₂.
- 1 point: I is an intermediate formed then consumed.
- 1 point: One A and two B units are conserved.
- 1 point: Compare predicted rate law with measured rates or other valid mechanistic evidence; balance alone is insufficient.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Why cancel an intermediate?
It has zero net production in a completed reaction sequence.
RECALL 2What must remain after summing?
The balanced overall chemical equation.
RECALL 3Does one matching result prove uniqueness?
No; multiple mechanisms may fit limited evidence.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do elementary steps combine into one reaction?
- Add steps → cancel identical species → check atoms and charge.
- A mechanism must reproduce the overall reaction and kinetic evidence.
Remember: A balanced sum does not prove a proposed mechanism is the unique actual pathway.
Conditions: Supplied proposed NO₂/CO mechanism. Elementary steps conserve atoms. A correct sum is necessary but not proof of a unique actual mechanism.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.7, objective 5.7.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 5: Kinetics, Topics 5.1–5.11. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Arrhenius calculations are not assessed in the current AP framework; temperature and activation energy are taught qualitatively here. Collection of intermediate-detection data is not assigned. Integrated rate laws explicitly use the monitored species’ disappearance constant, while event and normalized reaction rates are labeled separately. Pre-equilibrium models state their timescale assumptions and use free concentrations. Original illustrative data and geometry are not measured kinetics.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
Want to work through this with a tutor?
Bring your question about How do elementary steps combine into one reaction? Your explanation and answers remain free to access.