Which way does a reversible reaction move overall?
You will be able to: Calculate net change from competing forward and reverse rates.
Which way does a reversible reaction move overall?
At a doorway, eight people entering and five leaving each minute means three more people inside per minute. Reversible reactions also require subtracting opposing changes, while respecting the chemical coefficients.
A useful starting point: What do concentration and rate graphs show near equilibrium? →
Words and symbols before equations
- Net rate
- Forward contribution minus reverse contribution for the same convention.
- Reaction direction
- Toward products or reactants as the equation is written.
- Stoichiometric coefficient
- Balanced multiplier relating species changes.
- Normalized reaction rate
- Rate divided by its stoichiometric coefficient.
What this picture assumes
A₂ ⇌ 2A. Forward normalized rate held at 0.030 M/s. Both rates refer to the A₂ coefficient; net A production is twice their difference. Instantaneous rate comparison, not a time simulation.
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.
- Net normalized rate=0.02 M/s; A₂ change=-0.02 M/s and A change=0.04 M/s. Net dissociation.
- 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 A ⇌ B with one-to-one conversion, the net production rate of B is rf−rr. A positive result indicates net forward change; a negative result indicates net reverse change.
Both directions may still operate during a net change. Saying the system shifts right does not mean the reverse process shuts off.
When coefficients differ, compare rates using the same normalized convention. In A₂ ⇌ 2A, a dissociation event produces two A particles.
The doorway comparison explains a rate balance only. Molecules obey reaction-specific laws, conservation and interactions that the analogy does not describe.
A worked example, step by step
For A₂ ⇌ 2A, forward and reverse reaction rates are 0.030 and 0.010 M/s, each normalized to A₂. Find the net rate of A production.
- Subtract like rate conventions: rnet=0.030−0.010=0.020 M/s.
- Each net reaction produces two A.
- d[A]/dt=2×0.020=0.040 M/s.
- A₂ decreases at 0.020 M/s; A increases twice as fast, conserving A atoms.
Subtract rates on the same species or normalized basis before applying coefficients.
If reverse exceeds forward, is the forward rate necessarily zero?
Compare with an explanation
No. Both can be positive while the net change is reverse.
Predict. Change one thing. Explain.
Change the reverse normalized rate with the forward rate held fixed. Predict the sign of A production and identify the exact balanced point.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Net normalized rate=0.02 M/s; A₂ change=-0.02 M/s and A change=0.04 M/s. Net dissociation.
A₂ ⇌ 2A. Forward normalized rate held at 0.030 M/s. Both rates refer to the A₂ coefficient; net A production is twice their difference. Instantaneous rate comparison, not a time simulation.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using relative rates, particle conservation, the Q/K comparison or the stated dissolution equilibrium. 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 questionFor A₂ ⇌ 2A, rf=0.012 and rr=0.020 M/s normalized to A₂. Find the net reaction rate, both species changes and the direction.
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Compare with the answer and four-point rubric
- 1 point: rnet=−0.008 M/s.
- 1 point: d[A]/dt=−0.016 M/s.
- 1 point: d[A₂]/dt=+0.008 M/s.
- 1 point: The net direction is reverse while both directions can continue.
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 1Does a shift switch off one direction?
No.
RECALL 2What sets net direction?
The difference between consistently defined opposing rates.
RECALL 3Why use coefficients?
They connect reaction progress to each species change.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Which way does a reversible reaction move overall?
- For A ⇌ B: d[B]/dt=rf−rr.
- For A₂ ⇌ 2A: d[A]/dt=2(rf−rr), when rates are normalized to A₂.
Remember: Subtract rates on the same species or normalized basis before applying coefficients.
Conditions: A₂ ⇌ 2A. Forward normalized rate held at 0.030 M/s. Both rates refer to the A₂ coefficient; net A production is twice their difference. Instantaneous rate comparison, not a time simulation.
Refresh Kid · AP Chemistry Unit 7 · Objectives 7.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 7.2, objective 7.2.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 7: Equilibrium, Topics 7.1–7.12. 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. Converting between Kc and Kp and calculations for a dissolved species in equilibrium with its gas phase are excluded from assessed Unit 7 scope. Concentrations use mol/L and gas partial pressures use the stated pressure convention. Supplied constants are teaching data at fixed temperature unless otherwise specified. Ideal dilute-solution and ideal-gas approximations are stated. 3D views show inventories, not molecular trajectories, measured structures or proof of equilibrium from a single snapshot. Approximation checks are explicit; a small K alone does not justify neglecting every change.
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.
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