What changes immediately when you add a reactant?
You will be able to: Separate an imposed concentration change from the subsequent equilibrium response.
What changes immediately when you add a reactant?
Add a little reactant to a settled mixture without appreciably changing its volume. Its concentration jumps immediately. Only afterward does net reaction redistribute the mixture.
A useful starting point: What can a particle picture tell you about equilibrium? →
Words and symbols before equations
- Stress
- An imposed change to conditions or composition.
- Immediate change
- What the intervention directly changes before reaction responds.
- Readjustment
- Subsequent net conversion toward a new equilibrium.
- Le Châtelier’s principle
- A qualitative way to predict the response to a disturbance.
What this picture assumes
Ideal dilute concentrations in mol/L (M); fixed temperature, fixed volume except when explicitly changed, and no side reactions. Supplied K values use the stated AP concentration convention. A ⇌ B initially at equilibrium: [A]=0.20 M, [B]=0.60 M, K=3. Addition is followed by redistribution; endpoints only, no reaction times inferred.
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.
- Immediately Q=1; K=3 throughout. New [A]=0.3 M, [B]=0.9 M. Net conversion consumes 0.3 M A after the addition; it need not undo all of the disturbance.
- 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 K=3, adding A at fixed temperature lowers Q=[B]/[A]. Net forward conversion then raises B and consumes some of the added A.
The response does not necessarily remove all of the addition or restore every original concentration. A new total inventory generally creates new equilibrium amounts.
Removing a product similarly lowers Q and promotes net product replacement. Specify that volume and temperature are fixed before applying this comparison.
Observable color depends on which species absorbs and on concentration and path length. A color change is evidence to interpret, not a universal indicator of a particular direction.
A worked example, step by step
An equilibrium A⇌B mixture has [A]=0.20 M and [B]=0.60 M. Add enough A to raise [A] immediately to 0.60 M at fixed volume and T. Find the new equilibrium.
- K=0.60/0.20=3 initially.
- Immediately after addition Q=0.60/0.60=1, so net change is forward.
- New total concentration is 1.20 M; [B]=3[A] gives [A]=0.30 M.
- [B]=0.90 M. Some added A remains: final [A] is above its original 0.20 M.
A shift counteracts a disturbance; it does not necessarily restore the original amounts.
Does adding A change K at fixed temperature?
Compare with an explanation
No. It changes the composition and Q; the final quotient returns to the same K.
Predict. Change one thing. Explain.
Vary the added A while holding initial equilibrium, volume and T fixed. Compare all three states and identify what jumps versus changes through reaction.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Immediately Q=1; K=3 throughout. New [A]=0.3 M, [B]=0.9 M. Net conversion consumes 0.3 M A after the addition; it need not undo all of the disturbance.
Ideal dilute concentrations in mol/L (M); fixed temperature, fixed volume except when explicitly changed, and no side reactions. Supplied K values use the stated AP concentration convention. A ⇌ B initially at equilibrium: [A]=0.20 M, [B]=0.60 M, K=3. Addition is followed by redistribution; endpoints only, no reaction times inferred.
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 questionAt K=3, [A]=0.20 and [B]=0.60 M. Remove 0.20 M B with negligible volume change. Find immediate Q, direction and final concentrations.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Immediate A=0.20, B=0.40 M, so Q=2.
- 1 point: Q<K, so net forward change.
- 1 point: Total is 0.60 M: final A=0.15 M.
- 1 point: Final B=0.45 M; only part of the removed B is replaced.
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 1What changes first after an addition?
The directly altered species concentration.
RECALL 2Must original concentrations be restored?
No; conservation uses the new inventory.
RECALL 3How is color interpreted?
Through the identity and concentration of absorbing species under stated conditions.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
What changes immediately when you add a reactant?
- At fixed T, adding/removing species changes Q, not K.
- Separate before, immediate and final states.
Remember: A shift counteracts a disturbance; it does not necessarily restore the original amounts.
Conditions: Ideal dilute concentrations in mol/L (M); fixed temperature, fixed volume except when explicitly changed, and no side reactions. Supplied K values use the stated AP concentration convention. A ⇌ B initially at equilibrium: [A]=0.20 M, [B]=0.60 M, K=3. Addition is followed by redistribution; endpoints only, no reaction times inferred.
Refresh Kid · AP Chemistry Unit 7 · Objectives 7.9.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 7.9, objective 7.9.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.
Want to work through this with a tutor?
Bring your question about What changes immediately when you add a reactant? Your explanation and answers remain free to access.