Why can compression change a gas equilibrium?
You will be able to: Predict a gas response using partial pressures and distinguish inert-gas conditions.
Why can compression change a gas equilibrium?
Compressing a sealed gas mixture squeezes the same initial molecules into less space. Every reactive partial pressure rises before the mixture has time to react.
A useful starting point: What changes immediately when you add a reactant? →
Words and symbols before equations
- Compression
- Decrease in volume of a gas mixture.
- Partial pressure
- Pressure contributed by one gas species.
- Gas mole difference
- Product gas coefficients minus reactant gas coefficients.
- Inert gas
- A gas that does not react in the stated system.
What this picture assumes
Ideal gas at fixed T, A₂ ⇌ 2A, initially PA₂=0.25 atm, PA=0.50 atm, Kp=1. Uniform reactive pressure factors are 2, 1 and 0.5. Third case is inert-gas addition until volume doubles at fixed total pressure. Displays immediate values and predicted direction, not final pressures.
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.
- Reactive pressures scale by 2. Immediate Qp=2; Kp=1: net reverse. At fixed volume an inert gas changes total pressure without changing reactive partial pressures.
- 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 A₂(g)⇌2A(g), Qp=PA²/PA₂. Halving volume at fixed T immediately doubles each partial pressure, so Qp doubles, rather than staying unchanged.
If the system was at equilibrium, Qp now exceeds Kp and net association makes fewer gas particles. Compression favors the side with fewer gas moles when counts differ.
Equal gas-mole coefficients on both sides make a uniform pressure scaling cancel from Qp. Pure condensed phases do not enter the gas-mole count.
Adding inert gas at fixed volume raises total pressure but leaves reactive partial pressures unchanged, so no ideal-gas shift follows. At fixed total pressure, adding inert gas expands the volume; the resulting dilution can shift a reaction toward more gas moles.
A worked example, step by step
A₂⇌2A initially has PA₂=0.25 atm and PA=0.50 atm at equilibrium. Volume is halved at fixed T. Predict the immediate quotient and net direction.
- Initial Kp=0.50²/0.25=1.00.
- Compression doubles partial pressures to 0.50 and 1.00 atm.
- Immediate Qp=1.00²/0.50=2.00.
- Qp>Kp, so net change forms A₂, the side with fewer gas particles; Kp remains 1.00.
Total pressure alone cannot predict a shift; identify which reactive partial pressures change.
Does adding inert gas at fixed volume always shift toward fewer moles?
Compare with an explanation
No. Reactive partial pressures do not change in the ideal-gas model.
Predict. Change one thing. Explain.
Switch between halving volume, fixed-volume inert gas and doubling volume through constant-pressure inert-gas addition. Compare reactive partial pressures and Qp.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Reactive pressures scale by 2. Immediate Qp=2; Kp=1: net reverse. At fixed volume an inert gas changes total pressure without changing reactive partial pressures.
Ideal gas at fixed T, A₂ ⇌ 2A, initially PA₂=0.25 atm, PA=0.50 atm, Kp=1. Uniform reactive pressure factors are 2, 1 and 0.5. Third case is inert-gas addition until volume doubles at fixed total pressure. Displays immediate values and predicted direction, not final pressures.
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 questionExplain why adding inert gas can produce different results at fixed volume and at fixed total pressure for A₂⇌2A. State the ideal-gas and fixed-temperature assumptions.
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Compare with the answer and four-point rubric
- 1 point: At fixed V and T, reactive partial pressures niRT/V stay unchanged.
- 1 point: Qp and Kp are unchanged, so no shift is predicted.
- 1 point: At fixed total pressure, inert-gas addition expands V and lowers reactive partial pressures.
- 1 point: Qp falls for A₂⇌2A, so dissociation is favored; Kp stays fixed at the same T.
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 determines a gas quotient?
Reactive species partial pressures and their powers.
RECALL 2What does fixed-volume inert gas change?
Total pressure, but not ideal reactive partial pressures.
RECALL 3Does compression change K at fixed T?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why can compression change a gas equilibrium?
- For a uniform pressure factor f: Qp′=f^Δn Qp.
- Fixed-volume inert gas leaves ideal reactive partial pressures unchanged.
Remember: Total pressure alone cannot predict a shift; identify which reactive partial pressures change.
Conditions: Ideal gas at fixed T, A₂ ⇌ 2A, initially PA₂=0.25 atm, PA=0.50 atm, Kp=1. Uniform reactive pressure factors are 2, 1 and 0.5. Third case is inert-gas addition until volume doubles at fixed total pressure. Displays immediate values and predicted direction, not final pressures.
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.
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