How do you turn a balanced reaction into Q and K?
You will be able to: Write quotient expressions with correct powers and concentration or pressure inputs.
How do you turn a balanced reaction into Q and K?
A recipe with two portions of one ingredient cannot be described by simply adding every ingredient amount. Equilibrium likewise uses a specific composition expression tied to the balanced equation.
A useful starting point: Which way does a reversible reaction move overall? →
Words and symbols before equations
- Reaction quotient Q
- Composition expression evaluated using current values.
- Equilibrium constant K
- The same expression evaluated at equilibrium at a stated temperature.
- Exponent
- A power; [A]² means [A]×[A].
- Partial pressure
- Pressure attributable to one gas, not the total mixture pressure.
What this picture assumes
A₂ ⇌ 2A with current [A₂]=0.10 M fixed. The slider compares different current compositions, not a closed-system trajectory. Qc=[A]²/[A₂]; values use M inputs.
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.
- Qc=0.4. Doubling only [A] quadruples Qc; this is not automatically an equilibrium sample.
- 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), Qc=[A]²/[A₂]. Products go in the numerator and reactants in the denominator; coefficients become powers.
Qc can be calculated before, during or after adjustment. It equals Kc only when the mixture is at equilibrium at that temperature.
Qp uses species partial pressures instead: PA²/PA₂. Do not substitute total pressure for each gas or compare a Qc with a Kp.
We use the usual AP numerical concentration/pressure expressions with stated M or atm inputs. More rigorous thermodynamic constants use normalized activities; conversion between Kc and Kp is not required here.
A worked example, step by step
A₂ ⇌ 2A currently has [A]=0.20 M and [A₂]=0.10 M. Write and calculate Qc.
- Read the balanced coefficients: one A₂ and two A.
- Write Qc=[A]²/[A₂].
- Substitute (0.20)²/0.10=0.40.
- This is Qc; it is Kc only if the sample is known to be at equilibrium at the stated temperature.
Coefficients become powers, not factors in front of concentrations. Q is not automatically K.
What happens to Qc if only [A] doubles in A₂ ⇌ 2A?
Compare with an explanation
It becomes four times as large because [A] is squared.
Predict. Change one thing. Explain.
Change [A] while keeping [A₂] fixed. Double [A] and predict the quotient factor before checking the squared term.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Qc=0.4. Doubling only [A] quadruples Qc; this is not automatically an equilibrium sample.
A₂ ⇌ 2A with current [A₂]=0.10 M fixed. The slider compares different current compositions, not a closed-system trajectory. Qc=[A]²/[A₂]; values use M inputs.
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 A(g)+2B(g) ⇌ C(g), partial pressures are 0.50, 0.20 and 0.10 atm respectively. Write Qp, calculate it, and state what additional information lets you predict direction.
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Compare with the answer and four-point rubric
- 1 point: Qp=PC/(PA PB²).
- 1 point: Use individual partial pressures in atm.
- 1 point: Qp=0.10/(0.50×0.20²)=5.0.
- 1 point: Compare with Kp at the same temperature; Q alone does not give direction.
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 1When is Q equal to K?
At equilibrium under the stated conditions.
RECALL 2Where do exponents come from?
The balanced coefficients.
RECALL 3Why distinguish Kc and Kp?
Their input quantities differ; numerical values need not be equal.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do you turn a balanced reaction into Q and K?
- aA+bB ⇌ cC+dD: Qc=[C]^c[D]^d/([A]^a[B]^b).
- Use matching Qc/Kc or Qp/Kp conventions.
Remember: Coefficients become powers, not factors in front of concentrations. Q is not automatically K.
Conditions: A₂ ⇌ 2A with current [A₂]=0.10 M fixed. The slider compares different current compositions, not a closed-system trajectory. Qc=[A]²/[A₂]; values use M inputs.
Refresh Kid · AP Chemistry Unit 7 · Objectives 7.3.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 7.3, objective 7.3.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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