Can a positive ΔG° still allow forward change?
You will be able to: Separate standard-state favorability from the direction of a particular mixture.
Can a positive ΔG° still allow forward change?
A reaction whose equilibrium favors reactants can still make some product when it starts with almost none. The actual composition matters in addition to the standard-state number.
A useful starting point: How does free energy tell you which side equilibrium favors? →
Words and symbols before equations
- Q
- Current reaction quotient, using the same expression as K.
- ΔG
- Free energy change for the current composition.
- ΔG°
- Standard-state value; fixed for the given reaction and temperature.
- Q/K comparison
- Q<K favors net forward change; Q>K favors net reverse change.
What this picture assumes
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Fixed T=300 K and supplied K=0.10. The numerical bridge ΔG=RT ln(Q/K) illustrates current driving force; the core task is Q/K direction reasoning. Each slider setting is a supplied mixture, not a time step.
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.
- Q/K=0.1: net forward change favored. Standard ΔG° remains positive because K=0.10. Actual ΔG=-5.743 kJ/mol is a supporting quantitative bridge.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
ΔG° relates to the equilibrium constant, while Q describes the current mixture. A positive ΔG° means K<1, not that absolutely no forward reaction can occur.
Use Q<K, Q=K or Q>K to determine the net direction toward equilibrium. Both microscopic directions can continue; a net direction is not a claim that the other direction stops.
At equilibrium actual ΔG=0, but ΔG° need not be zero. The special case ΔG°=0 corresponds to K=1.
The optional quantitative bridge ΔG=RT ln(Q/K) illustrates this distinction. The assessment here focuses on explaining current direction using Q and K, rather than memorizing an additional substitution routine.
A worked example, step by step
At fixed temperature a supplied reaction has K=0.10. Its current Q is 0.010. Predict net direction despite positive ΔG°.
- K<1 corresponds to positive ΔG°.
- Compare Q with K: 0.010<0.10.
- The mixture has too little product relative to its equilibrium quotient, so net forward change raises Q.
- Actual ΔG is negative for this mixture; it reaches zero when Q reaches K.
Actual ΔG is zero at equilibrium. Standard ΔG° is zero only if K=1.
At equilibrium with K=100, is ΔG° zero?
Compare with an explanation
No. Actual ΔG=0, but ΔG°=−RT ln(100)<0.
Predict. Change one thing. Explain.
Keep K fixed and move the Q/K ratio through 0.1, 1 and 10. Explain how direction changes while the standard-state equilibrium preference stays fixed.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Q/K=0.1: net forward change favored. Standard ΔG° remains positive because K=0.10. Actual ΔG=-5.743 kJ/mol is a supporting quantitative bridge.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Fixed T=300 K and supplied K=0.10. The numerical bridge ΔG=RT ln(Q/K) illustrates current driving force; the core task is Q/K direction reasoning. Each slider setting is a supplied mixture, not a time step.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using energy and entropy contributions, electron and ion bookkeeping, or the stated cell reaction. 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 questionA mixture has K=100 and Q=1000. State the signs of standard ΔG° and actual ΔG, predict net direction and identify the equilibrium condition.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: K>1 gives ΔG°<0.
- 1 point: Q>K gives actual ΔG>0 for the written forward reaction.
- 1 point: Net change is reverse until the quotient approaches K.
- 1 point: At Q=K, actual ΔG=0, while standard ΔG° remains negative.
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 describes the current mixture?
Q.
RECALL 2What describes the equilibrium relation?
K.
RECALL 3Can standard and actual free-energy signs differ?
Yes, because composition can differ from standard conditions.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Can a positive ΔG° still allow forward change?
- Current direction: compare Q with K.
- At equilibrium Q=K and ΔG=0; ΔG°=−RT ln K may be nonzero.
Remember: Actual ΔG is zero at equilibrium. Standard ΔG° is zero only if K=1.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Fixed T=300 K and supplied K=0.10. The numerical bridge ΔG=RT ln(Q/K) illustrates current driving force; the core task is Q/K direction reasoning. Each slider setting is a supplied mixture, not a time step.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.5.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.5, objective 9.5.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 9: Thermodynamics and Electrochemistry, Topics 9.1–9.11. 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. Numerical thermodynamic examples state standard conditions, temperature, reaction scaling and unit conventions. Supplied data and schematic geometry are teaching models. Standard ΔG° describes standard-state favorability and relates to K; actual direction depends on composition. Thermodynamic favorability does not predict rate. Nonstandard cell potential is taught through Q, distance from equilibrium and qualitative Nernst reasoning; algorithmic substitution alone does not demonstrate the assessed understanding. Electrode positive/negative labeling is excluded from assessed scope. Oxidation at the anode and reduction at the cathode remain essential. Faraday calculations assume the stated current efficiency and electron stoichiometry. Rotatable particle models are schematic inventories, not measured molecular trajectories. Virtual models do not replace required supervised laboratory work.
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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