How can formation data give a reaction’s free energy?
You will be able to: Calculate ΔG°reaction from formation values with coefficients and phases.
How can formation data give a reaction’s free energy?
A formation-data table provides a common reference for comparing the starting and ending substances. You can combine those values without knowing every microscopic step of the reaction.
A useful starting point: When can temperature reverse thermodynamic favorability? →
Words and symbols before equations
- ΔGf°
- Standard Gibbs free energy change for forming one mole of a species from elements in their standard states.
- Reference element
- An element in its standard state, assigned ΔGf°=0.
- Reaction amount
- One mole of the balanced reaction as written.
- State quantity
- A quantity whose difference depends on endpoints, not the chosen calculation path.
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. Supplied ΔGf° at 298 K: H₂O(l)=−237 kJ/mol; H₂(g) and O₂(g) in their standard states have zero formation values. Base equation: H₂+½O₂→H₂O(l).
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.
- Water formation, multiplier 1: ΔG°=-237 kJ/mol reaction as written. Elemental standard-state formation values are zero; their absolute entropies are not set to zero.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Use ΔG°reaction=ΣνΔGf°products−ΣνΔGf°reactants. Every coefficient and phase belongs in the calculation.
The zero convention applies to formation free energy of elements in their standard states. It does not imply their absolute entropy is zero, and it does not apply to every allotrope or phase of an element.
Reversing the equation changes the sign of ΔG°. Scaling the equation scales ΔG° because more reaction amount changes more free energy.
The resulting standard value refers to the stated temperature and standard states. It is distinct from the activation barrier and from ΔG for an arbitrary nonstandard mixture.
A worked example, step by step
For H₂(g)+½O₂(g)→H₂O(l), use supplied ΔGf°(H₂O(l))=−237 kJ/mol and elemental standard-state values of zero at 298 K.
- Products contribute 1×(−237)=−237 kJ/mol reaction.
- Reactants contribute 1×0+½×0=0.
- ΔG°reaction=−237−0=−237 kJ/mol reaction.
- For 2H₂+O₂→2H₂O(l), double the value to −474 kJ/mol reaction as rewritten.
Formation free energy and absolute molar entropy have different reference conventions. Keep liquid and gaseous water distinct.
Is ΔGf° of every compound negative?
Compare with an explanation
No. Formation data can be positive, negative or zero according to the species and conditions.
Predict. Change one thing. Explain.
Change the reaction multiplier and reverse the water-formation equation. Predict ΔG° while keeping the tabulated per-mole formation value fixed.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Water formation, multiplier 1: ΔG°=-237 kJ/mol reaction as written. Elemental standard-state formation values are zero; their absolute entropies are not set to zero.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Supplied ΔGf° at 298 K: H₂O(l)=−237 kJ/mol; H₂(g) and O₂(g) in their standard states have zero formation values. Base equation: H₂+½O₂→H₂O(l).
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 questionFor A+2B→C, supplied formation values are A=−10, B=−20 and C=−80 kJ/mol. Find forward and reverse ΔG°.
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Compare with the answer and four-point rubric
- 1 point: Product total=−80 kJ/mol reaction.
- 1 point: Reactant total=−10+2(−20)=−50.
- 1 point: Forward ΔG°=−80−(−50)=−30 kJ/mol reaction.
- 1 point: Reverse ΔG°=+30 kJ/mol reaction; coefficients and states must match.
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 1How are formation values combined?
Products minus reactants, each weighted by coefficient.
RECALL 2What changes upon reversal?
The sign of the reaction free energy.
RECALL 3Does a negative value give a rate?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How can formation data give a reaction’s free energy?
- ΔG°reaction=ΣνΔGf°products−ΣνΔGf°reactants.
- ΔGf°=0 for an element in its specified standard state.
Remember: Formation free energy and absolute molar entropy have different reference conventions. Keep liquid and gaseous water distinct.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Supplied ΔGf° at 298 K: H₂O(l)=−237 kJ/mol; H₂(g) and O₂(g) in their standard states have zero formation values. Base equation: H₂+½O₂→H₂O(l).
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.3.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.3, objective 9.3.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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