What changes when you double or reverse a thermochemical equation?
You will be able to: Scale reaction amounts and reverse heat signs consistently.
What changes when you double or reverse a thermochemical equation?
If making one batch releases a certain energy, two identical batches release twice as much. Undoing the exact same change requires the opposite signed energy at the same conditions.
A useful starting point: What does a reaction enthalpy mean for an actual sample? →
Words and symbols before equations
- Scale factor
- Number multiplying all coefficients and the equation’s enthalpy.
- Reverse process
- The same states exchanged between reactant and product sides.
- Equation basis
- The particular stoichiometric quantities used to quote enthalpy.
What this picture assumes
Supplied gas-phase equation heat −92 kJ for N₂+3H₂→2NH₃ at fixed conditions. Scaling changes the displayed equation basis; reversing changes sign. Not a yield or kinetics model.
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.
- Scale=1; forward direction; quoted heat=-92 kJ. Coefficients express mole amounts, not fractions of an individual molecule.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Multiplying every coefficient by a factor also multiplies the quoted enthalpy for that equation by the factor. This changes the equation basis, not the underlying material behavior.
Reversing a process reverses the sign of ΔH while keeping its magnitude for the same amounts and conditions.
A fractional coefficient is valid in a thermochemical equation: it can express half a mole without claiming half of an individual molecule reacts.
Keep species phases unchanged when reversing. If a phase differs, include the required phase transition rather than treating the two processes as exact reverses.
A worked example, step by step
Given N₂(g)+3H₂(g) → 2NH₃(g), ΔH=−92 kJ for the displayed amounts, find ΔH for NH₃(g) → ½N₂(g)+3/2H₂(g).
- The target decomposes ammonia, so reverse the original equation and change −92 to +92 kJ.
- The reversed equation begins with 2 mol NH₃.
- Divide every coefficient and the energy by 2.
- Target ΔH=+46 kJ for decomposition of 1 mol NH₃ with the stated phases.
Scaling coefficients without scaling the quoted heat creates an inconsistent thermochemical equation.
Does ½N₂ mean a physically isolated half-molecule?
Compare with an explanation
No. Equation coefficients can represent macroscopic mole ratios.
Predict. Change one thing. Explain.
Select forward or reverse ammonia conversion and change the scale factor. Check both coefficients and the signed energy together.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Scale=1; forward direction; quoted heat=-92 kJ. Coefficients express mole amounts, not fractions of an individual molecule.
Supplied gas-phase equation heat −92 kJ for N₂+3H₂→2NH₃ at fixed conditions. Scaling changes the displayed equation basis; reversing changes sign. Not a yield or kinetics model.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using heat-flow signs, energy conservation, phase changes, bond inventories or the stated thermochemical path. 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 → 2B has ΔH=+48 kJ for the stated amounts and phases. Construct 6B → 3A and give its heat, explaining both operations and the role of phases.
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Compare with the answer and four-point rubric
- 1 point: Reverse to 2B → A, changing ΔH to −48 kJ.
- 1 point: Multiply coefficients by 3 to get 6B → 3A.
- 1 point: ΔH=−144 kJ for the displayed quantities.
- 1 point: Species phases must match to call it the exact reversed/scaled process.
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 happens to heat on reversal?
Its sign changes.
RECALL 2What happens on coefficient scaling?
The quoted heat scales by the same factor.
RECALL 3Why preserve the equation basis?
It states exactly which amount the energy describes.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
What changes when you double or reverse a thermochemical equation?
- Reverse equation → negate ΔH.
- Multiply all coefficients by a → multiply quoted ΔH by a.
Remember: Scaling coefficients without scaling the quoted heat creates an inconsistent thermochemical equation.
Conditions: Supplied gas-phase equation heat −92 kJ for N₂+3H₂→2NH₃ at fixed conditions. Scaling changes the displayed equation basis; reversing changes sign. Not a yield or kinetics model.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.6, objective 6.6.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 6: Thermochemistry, Topics 6.1–6.9. 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. Technical enthalpy/internal-energy distinctions and formal state-function terminology are not assessed in the current AP framework. Constant-pressure heat, conservation, phase-specific capacities, reaction amounts and Hess sums are taught here with explicit conditions. Supplied rounded data and original molecular geometry are teaching models, not experimental measurements. A phase transition preserves molecular identity; a bond-energy accounting path is not an actual reaction mechanism.
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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