What does a reaction enthalpy mean for an actual sample?
You will be able to: Connect a constant-pressure reaction enthalpy to the amount reacting.
What does a reaction enthalpy mean for an actual sample?
A fuel label can describe energy per mole, but a tiny sample releases less energy than a large one. A thermochemical equation needs both its balanced amounts and its energy change.
A useful starting point: How do you add warming and melting in one problem? →
Words and symbols before equations
- Reaction enthalpy, ΔrH
- Enthalpy change per mole of reaction as the equation is written.
- Reaction extent, ξ
- Amount of reaction; moles of a species consumed divided by its coefficient.
- qp
- Heat exchanged at constant pressure.
- Thermochemical equation
- A balanced, phase-labeled equation accompanied by its enthalpy.
What this picture assumes
Supplied 2A → B with ΔrH=−120 kJ per mole reaction. Complete specified transformation at constant pressure; actual heat scales with ξ=n(A)/2. A and B are abstract stoichiometric species.
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.
- 0.5 mol A corresponds to 0.25 mol reaction and q=-30 kJ. Total q changes with amount; the stated ΔrH does not.
- 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 the specified reaction at constant pressure with only pressure–volume work, q_p=ξΔrH. Negative means release and positive absorption.
For 2A → B, consuming 2 mol A corresponds to 1 mol of reaction as written. Consuming 1 mol A gives ξ=0.5 mol reaction.
The states and temperature matter. Producing H₂O(l) is energetically different from producing H₂O(g), even with identical atom counts.
This unit uses the AP constant-pressure convention. Technical enthalpy/internal-energy distinctions and work derivations are not assigned as exam requirements.
A worked example, step by step
A supplied equation 2A → B has ΔrH=−120 kJ per mol reaction. If 0.50 mol A reacts completely, find heat.
- Read the coefficient of A: two moles per mole of reaction.
- ξ=0.50/2=0.25 mol reaction.
- q_p=0.25×(−120)=−30 kJ.
- The sample releases 30 kJ; −120 is not automatically the heat for one mole A.
Do not divide by moles without identifying which species or balanced-equation amount the molar enthalpy refers to.
Does doubling the reacting sample double ΔrH itself?
Compare with an explanation
No. It doubles total q for a fixed reaction convention; the molar enthalpy stays the same.
Predict. Change one thing. Explain.
Change moles of A for the supplied 2A → B equation. Track reaction extent and heat; compare per-mole-A and per-mole-reaction values.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
0.5 mol A corresponds to 0.25 mol reaction and q=-30 kJ. Total q changes with amount; the stated ΔrH does not.
Supplied 2A → B with ΔrH=−120 kJ per mole reaction. Complete specified transformation at constant pressure; actual heat scales with ξ=n(A)/2. A and B are abstract stoichiometric species.
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 questionFor 3X → Y with ΔrH=−90 kJ/mol reaction, 0.60 mol X reacts completely at constant pressure. Calculate ξ, q, heat released per mole X and name a needed condition.
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Compare with the answer and four-point rubric
- 1 point: ξ=0.60/3=0.20 mol reaction.
- 1 point: q=0.20(−90)=−18 kJ.
- 1 point: Per mole X the release is 30 kJ, or signed heat −30 kJ/mol X.
- 1 point: Use the specified phases/temperature and constant-pressure conditions with only pressure–volume work.
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 is extensive here?
Total heat for the reacting amount.
RECALL 2What does the equation coefficient control?
Conversion between species moles and reaction extent.
RECALL 3Why label phases?
Different physical states have different enthalpies.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
What does a reaction enthalpy mean for an actual sample?
- q_p=ξΔrH at constant pressure under the stated work condition.
- ξ=n_consumed/stoichiometric coefficient.
Remember: Do not divide by moles without identifying which species or balanced-equation amount the molar enthalpy refers to.
Conditions: Supplied 2A → B with ΔrH=−120 kJ per mole reaction. Complete specified transformation at constant pressure; actual heat scales with ξ=n(A)/2. A and B are abstract stoichiometric species.
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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