How can known reactions reveal an unknown reaction heat?
You will be able to: Combine measured thermochemical equations and verify the target and heat sum.
How can known reactions reveal an unknown reaction heat?
Directly measuring one conversion may be difficult, while two related conversions are known. If the known equations combine to the target, their enthalpies provide a route to the unknown value.
A useful starting point: How do you prepare equations for a Hess’s law sum? →
Words and symbols before equations
- Thermochemical cycle
- Related processes connecting the same starting and ending states.
- Net reaction
- Equation left after matching species cancel.
- Energy conservation
- The signed accounting principle underlying the Hess sum.
What this picture assumes
Supplied same-condition data: C(graphite)+O₂(g)→CO₂(g), −393.5 kJ; C(graphite)+½O₂(g)→CO(g), −110.5 kJ. Matching species and phases cancel. Diagram contains relative enthalpy levels, not activation barriers.
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.
- Two-step sum: −110.5−283.0=−393.5 kJ, the same complete-combustion endpoint difference.
- 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 carbon combustion, C(graphite)+O₂(g) → CO₂(g) can be split into formation of CO followed by its combustion.
Subtracting the first stage is the same as adding its reverse. Write the reversal explicitly to preserve signs and species.
After addition, cancel carbon and matching oxygen amounts. Confirm that the remaining equation is CO+½O₂ → CO₂ before calculating its enthalpy.
The diagram compares endpoint levels and step changes. A vertical gap represents energy, not reaction duration or activation barrier.
A worked example, step by step
Given C(graphite)+O₂(g) → CO₂(g), ΔH=−393.5 kJ, and C(graphite)+½O₂(g) → CO(g), ΔH=−110.5 kJ, find the heat for CO(g)+½O₂(g) → CO₂(g).
- Keep complete combustion as supplied: −393.5 kJ.
- Reverse CO formation: CO(g) → C(graphite)+½O₂(g), +110.5 kJ.
- Add, cancel C and half of the O₂, leaving the target CO combustion.
- ΔH=−393.5+110.5=−283.0 kJ for one mole CO combusted.
Adding supplied enthalpies without first orienting the equations can produce the wrong process and sign.
Why does reversing CO formation add +110.5 kJ?
Compare with an explanation
It undoes the same process, so the signed heat is reversed.
Predict. Change one thing. Explain.
Switch between the two-step carbon-to-CO₂ path and the isolated CO-combustion target. Check that both routes reconcile the same three energy levels.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Two-step sum: −110.5−283.0=−393.5 kJ, the same complete-combustion endpoint difference.
Supplied same-condition data: C(graphite)+O₂(g)→CO₂(g), −393.5 kJ; C(graphite)+½O₂(g)→CO(g), −110.5 kJ. Matching species and phases cancel. Diagram contains relative enthalpy levels, not activation barriers.
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 questionGiven A→B, ΔH=+18 kJ and A→C, ΔH=−42 kJ, construct B→C. Show the needed reversal, net equation, total and what the path does not establish.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Reverse A→B to B→A with ΔH=−18 kJ.
- 1 point: Add A→C and cancel A to obtain B→C.
- 1 point: ΔH=−18−42=−60 kJ.
- 1 point: This thermochemical combination does not establish the actual kinetic mechanism or rate.
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 1Why combine equations first?
To ensure the energy sum describes the intended reaction.
RECALL 2What does reversing a step do?
Reverses both species order and heat sign.
RECALL 3What does a Hess diagram not show by itself?
Rate, duration or the actual mechanism.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How can known reactions reveal an unknown reaction heat?
- ΔH_target=ΣΔH_transformed steps.
- The net species and phases must match the target exactly.
Remember: Adding supplied enthalpies without first orienting the equations can produce the wrong process and sign.
Conditions: Supplied same-condition data: C(graphite)+O₂(g)→CO₂(g), −393.5 kJ; C(graphite)+½O₂(g)→CO(g), −110.5 kJ. Matching species and phases cancel. Diagram contains relative enthalpy levels, not activation barriers.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.9.A; 6.9.B · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.9, objective 6.9.A; 6.9.B. 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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