How do bond counts give a reaction-enthalpy estimate?
You will be able to: Count changed bonds and calculate an average-bond-enthalpy estimate.
How do bond counts give a reaction-enthalpy estimate?
Adding hydrogen across a carbon–carbon double bond changes several bonds at once. Counting only the double bond misses both the hydrogen bond broken and the new C–H bonds formed.
A useful starting point: Why does breaking a bond require energy? →
Words and symbols before equations
- Structural formula
- Representation identifying which atoms are connected and bond orders.
- Bond order
- Single, double or triple connection as represented in the structure.
- Changed-bond accounting
- Canceling identical bond counts from a complete reactant/product inventory.
- Estimate
- An approximate result because average values depend on chemical environment.
What this picture assumes
C₂H₄(g)+H₂(g)→C₂H₆(g). Supplied D(C=C)=614, D(H–H)=436, D(C–C)=347 kJ/mol bonds, selected average C–H. Four C–H terms cancel in the average-value arithmetic. Geometry is schematic; enthalpy is an estimate, not a mechanism.
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.
- C₂H₄+H₂→C₂H₆: ΔrH≈-123 kJ/mol reaction. Rotating or switching the endpoint view changes neither the balanced atom count nor the computed reaction estimate.
- 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 C₂H₄(g)+H₂(g) → C₂H₆(g), the full reactant inventory has four C–H, one C=C and one H–H bond; products have six C–H and one C–C bond.
Using a common average C–H value allows four C–H terms to cancel mathematically. The changed inventory breaks C=C and H–H and forms C–C plus two C–H.
Use positive tabulated D values in broken minus formed. Coefficients multiply the number of bonds as well as molecule amounts.
Average gas-phase values give an estimate. A liquid product, different molecular environments or precise experimental enthalpies can require additional information. Do not treat agreement to many decimals as physical accuracy.
A worked example, step by step
Estimate ΔH for the hydrogenation above using supplied average values: D(C=C)=614, D(H–H)=436, D(C–C)=347 and D(C–H)=413 kJ/mol bonds.
- Breaking sum=614+436=1050 kJ per mole reaction.
- Formation sum=347+2(413)=1173 kJ per mole reaction.
- ΔrH≈1050−1173=−123 kJ/mol reaction.
- This is exothermic and approximate; the four unchanged average C–H terms cancel, not all six product C–H bonds.
A double bond has its own tabulated dissociation energy; it is not automatically twice a single-bond value.
If the C–H average rises by 10 kJ/mol, how does this estimate change?
Compare with an explanation
Two extra C–H bonds form, so the estimated ΔrH becomes 20 kJ/mol more negative.
Predict. Change one thing. Explain.
Inspect ethene plus H₂ and ethane in 2D/3D. Change the supplied C–H average and predict how the estimate changes while bond counts stay fixed.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
C₂H₄+H₂→C₂H₆: ΔrH≈-123 kJ/mol reaction. Rotating or switching the endpoint view changes neither the balanced atom count nor the computed reaction estimate.
C₂H₄(g)+H₂(g)→C₂H₆(g). Supplied D(C=C)=614, D(H–H)=436, D(C–C)=347 kJ/mol bonds, selected average C–H. Four C–H terms cancel in the average-value arithmetic. Geometry is schematic; enthalpy is an estimate, not a mechanism.
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 the same gas-phase reaction, use D(C=C)=610, D(H–H)=440, D(C–C)=350 and D(C–H)=410 kJ/mol bonds. Calculate both sums and ΔrH, and explain one limitation.
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Compare with the answer and four-point rubric
- 1 point: Breaking=610+440=1050 kJ/mol reaction.
- 1 point: Forming=350+2(410)=1170 kJ/mol reaction.
- 1 point: ΔrH≈−120 kJ/mol reaction.
- 1 point: Average values approximate environment-specific gas-phase bond energies; phase corrections may also be needed for different states.
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 1Which bond counts cancel in the average model?
Four C–H bonds on both sides.
RECALL 2What is the sign order?
Broken minus formed.
RECALL 3What does rotating the model change?
Only the view; the molecular bond inventory stays fixed.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do bond counts give a reaction-enthalpy estimate?
- For this inventory: ΔrH≈D(C=C)+D(H–H)−D(C–C)−2D(C–H).
- All supplied species and average bond values are gas-phase.
Remember: A double bond has its own tabulated dissociation energy; it is not automatically twice a single-bond value.
Conditions: C₂H₄(g)+H₂(g)→C₂H₆(g). Supplied D(C=C)=614, D(H–H)=436, D(C–C)=347 kJ/mol bonds, selected average C–H. Four C–H terms cancel in the average-value arithmetic. Geometry is schematic; enthalpy is an estimate, not a mechanism.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.7, objective 6.7.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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