How do you prepare equations for a Hess’s law sum?
You will be able to: Reverse and scale supplied equations to match a target process.
How do you prepare equations for a Hess’s law sum?
To assemble a route to a destination, a map segment may need to be followed backward. A thermochemical equation can likewise be reversed or repeated before it is added to other equations.
A useful starting point: Why subtract reactant formation enthalpies from product values? →
Words and symbols before equations
- Hess’s law
- The overall enthalpy equals the sum for a sequence connecting the same states.
- Target equation
- The net process to construct.
- Intermediate cancellation
- Removing identical species amounts appearing on opposite sides of the sum.
What this picture assumes
Given 2A→B, −40 kJ; B→2C, +16 kJ for supplied amounts and consistent states. Display each transformed equation and its heat. A/B/C represent abstract consistent species; path is thermochemical, 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.
- Target A→C scaled by 1: ΔH=-12 kJ. Each transformed equation’s heat follows its reversal and scale.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Start by identifying a species that appears in the target and only one supplied equation. Orient and scale that equation so the target amount is correct.
Every reversal changes the sign of its enthalpy; every coefficient factor also multiplies its enthalpy. Record the operation beside the equation.
Then add equations and check all remaining species and phases against the target. A plausible heat value does not rescue a wrong chemical sum.
These equations describe a thermochemical path, not necessarily the actual reaction mechanism. The AP focus is on energy accounting; formal state-function terminology is not assessed here.
A worked example, step by step
Given 2A → B with ΔH=−40 kJ and B → 2C with ΔH=+16 kJ, construct C → A and find its heat. A, B and C denote supplied consistent species/states.
- Reverse and halve the second equation: C → ½B, ΔH=−8 kJ.
- Reverse and halve the first: ½B → A, ΔH=+20 kJ.
- Add and cancel ½B to obtain C → A.
- Total ΔH=−8+20=+12 kJ for the target amounts.
Cancel only identical species in identical physical states on opposite sides; do not cancel energy numbers instead of equations.
Does a Hess path prove the reaction occurs in those actual steps?
Compare with an explanation
No. It is an energy-accounting path connecting the same states.
Predict. Change one thing. Explain.
Select forward A→C or reverse C→A and change the amount. Follow the two displayed transformed equations, then check their cancellation.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Target A→C scaled by 1: ΔH=-12 kJ. Each transformed equation’s heat follows its reversal and scale.
Given 2A→B, −40 kJ; B→2C, +16 kJ for supplied amounts and consistent states. Display each transformed equation and its heat. A/B/C represent abstract consistent species; path is thermochemical, 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 questionGiven A→B, ΔH=+30 kJ and B→C, ΔH=−50 kJ, obtain 2C→2A. Show transformed equations, cancellation and total.
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Compare with the answer and four-point rubric
- 1 point: Reverse/double B→C: 2C→2B, ΔH=+100 kJ.
- 1 point: Reverse/double A→B: 2B→2A, ΔH=−60 kJ.
- 1 point: Cancel 2B and obtain 2C→2A.
- 1 point: Sum=+40 kJ for the target.
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 1When negate ΔH?
When reversing the whole process.
RECALL 2When multiply ΔH?
When multiplying all coefficients.
RECALL 3What is the final check?
The sum must equal the target with matching phases.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do you prepare equations for a Hess’s law sum?
- Transform each equation and its ΔH together.
- Check the net equation before accepting the summed heat.
Remember: Cancel only identical species in identical physical states on opposite sides; do not cancel energy numbers instead of equations.
Conditions: Given 2A→B, −40 kJ; B→2C, +16 kJ for supplied amounts and consistent states. Display each transformed equation and its heat. A/B/C represent abstract consistent species; path is thermochemical, not a mechanism.
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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