Hess's law then lets you add reactions like ledger lines. Signs first, arithmetic second. The tool below drills both books.
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Unit 6 heat ledgers, from your own notes
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Signs before arithmetic: the system's point of view
Enthalpy changes are booked from the system's side: exothermic reactions release heat, delta H negative, surroundings warm; endothermic reactions absorb, positive, surroundings cool. Half the unit's lost points are sign errors, so state the direction of heat flow in words before any number, and check every final answer against the story: did the beaker warm or chill?
Calorimetry reads the surroundings' book
A calorimeter traps a reaction's heat in a known mass of water and reads q as mass times specific heat times temperature change. The reaction's q is the water's with the sign flipped, and dividing by moles converts to per mole enthalpy. Distinguish heat from temperature explicitly, the same q warms less water further, because the exam builds distractors on exactly that confusion.
Hess's law: state functions ignore the route
Enthalpy is a state function, so Hess's law lets you reach a target reaction by adding known steps: reverse a step and flip its sign, scale a step and scale its enthalpy, then cancel species appearing on both sides. Formation enthalpies are the shortcut form, products minus reactants, each weighted by coefficients, with elements in standard states contributing zero.
Bond energies tell the same story from inside
Breaking bonds costs energy, forming bonds releases it, so a reaction's enthalpy approximates bonds broken minus bonds formed, using average bond energies. The estimate is rougher than formation data because averages blur molecular context, and the exam asks why the two methods disagree. Conceptually it explains exothermicity: products bound tighter than reactants leave the difference as heat.
Energy diagrams draw both books at once
A reaction profile puts reactants and products on shelves with delta H the height difference and activation energy the hill between, the kinetics link back to the previous unit. Heating curves do the same for phase changes: temperature climbs within a phase by q equals mc delta T, then plateaus while heat pays the transition toll. Reading which segment a question sits on, slope or plateau, decides which formula applies.
What to photograph for Thermochemistry
Your sign conventions and worked ledgers. Related: Unit 5, photo to quiz and pricing.
Sources used on this page
- College Board, AP Chemistry
- Enthalpy
- Calorimetry
- Hess's law
- Bond energy
- Active recall
- Spaced repetition
- Testing effect
- Forgetting curve
- Generation effect
- Judgment of learning
- Metacognition
- Desirable difficulty
- Distributed practice
- Formative assessment
- Flashcard
- Cloze test
- Multiple choice
- Test (assessment)
- Educational assessment
- Advanced Placement
- Curriculum
- Study skills
- Study guide
- Note-taking
- Overlearning
- Instructional scaffolding
- Item analysis
- Mastery learning
| Quantity | Which book it reads | The classic error |
|---|---|---|
| delta H sign | System's ledger | Booking from the water's side |
| q equals mc delta T | Surroundings' ledger | Forgetting the sign flip |
| Hess addition | Any path, same total | Scaling without scaling H |
| Formation enthalpies | Products minus reactants | Dropping coefficients |
| Bond energies | Broken minus formed | Reversing the subtraction |
| Heating curve | Slope or plateau | Using mc delta T on a plateau |
What does AP Chemistry Unit 6 cover?
Thermochemistry: endothermic and exothermic processes, calorimetry, enthalpy, Hess's law, formation and bond enthalpies, and energy diagrams.
How do I keep signs straight?
Book from the system's side and tell the story first: heat leaving means negative delta H and warmer surroundings. Check numbers against the story.
What does calorimetry actually measure?
The water's side of the ledger: q equals mass times specific heat times temperature change, with the reaction's q equal and opposite.
When do I use Hess's law versus formation enthalpies?
Hess adds given step reactions with flips and scales; formation data jumps straight to products minus reactants. They are the same principle in two dresses.
Why do bond energy estimates disagree with formation data?
Bond energies are averages across many molecules, so they blur each bond's real context; formation enthalpies carry the exact molecules.
Can I build questions from my own Unit 6 notes?
Yes. The questions follow the upload: photograph these pages or attach the PDF, and nothing outside them enters the set.
Last updated: 2026-08-15