Whose energy increases when a hand warmer feels warm?
You will be able to: Identify a system and use the direction of heat transfer to assign signs.
Whose energy increases when a hand warmer feels warm?
A sealed hand warmer warms your hand. The reacting chemicals and your hand do not gain the same energy: energy leaves one and enters the other. First decide which part you are describing.
A useful starting point: Review physical and chemical changes →
Words and symbols before equations
- System
- The part of the world selected for study, such as the reacting chemicals.
- Surroundings
- Everything outside that chosen system.
- Heat, q
- Energy transferred because of a temperature difference; positive into the system.
- Exothermic / endothermic
- A process releasing / absorbing heat under the stated conditions.
What this picture assumes
An isolated process–surroundings pair exchanges only heat. The boundary and signs are bookkeeping choices; arrow length is not an energy scale.
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.
- Process q=-400 J; surroundings q=400 J; sum=0 J. Heat leaves: exothermic.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Draw a boundary around the chosen system. An arrow crossing outward means q_system is negative; the receiving surroundings gain that energy.
For an insulated combined system and surroundings exchanging only heat, q_system + q_surroundings = 0. The transfers have opposite signs, not different magnitudes.
An exothermic reaction can warm nearby solution or skin. An endothermic process can cool its surroundings as energy flows into the process.
A temperature reading belongs to the material measured. A warming solution does not mean the chemical process absorbed heat. Name both the process and the measured surroundings before assigning signs.
A worked example, step by step
A reaction releases 480 J into nearby water. Treat the reaction as system and assume no other heat exchange. Find both q values.
- The word releases identifies energy leaving the reaction.
- q_system = −480 J.
- Conservation gives q_water = +480 J.
- The water gains energy while the reaction is exothermic; the two signs describe the same transfer from different boundaries.
Do not assign the reaction’s sign from the thermometer alone without identifying what the thermometer measures.
If the surroundings gain 250 J, what is q_system?
Compare with an explanation
−250 J when those are the only heat transfers.
Predict. Change one thing. Explain.
Move heat transfer through zero from negative to positive. Follow the arrow and predict the surroundings sign before reading the ledger.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Process q=-400 J; surroundings q=400 J; sum=0 J. Heat leaves: exothermic.
An isolated process–surroundings pair exchanges only heat. The boundary and signs are bookkeeping choices; arrow length is not an energy scale.
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 questionA chemical process transfers 750 J to a solution. Identify system and surroundings, assign both signs and explain the solution’s temperature trend assuming no phase change.
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Compare with the answer and four-point rubric
- 1 point: System: reacting process; surroundings measured: solution.
- 1 point: q_process=−750 J.
- 1 point: q_solution=+750 J if other transfers are negligible.
- 1 point: The solution warms because it gains energy within one phase; the process is exothermic.
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 define a boundary first?
Heat sign depends on which system receives or loses energy.
RECALL 2What does q<0 mean?
Energy leaves the selected system as heat.
RECALL 3Are heat and temperature the same quantity?
No. Heat is energy transfer; temperature describes thermal state.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Whose energy increases when a hand warmer feels warm?
- q_system + q_surroundings = 0 for heat exchange in an isolated pair.
- Heat into system: q>0; heat out: q<0.
Remember: Do not assign the reaction’s sign from the thermometer alone without identifying what the thermometer measures.
Conditions: An isolated process–surroundings pair exchanges only heat. The boundary and signs are bookkeeping choices; arrow length is not an energy scale.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.1, objective 6.1.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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