Endothermic and Exothermic Processes
Choose the system boundary and explain heat-transfer signs without equating temperature with stored energy.
AP Chemistry Topic 6.1 · CED printed p. 112 / PDF p. 119. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- System and surroundings as a chosen boundary
- Moles, balanced equations and limiting reactants
- Intermolecular versus intramolecular interactions
- Activation energy versus overall energy change
Before reading a thermometer, draw a boundary around what you call the system. Temperature changes often indicate changes in thermal energy, but temperature is not the total energy stored in a sample. A larger sample can take more transferred energy for the same temperature rise. A phase change can absorb energy with no temperature increase. Thus an unchanged thermometer does not prove that no energy transfer occurred; ask whether composition or phase is changing too.
Endothermic means energy is transferred into the chosen process/system as heat; exothermic means it transfers energy out. Heating a nonreacting sample is endothermic for that sample, while cooling is exothermic. Melting and vaporization require energy input; freezing and condensation release it. For a chemical transformation, the label follows the reacting species’ net energy change, not simply whether the surrounding liquid ends warm or cold.
When an exothermic reaction reduces the chemical potential energy of the reacting species, that energy can increase surrounding particles’ thermal motion or be transferred by work. An endothermic transformation gains energy from its surroundings. Energy is redistributed rather than created or destroyed. For the usual constant-pressure AP calorimetry model with negligible non-expansion work, reaction heat equals ΔH. If the surrounding solution warms, qsolution is positive while qreaction is negative. Choose the boundary before using the words gains or releases.
Dissolution has competing energetic contributions. Separating solute particles costs energy; making space/rearranging solvent interactions costs energy; forming solute–solvent attractions releases energy. Stronger new attractions can more than repay the separation costs, making dissolution exothermic. If they do not, it is endothermic. Dissolving an ionic solid does not automatically release energy, and no rule says that every physical process has ΔH =0. A measured warming or cooling can test the net balance, not reveal each contribution by itself.
Let's work through it
A supplied insulated-cup record shows that dissolving a solid lowers solution temperature from 24.0 to 19.5 °C, with cup effects temporarily neglected.
- Choose dissolution as the process of interest; the bulk solution’s thermal change is the measured surroundings response.
- The solution cools, so qsolution <0. Under the insulated heat-balance model, qdissolution =−qsolution >0.
- Dissolution is endothermic: separating/reorganizing initial particles costs more than the new solvation attractions release. The same cooling data do not separately measure each interaction term.
A supplied heating record gives a flat temperature while a pure solid melts at fixed pressure.
- Energy still enters the melting substance, so q is positive and melting is endothermic.
- The input changes interparticle potential energy and phase fraction, rather than raising temperature.
- The warmer external source loses energy; it is a different boundary with opposite heat sign.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
A solution warms during a reaction. Which sign belongs to the reaction under an insulated model?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
A solution warms during a reaction. Which sign belongs to the reaction under an insulated model?
Worked answer and reason
qreaction <0; it releases energy while the solution gains it.
Conservation relates equal and opposite heat changes across the chosen boundary.
Retrieval 2
Can energy enter a substance while its temperature stays constant?
Worked answer and reason
Yes, during a pure-substance phase transition at fixed pressure the energy changes particle interactions while phase fractions change.
Potential-energy change need not appear as a kinetic-temperature change.
Catch the tempting mistake
- A warm solution means the reaction absorbed heat.
- The warming solution gains heat from an exothermic reaction; label each boundary separately.
- Dissolution is always exothermic.
- The sign depends on separation costs versus new solvation interactions.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 1 · ap synthesis · multiple
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For the teacher
- Require system and measured surroundings labels on every sign explanation.
- When students equate temperature with energy, compare samples with different masses and a melting plateau.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.1.A
- Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.
- 6.1.A.1
- Temperature changes in a system indicate energy changes.
- 6.1.A.2
- Energy changes in a system can be described as endothermic and exothermic processes such as the heating or cooling of a substance, phase changes, or chemical transformations.
- 6.1.A.3
- When a chemical reaction occurs, the energy of the system either decreases (exothermic reaction), increases (endothermic reaction), or remains the same. For exothermic reactions, the energy lost by the reacting species (system) is gained by the surroundings, as heat transfer from or work done by the system. Likewise, for endothermic reactions, the system gains energy from the surroundings by heat transfer to or work done on the system.
- 6.1.A.4
- The formation of a solution may be an exothermic or endothermic process, depending on the relative strengths of intermolecular/interparticle interactions before and after the dissolution process.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Energy Basics — Scientific check: system boundaries, energy conservation, thermal transfer and heat capacities.
- OpenStax Chemistry 2e: Calorimetry — Scientific check: heat-balance equations, calorimeter heat capacity, reaction/solution signs; newly authored numerical data.
Glossary and deeper reading · 9 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Limiting reactant
The limiting reactant is consumed first under the specified complete-reaction model and limits the product amount. Identifying it requires comparing available amounts with the balanced equation's ratios. For example, abundant hydrogen cannot produce more water after the available oxygen has been consumed.
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Activation energy
Activation energy is the energy barrier associated with a reaction pathway in an introductory energy-profile model. It concerns the pathway and is distinct from the reactant-to-product energy change. For example, a thermodynamically favored reaction can remain slow when its activation barrier is large.
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Phase change
A phase change alters a substance's physical state without changing its chemical identity. It involves a change in particle arrangement and interactions and can transfer energy without raising temperature during a pure-substance transition at fixed pressure. For example, vaporizing liquid water separates water molecules into a gas.
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Endothermic process
An endothermic process absorbs heat into the chosen system. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is positive. For example, melting ice absorbs heat while the phase change occurs.
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Exothermic process
An exothermic process transfers heat from the chosen system to its surroundings. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is negative. For example, freezing water releases heat to the surroundings.
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Calorimetry
Calorimetry determines heat transfer from measured thermal changes in a specified setup. The calculation accounts for the measured material and any supplied calorimeter heat capacity using an energy balance. For example, warming solution can reveal heat released by a reaction when the reaction and surroundings are correctly distinguished.
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Solute
A solute is a component dispersed in a solvent to form a solution. It need not be a solid, and a solution need not be liquid. For example, carbon dioxide is a gaseous solute in carbonated water.
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Solvent
A solvent is the component used as the dissolving medium in a solution. It is often present in the greater amount, though the designation also depends on the mixture's context. For example, water is the solvent in a dilute aqueous sugar solution.