Energy of Phase Changes
Account separately for warming/cooling and fixed-pressure phase-transition energy.
AP Chemistry Topic 6.5 · CED printed p. 117 / PDF p. 124. 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
Melting and boiling increase separation or rearrange the interactions among particles, so energy must enter the substance. Freezing and condensation release energy as stronger/coordinated attractions are established. For a pure substance with two phases coexisting at fixed pressure in equilibrium, temperature stays constant during the transition; the energy changes potential/interparticle energy and phase fraction rather than mean kinetic temperature. In boiling water, H2O molecules remain H2O: O–H covalent bonds are not what the vaporization enthalpy breaks. A plateau model assumes equilibrium and fixed pressure; impurities or non-equilibrium superheating can complicate real traces.
For a phase segment, . If ΔHvap is positive, condensation at the same conditions has ΔHcond =−ΔHvap; freezing similarly reverses fusion. Doubling moles doubles the phase heat. A heating curve can contain both slopes and plateaus, so calculate each segment with the appropriate equation and add signed terms. Never use an overall temperature change to replace a phase term. Molar phase enthalpy belongs to specified temperature/pressure conditions and phase identities.
Let's work through it
At its melting temperature, 18.0 g ice melts completely. Given molar mass 18.0 g mol−1 and ΔHfus =6.01 kJ mol−1, find heat. Then the liquid warms by 10.0 °C with c =4.18 J g−1 °C−1.
- n =18.0/18.0 =1.00 mol; melting heat is +6.01 kJ despite ΔT =0 during melting.
- Liquid warming heat is 18.0×4.18×10.0 =752 J =0.752 kJ.
- Total input is 6.01+0.752 =6.76 kJ. Freezing the same amount at the same transition conditions would release −6.01 kJ, with no warming term unless another temperature segment occurs.
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
How can a melting plateau absorb energy at constant temperature?
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
How can a melting plateau absorb energy at constant temperature?
Worked answer and reason
Energy changes interparticle interactions and the amounts of each phase, rather than increasing average kinetic temperature.
Energy has potential as well as kinetic contributions.
Retrieval 2
If vaporizing 0.50 mol absorbs 20 kJ, what does condensing 1.00 mol release under the same conditions?
Worked answer and reason
40 kJ is released, so q =−40 kJ for the condensing substance.
Heat scales with moles and reverses sign for the reverse phase process.
Catch the tempting mistake
- Vaporization breaks O–H bonds in water.
- It separates intact water molecules by overcoming intermolecular attractions.
- A plateau means no heat transfer.
- The phase enthalpy changes even when temperature is fixed.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 7 · ap application · multiple
- Unit 6 problem 8 · ap application · data
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For the teacher
- Have students color-code slope versus plateau terms on a heating curve.
- State pure-substance, fixed-pressure equilibrium conditions when teaching a plateau.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.5.A
- Explain changes in the heat q absorbed or released by a system undergoing a phase transition based on the amount of the substance in moles and the molar enthalpy of the phase transition.
- 6.5.A.1
- Energy must be transferred to a system to cause a substance to melt (or boil). The energy of the system therefore increases as the system undergoes a solid-to-liquid (or liquid-to-gas) phase transition. Likewise, a system releases energy when it freezes (or condenses). The energy of the system decreases as the system undergoes a liquid-to-solid (or gas-to-liquid) phase transition. The temperature of a pure substance remains constant during a phase change.
- 6.5.A.2
- The energy absorbed during a phase change is equal to the energy released during a complementary phase change in the opposite direction. For example, the molar enthalpy of condensation of a substance is equal to the negative of its molar enthalpy of vaporization. Similarly, the molar enthalpy of fusion can be used to calculate the energy absorbed when melting a substance and the energy released when freezing a substance.
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: Calorimetry — Scientific check: heat-balance equations, calorimeter heat capacity, reaction/solution signs; newly authored numerical data.
- OpenStax Chemistry 2e: Phase Transitions — Scientific check: phase enthalpies, fixed-pressure pure-substance heating curves and interaction changes.
Glossary and deeper reading · 7 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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Condensation
Condensation is a phase change from gas to liquid. It releases energy to the surroundings as particles form more closely associated liquid arrangements. For example, water vapor condensing on a cooler surface forms liquid droplets.
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Pure substance
A pure substance has one chemical identity and a fixed composition. It may be an element or a compound, so containing more than one element does not make it a mixture. For example, pure water contains hydrogen and oxygen in a fixed ratio.
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Enthalpy
Enthalpy is a state function defined from internal energy, pressure, and volume. At constant pressure with only pressure–volume work, an enthalpy change equals the heat transferred to the system. For example, a negative reaction enthalpy indicates heat release for the written reaction under those conditions.