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6.5Thermochemistry

Energy of Phase Changes — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 10 of 14 1 quiz

📚 AP CHEMISTRY Hub / Topical Revision
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Lesson summary

6.5 Energy of Phase Changes — AP CHEMISTRY notes: Describe the energy changes in phase transitions and interpret heating curves.ChangeNameEnergySolid to liquidMelting (fusion)AbsorbedLiquid to solidFreezingReleasedLiquid to gasVaporisationAbsorbedGas to liquidCondensationReleased…

  • Heat of fusion
  • Heat of vaporisation
  • Reverse changes
  • Sublimation
  • Sloping segments
  • Flat segments

Section 1 of 9Phase changes and energy

ChangeNameEnergy
Solid to liquidMelting (fusion)Absorbed
Liquid to solidFreezingReleased
Liquid to gasVaporisationAbsorbed
Gas to liquidCondensationReleased
Solid to gasSublimationAbsorbed
Gas to solidDepositionReleased

Section 2 of 9Heats of phase change

1Heat of fusion
  • Heat of fusion the energy to melt a solid at its melting point; for ice it is 6.01 kJ mol-1
2Heat of vaporisation
  • Heat of vaporisation the energy to evaporate a liquid and a measure of the strength of intermolecular forces
3Reverse changes
  • Reverse changes freezing and condensation have equal magnitude and opposite sign to fusion and vaporisation
4Sublimation
  • ΔHsub = ΔHfus + ΔHvap

Section 3 of 9Heating curve

Sloping segments
  • Sloping segments the temperature rises and average kinetic energy increases
  • Heat is calculated from q = mcΔT.
  • A larger specific heat capacity gives a smaller slope.
Flat segments
  • Flat segments the temperature is constant during melting and boiling
  • Added heat increases potential energy by overcoming intermolecular forces, while average kinetic energy is constant.
  • Heat for a phase change at constant temperature is q = nΔH.
Vaporisation segment
  • Vaporisation segment longer than fusion because more energy is needed to separate particles completely

Section 4 of 9Cooling curve

Cooling curve
  • Cooling curve the reverse of a heating curve, from gas to solid
Flat segments
  • Flat segments condensation and freezing, where energy is released at constant temperature

Section 5 of 9Evaporation versus boiling

Evaporation
  • Evaporation occurs at the surface at any temperature and cools the liquid
  • Skin feels cold when water evaporates because the heat of vaporisation is drawn from the body.
Boiling
  • Boiling occurs throughout the liquid at the boiling point

Section 6 of 9Everyday examples

Dew
  • Dew condensation of water vapour
Frost and snow
  • Frost and snow deposition of water vapour as solid
Iodine
  • Iodine sublimes when heated

Section 7 of 9More from the specification

More from the specification
Enthalpy changes during vaporization
  • Enthalpy changes during vaporization latent heat input increases potential energy without altering temperature within a boiling liquid-vapour system.
Substance
  • A material with only one type of particle.
Surroundings
  • Everything other than the reacting chemicals.
Calorimeter
  • An insulated container used to measure temperature changes in reactions.
Calorimetry
  • Measuring heat energy changes

Section 8 of 9Key terms

TermMeaning
Heat of fusionEnergy to melt one mole of a solid at its melting point.
Heat of vaporisationEnergy to vaporise one mole of liquid at its boiling point.
SublimationDirect change from solid to gas.
DepositionDirect change from gas to solid.
Heating curvePlot of temperature against heat added.

Section 9 of 9Exam technique

Avoid these mistakes
Saying temperature rises during melting — it stays constant while potential energy increases.
Using q = mcΔT during a phase change — use q = nΔH instead.
Thinking vaporisation needs less energy than fusion — vaporisation needs more.

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