Energy of Phase Changes

Learning Objective 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.

Quick Notes

Phase changes involve energy transfer without a change in temperature.
The energy change depends on the number of moles and the molar enthalpy of the phase change.
Key equations:
- For melting/freezing: q = nΔH_fus
- For vaporization/condensation: q = nΔH_vap
- where

Full Notes

Energy and Phase Changes

When a substance changes phase — for example, from solid to liquid or liquid to gas — it absorbs or releases energy. This energy is involved in the breaking or forming of forces holding the substance together, not in changing its temperature.

Melting (fusion) and boiling (vaporization) are endothermic – energy is absorbed.
Freezing and condensation are exothermic – energy is released.

ExampleThe melting of ice (solid to liquid) is endothermic and the freezing of water (liquid to solid) is exothermic.

AP Chemistry diagram of ice melting endothermic process

AP Chemistry diagram of ice freezing exothermic process

During these transitions, temperature remains constant, even though heat is being transferred.

Heating Curves and Temperature Plateaus

Heating and cooling curves show flat portions (plateaus) that represent phase changes. During these intervals, all energy transfer is involved in breaking or forming intermolecular forces — not changing temperature.

ExampleIf we plot temperature over time while heating ice we can see two flat lines or plateaus where the temperature doesn’t change – these represent the phase changes of melting and boiling.

AP Chemistry heating curve diagram showing melting and boiling plateaus

Calculating Heat for Phase Changes

We can use the number of moles and the appropriate molar enthalpy value to calculate heat transfer during phase changes:

q = nΔH_fus for melting or freezing
q = nΔH_vap for boiling or condensing

Where:

q is the heat absorbed (+) or released (−) in joules (J)

n is the amount in moles

ΔH_fus is the molar enthalpy of fusion (J/mol)

ΔH_vap is the molar enthalpy of vaporization (J/mol)

These ΔH values are typically provided and found in data tables.

Direction Matters!

The sign of q depends on the direction of the phase change:

Melting and vaporization: q is positive (energy in)
Freezing and condensation: q is negative (energy out)

Also:

ΔH_condensation = −ΔH_vaporization
ΔH_freezing = −ΔH_fusion

This reflects the fact that energy changes are equal and opposite for complementary processes.

Summary

Phase changes involve energy transfer whilst temperature stays constant
The amount of energy (q) absorbed or released is proportional to the number of moles and the molar enthalpy of the phase transition.
Melting and vaporization are endothermic, while freezing and condensation are exothermic. Complementary phase changes involve energy changes that are equal in magnitude and opposite in sign.