6.5 Energy of Phase Changes

Phase changes require energy transfer

Phase changes (melting, boiling, freezing, condensing) involve energy transfer without a temperature change, because the energy is used to overcome or form intermolecular forces (IMFs), not to change kinetic energy. During heating curves, the flat plateaus represent phase changes; the sloped segments represent temperature change within a single phase.

Heats of fusion and vaporization

Molar heat of fusion (ΔH_fus) is energy needed to melt one mole of solid; molar heat of vaporization (ΔH_vap) is energy needed to vaporize one mole of liquid. ΔH_vap is always larger than ΔH_fus because vaporization completely separates particles, breaking more/stronger IMFs than melting does.

Stronger IMFs, more energy needed

Substances with stronger intermolecular forces (hydrogen bonding, dipole-dipole, larger dispersion forces) have higher melting/boiling points and larger ΔH_fus/ΔH_vap values, since more energy is required to separate particles. This connects molecular structure to macroscopic thermal behavior.

Key terms

Molar heat of fusion (ΔH_fus)

Energy required to convert one mole of solid into liquid at constant temperature.

Molar heat of vaporization (ΔH_vap)

Energy required to convert one mole of liquid into gas at constant temperature; larger than ΔH_fus.

Heating curve plateau

Flat region during a phase change where temperature stays constant as energy overcomes IMFs.

Why does temperature stay constant during phase changes?

Because added/removed energy changes potential energy (IMFs) instead of kinetic energy (temperature).

Effect of stronger IMFs on phase change energy

Stronger intermolecular forces increase melting/boiling points and require larger ΔH_fus/ΔH_vap values.

Try an AP-style question

How much energy melts 36.0 g of ice at 0 °C? (ΔH_fus = 6.01 kJ/mol)

1. 12.0 kJ
2. 216 kJ
3. 3.01 kJ
4. 6.01 kJ