PHASE CHANGES

Lesson 6.6

I. Enthalpy of Phase Changes

� Phase changes b/w a solid, liquid, or gas

0°C

100°C

melting

vaporization

solid

liquid

gas

freezing

condensation

solid

liquid

gas

I. Enthalpy of Phase Changes

� 1) Heat of Fusion – the enthalpy change from melting a substance.

� Symbol: ΔHfus

� Ex: H2O(s) à H2O(l)ΔHfus = +6 kJ/mol

I. Enthalpy of Phase Changes

� Melting = endothermic; heat added� Freezing = exothermic; heat lost

melting, +ΔH

freezing, -ΔH

I. Enthalpy of Phase Changes

� 2) Heat of Vaporization – the enthalpy change from boiling a substance.

� Symbol: ΔHvap

� Ex: H2O(l) à H2O(g)� ΔHvap = +41 kJ/mol

I. Enthalpy of Phase Changes

� Boiling = endothermic� Condensing = exothermic

boiling, +ΔH

condensing, -ΔH

II. Heating Curves

� Heating curves (aka phase-change diagrams) show the relationship of heat and temperature.

II. Heating Curves

� Ex#1:10 grams of water cools from 110oC to 20oC. How much heat was lost?

II. Heating Curves

� Ex#2:40 grams of water warms from –15°C to 30°C. How much heat was gained?