http://www.ck12.org Chapter 17. Thermochemistry
17.3 Heat and Changes of State
Lesson Objectives
- Describe the enthalpy change that occurs as a substance changes between the solid and liquid states.
- Describe the enthalpy change that occurs as a substance changes between the liquid and gas states.
- Calculate the enthalpy change involved in the change of state for any amount of a given substance.
- Calculate the enthalpy changes involved as substances dissolve in water.
Lesson Vocabulary
- molar heat of condensation
- molar heat of fusion
- molar heat of solidification
- molar heat of solution
- molar heat of vaporization
Check Your Understanding
Recalling Prior Knowledge
- What happens to the temperature of a system during a change of state?
- What information is contained in a heating or cooling curve?
Enthalpy changes accompany physical processes as well as chemical reactions. In this lesson, you will examine
the heat absorbed or released during changes of state as well as the enthalpy change that occurs when a solute is
dissolved into a solvent.
Heats of Fusion and Solidification
Suppose you hold an ice cube in your hand. It feels cold because heat energy leaves your hand and enters the ice
cube. What happens to the ice cube? It melts. However, as you learned in an earlier chapter, the temperature during
a phase change remains constant. So the heat that is being lost by your hand does not raise the temperature of the ice
above its melting temperature of 0°C. Rather, all the heat goes into the change of state. Energy is absorbed during
the process of changing ice into water. The water that is produced also remains at 0°C until all of the ice has melted.
All solids absorb heat as they melt to become liquids. The gain of heat in this endothermic process goes into changing
the state rather than changing the temperature. Themolar heat of fusion(∆Hf us)of a substance is the heat absorbed
by one mole of that substance as it is converted from a solid to a liquid. Since the melting of any substance absorbs