http://www.ck12.org Chapter 17. Thermochemistry
17.3 Enthalpy and Phase Transitions
Lesson Objectives
- Explain why enthalpy increases or decreases during a given transition from one physical state to another.
- Calculate enthalpy changes that occur during changes of state.
- Calculate enthalpy changes that occur when materials are dissolved in water.
Lesson Vocabulary
- molar heat of fusion: The heat absorbed by one mole of a substance as it is converted from a solid to a liquid.
- molar heat of vaporization: The heat absorbed by one mole of a substance as it is converted from a liquid to
a gas. - heat of solution: Indicates how much energy is absorbed or released when one mole of a substance is fully
dissolved in a specific solvent.
Check Your Understanding
Recalling Prior Knowledge
- What phase transitions can a substance undergo?
- What are the energy requirements for achieving a change of state?
Introduction
When your ice cream melts on a hot day, there is a change in internal energy as well as a change in enthalpy
occurring. An enthalpy change also occurs when the moisture in the air condenses onto the cold windshield of a car.
These changes of state can be described by the changes in enthalpy that accompany them. In this lesson, you will
learn how to calculate and quantify changes of energy as related to changes of state.
Summary of Phase Transition Processes
Figure17.6 summarizes the basic transitions that a substance may undergo as a result of changes in temperature
or pressure. For any phase change, energy will be either released (for exothermic processes) or absorbed from the
surroundings (for endothermic processes). Processes in which intermolecular interactions are weakened or broken