Lecture Slides Chapter 11

Properties of Solutions: Concentrations and Colligative Properties

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. It is a key property that determines how readily a liquid will evaporate.

• Volatile substances are those that have a significant vapor pressure at a given temperature, meaning they evaporate easily.

• Nonvolatile substances have negligible vapor pressure and do not evaporate easily.

• Vapor pressure is a partial pressure of the vapor phase when the rate of evaporation equals the rate of condensation (dynamic equilibrium).

Example: Water in a closed container will reach a point where the rate of water molecules leaving the liquid (evaporation) equals the rate of molecules returning (condensation).

Vaporization and Condensation

Vaporization is the process by which molecules transition from the liquid phase to the gas phase. Condensation is the reverse process, where molecules move from the gas phase to the liquid phase.

• At the surface of a liquid, high-energy molecules may escape into the vapor phase if they have enough energy to overcome intermolecular attractions.

• The larger the surface area, the faster the rate of evaporation.

• Evaporation leads to the formation of vapor above the liquid.

Example: Water molecules at the surface of a beaker can escape into the air, becoming water vapor.

Distribution of Thermal Energy

Only a small fraction of molecules in a liquid have enough energy to escape into the vapor phase at any given time. As temperature increases, a greater fraction of molecules have sufficient energy to vaporize.

• The higher the temperature, the faster the rate of evaporation.

Example: Heating water increases the number of molecules with enough kinetic energy to escape as vapor.

Vaporization Process Summarized

• The rate of vaporization increases with increasing temperature.

• The rate of vaporization increases with increasing surface area.

• The rate of vaporization increases with decreasing strength of intermolecular forces.

Energetics of Vaporization

When high-energy molecules are lost from the liquid, the average energy of the remaining liquid is lowered.

• If energy is not replaced, the temperature of the liquid decreases.

• Vaporization is an endothermic process (absorbs energy).

• Condensation is an exothermic process (releases energy).

• Vaporization requires input of energy to overcome the attractive forces between molecules.

Example: Sweating cools the body because vaporization of sweat absorbs heat from the skin.

Additional info: The enthalpy of vaporization ($\Delta H_{vap}$) quantifies the energy required to vaporize one mole of a liquid at constant pressure. This value is always positive for vaporization and negative for condensation.