Section 10.4

Introduction

Introduction Necessary vocabulary for 10.4 Phase- Any part of a system that has uniform composition and properties. Condensation- The process by which a gas changes to a liquid. Equilibrium- A dynamic condition in which two opposing changes occur at equal rates in a closed system. Equilibrium Vapor Pressure- The pressure exerted by vapor in equilibrium with its corresponding liquid at a given temperature. Volatile Liquids- Liquids that evaporate readily. Boiling- A conversion of a liquid to a vapor within the liquid as well as at its surface.

Introduction 7) Boiling Point- The temperature at which the equilibrium vapor pressure of the liquid equals the atmospheric pressure. 8) Molar Enthalpy Of Vaporization- The amount of energy as heat that is needed to vaporize one mole of liquid at the liquid’s boiling point at constant pressure. 9) Freezing- The physical change of a liquid to a solid. 10) Freezing Point- The temperature at which the solid and liquid are in equilibrium at one atm.

Introduction 11) Molar Enthalpy Of Fusion- The amount of energy as heat required to melt one mole of solid at the solid’s melting point. 12) Sublimation- The change of state from solid directly to a gas. 13) Deposition- The change of state from a gas directly to a solid.

Changes of State

Changes Of State And Equilibrium Increasing the liquid’s temperature increases the average kinetic energy of the liquid’s molecules. This energy change increases the number of molecules that have enough energy to escape from the liquid phase into the vapor phase. The resulting increased evaporation rate increases the number of molecules in the vapor phase which in turn increases the equilibrium vapor pressure. Nonvolatile liquids do not evaporate readily and have relatively strong attractive forces between their particles. Ex. Molten Ionic Compounds Volatile liquids evaporate readily and have weak forces of attraction between their particles. Ex. Ether

Boiling If the temperature of the liquid is increased, the equilibrium vapor pressure also increases. The lower the atmospheric pressure, the lower the boiling point is. At the boiling point, all of the energy absorbed is used to evaporate the liquid and the temperature remains constant as long as the pressure does not change. If the pressure above the liquid being heated is increased, the temperature of the liquid will rise until the vapor pressure equals the new pressure and the liquid boils once again. At normal atmospheric pressure, the boiling point of water is exactly 100 degrees celsius

Energy And Boiling Energy must be added continuously in order to keep a liquid boiling. The temperature at the boiling point remains constant despite the continuous addition of energy.

Molar Enthalpy of Vaporization The magnitude of the molar enthalpy of vaporization is a measure of the attraction between particles of the liquid. The stronger this attraction is, the more energy that is required to overcome it which results in a higher molar enthalpy of vaporization. Each liquid has a characteristic molar enthalpy of vaporization. Water has an unusually high molar enthalpy which makes water a very effective cooling agent.

Freezing and Melting Liquid ----> Solid + Energy At freezing point, particles of the liquid and the solid have the same average kinetic energy. Solid + Energy ----> Liquid Solid + Energy ←→ Liquid Adding energy in the form of heat to such a system shifts the equilibrium to the right.

Molar Enthalpy of Fusion The energy absorbed increases the solid’s potential energy as its particles are pulled apart, overcoming the attractive forces holding them together. At the same time, there is a significant decrease in particle order as the substance makes the transformation from solid to liquid. The magnitude of the molar enthalpy of fusion depends on the attraction between the solid particles.

Sublimation and Deposition At sufficiently low temperature and pressure conditions, a liquid cannot exist. Under such conditions, a solid substance exists in equilibrium with its vapor instead of its liquid. Here is the equation that represents this. Solid + Energy ←→ Vapor A real world example of what we’ve learned would be sublimation is the reason a thin layer of snow eventually “disappears.”

Questions for the Test ;) 1. The amount of energy as heat that is needed to vaporize one mole of liquid at the liquid’s boiling point at constant pressure. Boiling Point Phase Equilibrium Vapor Pressure Volatile Liquids 2. T/F Nonvolatile liquids evaporate readily 3. If the temperature of the liquid is increased, the equilibrium pressure _____. Increases Decreases 4. ___________- A solid going straight to a gas.

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