SectionC11_BB(1) - Tagged

Phase Transitions Overview

• Definition: Change of phase without change of composition.

• Transitions between states: Solid, liquid, and gas.

• Caused by changes in external conditions such as temperature (T) or pressure (P).

• Examples: Ice to warm water; liquid/solid transition in cold water.

Water Phase Diagram

• Solid form of water (ice) is disfavored at higher pressure.

• Liquid state is favorable at lower temperatures.

• Triple Point: Occurs at 0.01°C and 0.006 atm where solid, liquid, and vapor coexist.

• Diagram characteristics:

  • Ice, liquid water, and water vapor regions clearly defined.

Special Properties of Water

• Ice floats due to lower density compared to liquid water.

• Water is denser as a liquid than as ice, which is atypical:

  • Most substances have a solid/liquid line with a positive slope.

  • In comparison, the water phase diagram has a negative slope for solid/liquid transition.

CO2 Phase Diagram

• Solid CO2 (dry ice) is denser than its liquid form.

• Liquid CO2 is disfavored under higher pressure, favoring the solid state instead.

• The solid/liquid line exhibits a positive slope.

• Triple Point of CO2: -57°C and 5.1 atm.

Special Properties of Water - Hydrogen Bonding

• Water's hydrogen bonds provide more order than those in most other liquids.

• Water molecules tend to align with neighboring molecules due to these bonds:

  • Ionic characters contribute to this transient alignment.

Ice Formation and Structure

• Ice forms a tetrahedral lattice due to strong directional hydrogen bonds.

• Lower density of ice arises from increased space between molecules compared to liquid water.

Enthalpy of Phase Transition

• Phase transitions involve energy changes, frequently supplied as heat (q).

• Temperature remains constant during processes such as melting or boiling.

  • Example of melting point: Solid water (ice) converts to liquid without changing temperature.

Latent Heat and Phase Changes

• Phase transitions are reversible, characterized by latent heat.

• Latent heat ( q) can be described as:

  • For melting: Heat required to convert 1 mol solid to liquid at melting point.

  • For boiling: Heat required to convert 1 mol liquid to gas at boiling point.

Enthalpy Values of Water Phase Changes

• Enthalpy of Fusion (ΔfusH):

  • Energy required to convert 1 mol of solid (ice) to liquid at the melting point (0°C) = 6.01 kJ mol-1.

  • Conditions: 0°C and 1 bar.

• Enthalpy of Vaporization (ΔvapH):

  • Energy required to convert 1 mol of liquid (water) to gas (steam) at boiling point (100°C) = 40.7 kJ mol-1.

  • Conditions: 100°C and 1 bar.