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.