Copy of Latent Heat and Physical Change1

Change of Phase and Latent Heat

Key Concepts

  • Distinction Between Changes:

    • Physical Changes: Changes that do not alter the chemical composition of a substance.

    • Chemical Changes: Changes that result in the formation of new chemical substances.

  • Phase Changes of Matter:

    • Phases: Solid, Liquid, Gas

    • Change of phase occurs with the addition or removal of heat, leading to changes in temperature.

    • Example of phase changes: Solid to liquid (melting), liquid to gas (vaporization).

Heat Theory

  • Latent Heat:

    • Refers to the heat required to change a substance from one phase to another without changing its temperature.

    • Phases change at specific temperature and pressure with the absorption or release of heat.

  • Effects of Temperature and Pressure:

    • Increasing temperature can lead to melting of ice and boiling of water.

    • Lowering pressure may cause sublimation of ice directly to water vapor.

Thermodynamics

  • Definition of Phase Change:

    • Also called change of state. The four phases are: Solid, Liquid, Gas, Plasma.

  • Heat and Phase Changes:

    • During phase changes, heat energy is either absorbed or released but the temperature remains constant.

    • Exemplified: Ice melting into water involves absorption of heat without raising water temperature until all ice has melted.

Heat vs Temperature

  • Heat: Form of energy that measures the total thermal energy of molecules, typically measured in Joules.

  • Temperature: Measures the average kinetic energy of molecules, expressed in Kelvin (K), Celsius (°C), or Fahrenheit (°F).

    • Heat transfer causes temperature change, whereas temperature changes are that effect.

Phase Changes Overview

Types of Phase Changes

  • Melting: Solid to liquid; heat absorbed.

  • Freezing: Liquid to solid; heat released.

  • Evaporation: Liquid to gas; heat absorbed.

  • Condensation: Gas to liquid; heat released.

  • Sublimation: Solid to gas; heat absorbed.

Mechanism of Phase Changes

  • Breaking and Building Bonds:

    • During melting, energy breaks intermolecular bonds; during freezing, energy removal allows molecules to bond again.

Phase Change Diagram

  • Heat energy alters molecular bonding during phase changes:

    1. Heat increases energy during melting and boiling.

    2. Heat decreases energy during freezing and condensation.

Thermodynamics Insight

  • Energy Input/Output: During a phase change, temperature remains constant, though intermolecular forces change.

    • Example: Before melting, ice’s molecules vibrate; post melting, water’s molecules vibrate similarly, maintaining the average kinetic energy.

Latent Heat

Definition & Importance

  • Latent Heat: Energy needed for phase changes where no temperature change occurs.

    • Melting (heat absorbed, no temp change) and boiling (heat absorbed, no temp change).

Measurements

  • Latent Heat of Fusion (Lf): Energy required to turn a solid into a liquid.

  • Latent Heat of Vaporization (Lv): Energy required to turn a liquid into a gas.

    • Examples: Water Lf = 333 J/g, Lv = 2260 J/g.

Practical Applications

  • Calculating Energy Requirements:

    • To melt ice: Q = mL where L = latent heat; Example: Melting 10 g of ice requires Q = (10g)(333 J/g) = 3330 J.

    • To evaporate water: Q = (10g)(2260 J/g) = 22600 J.

Specific Heat Capacity

  • Definition: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

    • Unit: J/g⋅°C

    • Higher specific heat means more energy is required to increase temperature.