Lecture on Latent Heat of Fusion and Vaporization

Key Concepts

Latent Heat of Fusion

• Definition: The energy required to change a solid into a liquid or vice versa without changing its temperature.
• Example: Melting ice on a hot day or freezing water in a freezer.
• Process: Solid to liquid or liquid to solid.
• Energy involved: Absorbed or released during the transition.

Latent Heat of Vaporization

• Definition: The energy required to change a liquid into a gas or vice versa.
• Example: Boiling water on a stove or condensation on a cold mirror after a shower.
• Process: Liquid to gas or gas to liquid.
• Energy involved: Absorbed or released during the transition.

Application to Water

Heat of Fusion for Water

• Melting Point: 0°C for water.
• Energy Required: 334 Joules per gram to convert ice to liquid water.
• Process: Endothermic (energy absorbed, positive sign).
• Reverse Process: Freezing water releases 334 Joules per gram (exothermic, negative sign).

Heat of Vaporization for Water

• Boiling Point: 100°C for water.
• Energy Required: 2260 Joules per gram to convert liquid water to vapor.
• Process: Endothermic (energy absorbed, positive sign).
• Reverse Process: Condensing vapor releases 2260 Joules per gram (exothermic, negative sign).

Comparison of Energy Requirements

• Heat of vaporization for water is roughly seven times the heat of fusion.

Examples and Calculations

Example 1: Converting Water to Vapor

• Scenario: 100 grams of water to vapor at 100°C.
• Energy Calculation: 100g x 2260 J/g = 226,000 Joules required.
• Sign: Positive (endothermic process).

Example 2: Converting Water to Ice

• Scenario: 250 grams of water to ice at 0°C.
• Energy Calculation: 250g x -334 J/g = -83,500 Joules released.
• Sign: Negative (exothermic process).

Example 3: Converting Ice to Water

• Scenario: 10 grams of ice to water at 0°C.
• Energy Calculation: 10g x 334 J/g = 3,340 Joules absorbed.
• Sign: Positive (endothermic process).

Example 4: Converting Water Vapor to Water

• Scenario: 200 grams of water vapor to water.
• Energy Calculation: 200g x -2260 J/g = -452,000 Joules released.
• Sign: Negative (exothermic process).

Conclusion

• Understanding these concepts is crucial for solving thermochemistry problems.
• Different substances have different heat of fusion and vaporization values.
• The examples provided show practical applications of these concepts in solving chemistry problems.