1. Definition and Forms of Latent Heat
– **Definition**: Latent heat is the energy absorbed or released during a phase change at constant temperature.
– **Forms of Latent Heat**: Includes latent heat of fusion (melting) and latent heat of vaporization (boiling).
– **Endothermic Processes**: Both melting and vaporization require energy input to overcome molecular forces.
– **Condensation Energy**: Energy released during condensation can be transferred as sensible heat.
– **Safety Concerns**: High latent heat of steam makes it hazardous compared to boiling water.
2. Measurement and Quantification of Latent Heat
– **Specific Latent Heat**: Measures energy required for phase change of 1 kg of substance, expressed as ( L = rac{Q}{m} ).
– **Heat of Fusion**: Modern value for melting ice is approximately 334 kJ/kg.
– **Heat of Vaporization**: Black’s findings indicated 830 degrees of heat needed for vaporization.
– **Specific Latent Heats Table**:
– Ethyl alcohol: SLH of fusion = 108 kJ/kg, SLH of vaporization = 855 kJ/kg.
– Ammonia: SLH of fusion = 332.17 kJ/kg, SLH of vaporization = 1369 kJ/kg.
– Water: SLH of fusion = 334 kJ/kg, SLH of vaporization = 2264.705 kJ/kg.
– Carbon dioxide: SLH of fusion = 184 kJ/kg, SLH of vaporization = 574 kJ/kg.
– Lead: SLH of fusion = 23.0 kJ/kg, SLH of vaporization = 871 kJ/kg.
– **Variation with Temperature**: Latent heat of vaporization decreases as temperature or pressure increases.
3. Historical Context and Contributions
– **Cullen’s Experiment (1748)**: Demonstrated cooling through evaporation using diethyl ether.
– **Franklin and Hadley (1758)**: Showed that evaporation can lower temperatures significantly.
– **Introduction of Latent Heat**: Term introduced by Joseph Black for calorimetry studies in the 1750s.
– **Black’s Experiments (1762)**: Comparison of heat absorption in ice melting vs. warming water.
– **Influence on Distillation**: Black’s research aimed to optimize fuel and water use in whisky production.
4. Meteorological Applications of Latent Heat
– **Latent Heat Flux**: Energy transfer from Earth’s surface to atmosphere due to evaporation and condensation.
– **Surface Energy Budget**: Latent heat flux is a critical component of energy balance.
– **Evaporation and Transpiration**: Key processes in the water cycle influenced by latent heat.
– **Climate Impact**: Latent heat plays a significant role in weather patterns and climate systems.
– **Measurement Techniques**: Commonly measured using Bowen ratio or eddy covariance methods.
5. Related Concepts and Theoretical Framework
– **Thermal Radiation**: Heating at constant temperature can cause expansion or pressure increase.
– **Specific Heat Capacity**: Related property influencing energy changes during phase transitions.
– **Important References**: Key texts include Perrot’s “A to Z of Thermodynamics” and Clark’s “Essential Dictionary of Science.”
– **Ton of Refrigeration**: Defined as the power to freeze/melt 2000 lb of water in 24 hours.
– **Related Concepts**: Bowen ratio, eddy covariance flux, sublimation, enthalpy of fusion, and vaporization. https://en.wikipedia.org/wiki/Latent_heat
