Latent Heat: Phase Changes Explained
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When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise. The thermal energy may cause the mass to change from one phase, or state, to another. The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.
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Latent Heat: Phase Changes Explained
- 1. Latent Heat
- 2. Change of State Matter exists in 3 states: •Solid •Liquid •Gas water steam fusion (at melting point) vaporization (takes place at boiling point) solidification condensation Consider water: at freezing point at boiling point ice
- 3. Heats of Transformation When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise. The thermal energy may cause the mass to change from one phase, or state, to another. The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.
- 4. Heat of Fusion To melt a solid means to change its from the solid state to the liquid state. This process requires energy because the molecules of the solid must be freed from their rigid structure. To freeze a liquid to form a solid is the reverse of melting and requires that energy be removed from the liquid so that the molecules can settle into a rigid structure.
- 5. Heat of Fusion When the phase change is from solid to liquid, the sample must absorb heat; when the phase change is from a liquid to solid, the sample must release heat. The heat of transformation for these phase changes is called the heat of fusion, Lf. Water: Lf = 334 J/g = 79.5 cal/g fLmQ ⋅=
- 6. Heat of Vaporization To vaporize a liquid means to change it from the liquid state to the vapor or gas state. This process requires energy because the molecules must be freed from the liquid state. Condensing a gas to a liquid is the reverse of vaporizing; it requires that energy be removed from the gas so that the molecules can cluster together instead of flying away from each other. The heat of transformation for these phase changes is called the heat of vaporization, Lv. Water: Lv = 2256 J/g = 539 cal/g vLmQ ⋅=
- 8. Copyright 1999, PRENTICE HALL Chapter 118 Gas-Liquid Equilibration
- 9. SAMPLE PROBLEMS 1. In a water distilling store, 1.00 L ( 1.00kg ) of boiling water is to be changed to steam. How much heat will be needed to do that? 2. A freezer motor can remove 515 000J of heat from water at 0 °C. How much water will turn to ice?
- 10. SUMMARY Latent heat capacity – is the amount of heat that will change the state of a substance at a specific temperature. When heat is added to a substance, melting, vaporization or sublimation may occur. When heat is extracted from a substance, freezing, condensation and deposition may occur.
- 12. Copyright 1999, PRENTICE HALL Chapter 1112 Phase ChangesPhase Changes Energy Changes Accompanying PhaseEnergy Changes Accompanying Phase ChangesChanges
- 14. Copyright 1999, PRENTICE HALL Chapter 1114 Phase ChangesPhase Changes Heating CurvesHeating Curves
- 15. Copyright 1999, PRENTICE HALL Chapter 1115 Heating Curve Illustrated
- 18. Phase Change Diagram - Water
- 19. vaporization energy in 2260 kJ fusion energy in 334 kJ condensation energy out 2260 kJ solidification energy out 334 kJ steam (1 kg) water (1 kg) ice (1 kg) Summary: Change of State
- 20. Energy involved in heating 1 kg of water ice steam(ice and water) melting (334) (420) water boiling (water and stream) (2260) energy / kJ Summary: from ice to steam 0 100
- 21. Heat of Combustion (HC) The amount of heat released per unit mass or unit volume of a substance when the substance is completely burned. Heats of combustion are used as a basis for comparing the heating value of fuels, since the fuel that produces the greater amount of heat for a given cost is the more economic. Equation: CHmQ ⋅=
- 22. Phase Change Example Heat is added to 0.5 kg of water at 20° C. How many joules of heat energy are required to change the water to steam at 110° C? Heat has to be added to the water to raise its temperature from 20° C to the boiling point 100° C. Heat has to be added to vaporize all of the water that is at 100° C. ( ) JQ CC Ckg J kgTcmQ oo o 167440 2010041865.0 = −⋅ ⋅ ⋅=∆⋅⋅=
- 23. Phase Change Example Heat has to be added to raise the temperature of all of the steam from 100° C to 110° C. Total heat energy: Q = 167440 J + 1130000 J + 10050 J = 1307490 J J1130000Q kg J 2260000kg5.0LvmQ = ⋅=⋅= ( ) J10050Q C100C110 Ckg J 2010kg5.0TcmQ oo o = −⋅ ⋅ ⋅=∆⋅⋅=
- 24. Final Temperature Example A 0.3 kg piece of ice at 0° C is placed in 1 kg of water at 40° C in an insulated container. If no heat is lost to the container, what is the final temperature of the water? Q lost water = Q gained ice + Q gained ice water (m·c·∆T)water = m·Lf + (m·c·∆T)ice water ( ) ( ) FFo o Fo F o o T kg J 1255.8J100200T C J 4186J167440 C0T Ckg J 4186kg0.3 kg J 334000kg0.3TC40 Ckg J 4186kg1 ⋅+=⋅− −⋅ ⋅ ⋅+ ⋅=−⋅ ⋅ ⋅