LECTURE 9 LATENT HEAT & PHASE CHANGE Lecture Instructor: Kazumi Tolich
Lecture 9 2! Reading chapter 17-5 to 17-6.! Latent heats " Latent heat of fusion " Latent heat of vaporization " Latent heat of sublimation! Phase change and energy conservation
Latent heats 3! When two phases coexist, the temperature remains the same even if a small amount of heat is added. Instead of raising the temperature, the heat goes into changing the phase of the material such as! melting (solid to liquid)! vaporization (liquid to gas)! sublimation (solid to gas)
Latent heats: 2 4! Adding heat does not change the temperature, but instead is used to break the intermolecular attractions.! The energy required to convert a substance of mass m from one phase to another is given by Q = ml where L is the latent heat.
Latent heats: 3 5! The latent heat to melt (or fuse) is the latent heat of fusion, L f.! The latent heat to convert a liquid to a gas is the latent heat of vaporization, L v.! The latent heat to convert a solid to a gas is the latent heat of sublimation, L s.
Water protects buds from cold weather 6! When temperature falls a few degrees below 0 C, fruit crop is in danger of being ruined.! To protect the buds, farmers spray the trees with water.! Water has large latent heat of fusion.! Before the buds can freeze, the water must be cooled to 0 C and then freeze.! In this process, the water gives up heat and keeps the temperature of the buds from going below 0 C.! The layer of ice over the buds acts like insulation because ice is not a good conductor of heat.
Demo: 1 7! CO 2 sublimation! Demonstration of sublimation of CO 2
Example: 1 8! A heat transfer of Q = 9.5 10 5 J is required to convert a block of ice at T i = -12 C to T f = 12 C. What was the mass of the block of ice?
9 Clicker question: 1
Example: 2 10! Steam can cause more serious burns than water at the same temperature. Flesh becomes badly damaged when its temperature reaches 50.0 C. a) Calculate the heat released as m = 12.5 g of liquid water at T i = 100 C is cooled to T f = 50.0 C. b) Calculate the heat released as m = 12.5 g of steam at T i = 100 C is condensed and cooled to T f = 50.0 C. c) Calculate the mass of flesh that can be heated from T i = 37.0 C (normal body temperature) to T f = 50.0 C. The average specific heat of flesh is c flesh = 3500 J/kg#K.
Feeling cold after swimming 11! You may feel cold when you are wet after swimming on a hot dry day.! The evaporation of the water on your skin cools you down.! When the humidity is high, evaporation proceeds more slowly because there is already a lot of water vapor in the air.! You may also feel colder when you are taking a shower with a bathroom door open compared to when the door is closed.
Phase changes and energy conservation 12! If heat is exchanged within a system, but not with the external world, the total energy of the system is constant.! The heat flow within the system can cause changes in temperature and phases.! Set the magnitude of the heat lost by one part of the system equal to the magnitude of the heat gained by another.
13 Clicker question: 2
Example: 3 14! An ice cube of a mass m I = 0.075 kg at T Ii = 0.0 C is dropped into a Styrofoam cup holding water of a mass m W = 0.33 kg at T w = 14 C. a) Find the final temperature of the system, T f, and the amount of ice melted. b) Find the initial temperature of the water that would be enough to just barely melt all of the ice.
Example: 4 15! An aluminum cylinder of a mass m A = 155 g is removed from a liquid nitrogen bath, where it has been cooled to T Ai = -196 C. The cylinder is immediately placed in an insulated cup containing m W = 80 g of water at T Wi = 15.0 C. What is the equilibrium temperature of this system, T f? If you answer is 0 C, determine the amount of water that has frozen. The average specific heat of aluminum over this temperature range is c A = 653 J/kg#K.
Example: 5 16! An iron block with a mass m I = 825 g is heated to T Ii = 352 C and placed in a insulated container (of negligible heat capacity) containing water of a mass m W = 40.0 g at T Wi = 20.0 C. What is the equilibrium temperature of this system, T f? If your answer is 100 C, determine the amount of water that has evaporated. The average specific heat of iron over this temperature range is c I = 560 J/kg#K.