LECTURE 10 PHASES, EVAPORATION & LATENT HEAT. Lecture Instructor: Kazumi Tolich

Transcription

1 LECTURE 10 PHASES, EVAPORATION & LATENT HEAT Lecture Instructor: Kazumi Tolich

2 Lecture 10 2 Reading chapter 17-4 to Phase equilibrium Evaporation Latent heats n Latent heat of fusion n Latent heat of vaporization n Latent heat of sublimation

3 Phase equilibrium 3 If a substance has two or more phases that coexist in a steady and stable fashion, the substance is in phase equilibrium. The pressure of the gas when equilibrium is reached is called equilibrium vapor pressure.

4 Vapor-pressure curve 4 A plot of the equilibrium vapor pressure versus temperature is called vapor-pressure curve. A liquid boils at the temperature at which its vapor pressure equals the external pressure.

5 Pressure cookers 5 The pressure inside the cooker is higher than atmospheric pressure, so the water boils at the higher temperature. Food cooks faster at the higher temperature.

6 Phase diagram 6 A fusion curve indicates where the solid and liquid phases are in equilibrium. A sublimation curve indicates where the solid and gas phases are in equilibrium. A plot showing a vapor-pressure curve, a fusion curve, and a sublimation curve is called a phase diagram.

7 Phase diagram: 2 7 The vapor-pressure curve comes to an end at the critical point. Beyond the critical point, there is no distinction between liquid and gas. At triple point, all three phases, solid, liquid, and gas, are in equilibrium. In water, the triple point occur at T = K and P = Pa.

8 Slope of the fusion line 8 In most substances, the fusion line has a positive slope. As the pressure increases, the melting temperature of the substance also increases because a solid is denser than the corresponding liquid. In water, the slope is negative because ice is less dense. Typical substance Water

9 Example: 1 9 The phase diagram for water is shown. a) What is the temperature T 1? b) What is the temperature T 2? c) What happens to the melting/ freezing temperature if atmospheric pressure is decreased? d) What happens to the boiling/ condensation temperature if atmospheric pressure is increased?

10 Evaporation 10 If you leave a cup of water out at room temperature, the water eventually evaporates. The temperature of the water reflects the average kinetic energy of the water molecules. The most energetic molecules have enough energy to break loose from the molecular bonds at the surface of the water. As these highest-energy molecules leave the water, the average energy of those left behind decreases. Evaporation is a cooling process.

11 Demo: 1 11 Freezing water by evaporation When the pressure is lowered, water boils at a lower temperature. Evaporation of water lowers the temperature of water itself. The water reaches the triple point.

12 Evaporating atmosphere 12 The average kinetic energy of the molecules is proportional to temperature. For the temperature of Earth s atmosphere, most of N 2 and O 2 are moving at speed much less than the escape speed. Having higher speed, most of H 2 and He have escaped. The escape speed at the surface of the moon is 2.3 km/s, so all gas molecules/atoms escaped, and the moon does not have atmosphere. H 2 He O 2

13 Latent heats 13 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)

14 Latent heats: 2 14 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.

15 Latent heats: 3 15 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.

16 Demo: 2 16 CO 2 sublimation Demonstration of sublimation of CO 2

17 Example: 2 17 A heat transfer of Q = 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?

18 18 Clicker question: 1

19 Example: 3 19 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.

20 20 Clicker question: 2

21 Feeling cold after swimming 21 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.

22 Water protects buds from cold weather 22 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.