LECTURE 10 PHASES, EVAPORATION & LATENT HEAT Lecture Instructor: Kazumi Tolich Lecture 10 2 ¨ Reading chapter 13-5 to 13-6 & 14-3. ¤ Phase equilibrium ¤ Evaporation ¤ Latent heats n Latent heat of fusion n Latent heat of vaporization n Latent heat of sublimation 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. 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. 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. 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. ¨ 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 = 273.16 K and P = 611.2 Pa. Slope of the fusion line 7 ¨ 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 Example: 1 8 ¨ The phase diagram for water is shown. a) What is the temperature T1? b) What is the temperature T2? 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? Evaporation 9 ¨ 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. Demo: 1 10 ¨ Freezing water by evaporation ¤ When the pressure is lowered, water boils at a lower temperature. ¤ Evaporation of water lowers the temperature of water itself. Evaporating atmosphere 11 ¨ The average kinetic energy of the molecules is proportional to temperature. H2 He ¨ For the temperature of Earth’s atmosphere, O2 most of N2 and O2 are moving at speed much less than the escape speed. ¨ Having higher speed, most of H2 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. Latent heats 12 ¨ 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, breaking the intermolecular attractions. Latent heats: 2 13 ¨ The energy required to convert a substance of mass m from one phase to another is given by Q = mL where L is the coefficient of latent heat. ¤ The latent heat to melt (or fuse) is the latent heat of fusion, Lf. ¤ The latent heat to convert a liquid to a gas is the latent heat of vaporization, Lv. ¤ The latent heat to convert a solid to a gas is the latent heat of sublimation, Ls. Demo: 2 14 ¨ CO2 sublimation ¤ Demonstration of sublimation of CO2 Example: 2 15 ¨ A heat transfer of Q = 9.5 × 105 J is required to convert a block of ice at Ti = -12 °C to Tf = 12 °C. What was the mass of the block of ice? Clicker question: 1 16 Example: 3 17 ¨ 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 Ti = 100 °C is cooled to Tf = 50.0 °C. b) Calculate the heat released as m = 12.5 g of steam at Ti = 100 °C is condensed and cooled to Tf = 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 cflesh = 3500 J/kgK. Clicker question: 2 18 Water protects buds from cold weather 19 ¨ 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. .