Warm-up: 2/6/15 2/9/2015 LATENT HEAT AND PHASE CHANGES INTERNAL ENERGY

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1 If thermal energy is added to the water at a rate of 84 W, how much time would it take to bring the vat of water from room temperature (21.0 C) to boiling (100.0 C)? Warm-up: 2/6/15 { George Washington ran a distillery at his Mount Vernon estate. The vats where water was heated held 795 L of water. The current method of heating the water is through the use of an immersion heater hot wires with current running through them that have been submersed in the water. LATENT HEAT AND PHASE CHANGES A.S Due Thursday 2/12 There will be a quiz on Thursday INTERNAL ENERGY The total of the potential energy and random kinetic energy of all the particles in the substance Potential energy comes from bonds and intermolecular forces Typically, within a particular phase, the farther apart the molecules/atoms are, the higher the potential energy will be Average kinetic energy of the particles in the substance is related to the absolute (Kelvin) temperature of the substance 1

2 SKIT ASSIGNMENT Groups assigned Create a skit to model the molecular behavior of the 3 main states of matter as the temperature rises MACROSCOPIC CHARACTERISTICS OF MATTER Solid Liquid Gas Shape Volume Compressibility Diffusion MICROSCOPIC CHARACTERISTICS OF MATTER Solid Liquid Gas Kinetic Energy Potential Energy Mean molecular separation Molecules per m 3 2

3 WARM-UP: 2/9/15 Copy the diagram, and label each arrow with the correct name for the phase change that occurs in the direction indicated: SOLID LIQUID GAS QUANTIFYING PHASE CHANGES Latent Heat: The energy required to achieve the change of phase of a substance Energy added/removed is used to change the potential energy of the particles in the substance. The average kinetic energy remains constant, which means that the temperature will remain constant throughout the entirety of the phase change QUANTIFYING PHASE CHANGES Specific Latent Heat of Fusion The energy required to change the phase of 1 kg of substance from a solid to a liquid without any temperature change Add energy melt Remove energy freeze 3

4 QUANTIFYING PHASE CHANGES Specific Latent Heat of Vaporization The energy required to change the phase of 1 kg of substance from a liquid to a gas without any temperature change Add energy vaporize Remove energy condense QUANTIFYING PHASE CHANGE Q = ml Q = thermal energy added or removed (depending on phase change) Units typically in joules / J m = mass / kg L = specific latent heat / J kg -1 L f specific latent heat of fusion L v specific latent heat of vaporization SAMPLE PROBLEM 2.0 kg of solid water (ice) at exactly 0.0 C is to be changed into liquid water at this temperature. Calculate the amount of energy needed to be added to the water to melt it. (L f = 3.34 x 10 5 J kg -1 ) How much energy is required to raise the temperature of the same 2.0 kg of water, now that it s fully melted, to its boiling point? The same 2.0 kg of water now is boiled until it vaporizes completely into steam at 100. C. How much energy must be added to the water to just vaporize it? (L v = 2.26 x 10 6 J kg -1 ) 4

5 LATENT HEAT AND CALORIMETRY Quite often, total energy added involves both specific heat and specific latent heat quantities. For example, similar to the previous sample: How much energy must be added to a 3.2 kg of sample of ice, originally at 0.0 C, so that it becomes steam at C? Turn to you neighbors: 1 minute discuss how you would approach this problem PROBLEM SOLVING TIPS WHEN PROBLEMS INCLUDE SPECIFIC HEAT AND LATENT HEAT Make a column for each change that is occurring as the energy is added or removed. A change is either: phase change or change in temperature Put a brief heading at the top of each column (i.e. melt liquid water vaporize ) Under the description, write the equation that you will use to find the thermal energy for that segment Write your variables and constants for each column that you will use in the equations you listed. Solve for each individual amount of energy Add them all together! (and circle your answer you re done!) CALORIMETRY EXAMPLE Steam at 100 C is bubbled into kg of water at 30 C in a calorimeter cup. How much steam will have been added when the water in the cup reaches 51 C? (Ignore the effect of the cup.) Step 1: 2 columns gaining energy and losing energy Step 2: In each column, determine if there is a temperature change or phase change occurring Step 3: Set up the values (energy gained = energy lost) as you would under a calorimetry problem involving only temperature changes, but this time you ll have a phase change to deal with as well. 5

6 STEAM AT 100. C IS BUBBLED INTO KG OF WATER AT 30.0 C IN A CALORIMETER CUP. HOW MUCH STEAM WILL HAVE BEEN ADDED WHEN THE WATER IN THE CUP REACHES 51.0 C? (IGNORE THE EFFECT OF THE CUP.) Losing Energy Gaining Energy Steam condensing Hot water cooling Cool water warming Q = ml v Q = mcdt Q = mcdt m =?? m =?? m = kg L = 2.26 x 10 6 J kg -1 c = 4186 J kg -1 C -1 c = 4186 J kg -1 C -1 T i = 100. C T i = 30.0 C T f = 51.0 C T f = 51.0 C STEAM AT 100. C IS BUBBLED INTO KG OF WATER AT 30.0 C IN A CALORIMETER CUP. HOW MUCH STEAM WILL HAVE BEEN ADDED WHEN THE WATER IN THE CUP REACHES 51.0 C? (IGNORE THE EFFECT OF THE CUP.) ml v + mc T = mc T m 2.26x m 4186 ( ) = (0.330)(4186)( ) m 2.26x (m)(205114) = m = m = kg SAMPLE PROBLEM #3 A volume of 0.80 L of water at 19 C is put into an aluminum icecube tray of mass kg at the same temperature. How much energy must be removed from this system by the refrigerator to turn the water into ice at -9.0 C? 6