WARM-UP: 3/21/16 SOLIDGASLIQUID Copy the diagram, and label each arrow with the correct name for the phase change that occurs in the direction indicated:

Read pages WA: Latent Heat – due Thursday LATENT HEAT AND PHASE CHANGES

GRAB A BIG WHITEBOARD! (1 PER GROUP) Make 2 columns In column 1, list each phase change that requires the addition of thermal energy. Top to Bottom, they should be in order of increasing amount of energy that must be added to make the phase change. In column 2, list each phase change that requires the removal of thermal energy. Top to Bottom, they should be in order of increasing amount of energy that must be removed in order to make the phase change.

QUANTIFYING PHASE CHANGES Latent Heat: The energy required to achieve the change of phase of a substance What changes in the substance as the energy is added or removed? 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

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

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 )

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 3.20 kg of sample of ice, originally at 0.0°C, until it is 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!)

HOW MUCH ENERGY MUST BE ADDED TO A 3.20 KG OF SAMPLE OF ICE, ORIGINALLY AT 0.0°C, UNTIL IT IS STEAM AT °C? Melt IceHeat Liquid WaterVaporize to steamHeat Steam Q=mL Q=mc  T Q=mL Q=mc  T m= 3.20 kg L =c =L =c =  T = 100.0°  T = 15.0°

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.

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 EnergyGaining Energy Steam condensingHot water coolingCool water warming Q = mL v Q = mc  T m = ?? m = kg L = 2.26 x 10 6 J kg -1 c = 4186 J kg -1 °C -1 T i = 100. °CT i = 30.0 °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.)

SAMPLE PROBLEM #3 A volume of 0.80 L of water at 19°C is put into an aluminum ice- cube 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?