CHEMISTRY
Nov 16, 2009
Warm Up
• Name the different phase changes of a
matter
– For example: Solid  Liquid (Melting)
Agenda
• Unit 4 Phase Changes Notes and Heat
Transfer
• Collect Progress Report
• Homework
– Phase Change Worksheet
– Last Day for Make up Work : Wednesday
– Research Project: Due Thursday/Friday
Phase Changes
• Explain how the addition and removal of
energy can cause a phase change
• To learn about interactions among water
molecules.
• To understand and use heat of fusion and
heat of vaporization.
• Interpret a heating/cooling curve for water
Phase Changes
• Refer to diagram
– Six possible transitions between phases
Phase Changes that Require
Energy
• What happens to molecules in a solid as it
melts?
• Melting
– The amount of energy (heat of fusion)
required to melt one mole of a solid depends
on the strength of the forces keeping the
particles together (Intermolecular force).
Phase changes that require energy
• When liquid water is heated, some
molecules escape from the liquid and
enter the gas phase.
Phase changes that require energy
• If a substance is usually a liquid at room
temperature (as water is), the gas phase is
called a vapor.
• Vaporization is the process by which a
liquid changes into a gas or vapor.
• As temperature increases, water molecules
gain kinetic energy
– At Boiling point, molecules throughout the
liquid have the energy to enter the gas or vapor
phase.
Phase changes that require energy
• The process by which a solid changes
directly into a gas without first becoming a
liquid is called Sublimation.
– Solid air fresheners and dry ice are examples
of solids that sublime.
Phase changes that release energy
• Some phase changes release energy into their
surroundings.
• For example, when a vapor loses energy, it may
change into a liquid.
• Condensation is the process by which a gas or
vapor becomes a liquid. It is the reverse of
vaporization.
Phase changes that release
energy
• Water vapor undergoes condensation when
its molecules lose energy, their velocity
decreases.
• The freezing point is the temperature at
which a liquid becomes a crystalline solid.
• When a substance changes from a gas or
vapor directly into a solid without first
becoming a liquid, the process is called
deposition.
– Deposition is the reverse of sublimation. Frost
is an example of water deposition.
Phase Change Diagram
• Potential energy (Ep)
– Stored energy
• Energy of position
• Chemical energy (gas or food)
• Electrical energy (batteries)
• Kinetic energy (Ek)
– Motion
• Mechanical energy
• Radiant (Ed)
– Heat/Light/Sound
Summary: Phase changes
(Refer to handout: Label phases, Label energy change, draw atomic diagram at each phase
Temperature
Heating Curve
Energy or Time
)
Practice
Substance Freezing point (oC) Boiling Point (oC)
Water
0.0
100.0
Gallium
23.0
89.0
Iron
723.0
2780.0
• At room temperature (27 oC), Iron is a solid,
mixture, liquid or gas?
• At 800 oC, Iron is a solid, mixture, liquid or gas?
• During the process of heating water from 27 to 85
oC :
– Did the potential energy change? Kinetic energy?
– Is it an endothermic or exothermic reaction?
Homework
• Phase Change Worksheet
Methods of Heat Transfer
• http://www.wisconline.com/objects/index_tj.asp?objid=SC
E304
Methods of Heat Transfer
• Conduction:
– Transfer of heat between substances that are in direct
contact with each other
• Occurs mainly in solid
• Better conductor  More rapid heat transfer
– Examples of good and poor conductors?
• Convection:
– Up and down movement (circulation) of gases and liquids
caused by heat transfer
• Does not occur in solid (molecules not free to move around)
• Examples of convection?
• Radiation:
– Electromagnetic waves traveling through space
• Does not require a medium to transfer heat
– Waves transfer heat to the object
• Examples of radiation heat transfer?
Practice
Heat Transfer
• Boiling water over a campfire
• Melting a tub of ice cream on the kitchen counter
• Electric Stove versus Gas Stove
– Which stove will boil water faster? Why?
• Why is the second floor usually warmer than the first
floor? Why?
Warm Up
• Energy Conversions
– Flash Light
• A 20 g sample of water is put into a
calorimeter and heated until its
temperature increases from 60 to 80oC. If
the specific heat of water is 4.184J/g.oC,
calculate the heat absorbed by the water.
Conversions
• Converting between oC & oF
– ºC = 5/9(ºF – 32)
– ºF = 9/5 (ºC) + 32
• Converting between oC & K
– ºC = K – 273
– K = ºC + 273
• Practice Problems
– Convert 37 ºC to K
– Convert 100 oF to ºC
– Convert 50 oF to K
Conversions
• The breakfast shown in the photograph
contains 230 nutritional Calories.
• How much energy in joules will this healthy
breakfast supply?
Conversion factor
1 Calorie = 1000 calories
1 cal = 4.19 Joules
Calorimetry Practice Problems
• What amount of heat would be given off by
3.0 x 103 g of water in order to lower its
temperature from 95oC to 12oC?
Practice Problem
• If 5603 joules of heat is added to 5.6 g of
water at a temperature of 15oC, what will
the final temperature be?
• What amount of heat would be given off by
7.0 x 103 g of water in order to lower its
temperature by 5oC?
Practice Problem
• What is the specific heat of lead that has a
mass of 30 g and undergoes a 250oC
change while absorbing 229.5 calories?
• Copper has a specific heat of 0.387 J/g.oC.
What is the mass of a piece of copper that
undergoes a 25oC temperature change
when it absorbs 755 J of energy?
Conversions
• Converting between oC & oF
– ºC = 5/9(ºF – 32)
– ºF = 9/5 (ºC) + 32
• Converting between oC & K
– ºC = K – 273
– K = ºC + 273
• Practice Problems
– Convert 37 ºC to K
– Convert 100 oF to ºC
– Convert 50 oF to K
Conversions
• The breakfast shown in the photograph
contains 230 nutritional Calories.
• How much energy in joules will this healthy
breakfast supply?
Conversion factor
1 Calorie = 1000 calories
1 cal = 4.19 Joules
Practice Problems
• Refer to handout
– Specific heat problems
CHEMISTRY
September 29, 2010
Warm Up
• Study for Quiz
– 5 minutes
Agenda
• Quiz
• Homework
– Research Project Presentation
• Due Thursday/Friday
– Extra Credit: Dress Up
Quiz
20. Draw what happens to kinetic energy in
water as you cool it from 90oC to 10oC (Use
circles to represent molecules! Use “whoosies”
to represent speed of molecules)
21. What does it mean when an energy resource
is said to be “renewable”? Provide 2 examples
22. What are “fossil fuels”? Which nonrenewable energy resources are “fossil fuels”?