Study Buddy: Energy and Phase Changes
Terms
Heat
Description
Amount of thermal energy transferred from one
system to another
Picture
http://commons.wikimedia.org/wiki/File:Hand_left.svg
http://www.clker.com/cliparts/i/7/R/z/J/P/ice-cube-md.png
Describes the total amount of energy in a system.
Equals the sum of kinetic and potential energies.
Total Internal
Energy
Kinetic Energy: due to motion of particles
Exothermic
Process
Vapor
Pressure
low
PE or Ep
Potential Energy: due to relative position of particles
Endothermic
Process
KE or Ek
high
Symbol/Unit
Q/ Joules (J)
or calorie
(cal)
IE/ Joules (J)
or calorie
(cal)
System absorbs energy from surroundings
Phase changes: melting, vaporizing, subliming
http://sweetclipart.com/cooking-pot-coloring-page-2015
high
low
far apart
close
Stove top or fire
System releases heat to surroundings
Phase changes: freezing, condensing, depositing
Pressure exerted by the vapor particles over a liquid
or solid substance at equilibrium.
+Q for
system
-Q for system
Bowl of ice
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com/physics/Chemistry_Ph
ysics/physics15.htm
How does thermal energy transfer cause phase changes?
As a solid or liquid is heated, the thermal energy is used to increase the kinetic and potential energy of
the particles (particles moving faster and farther apart). When a liquid or gas is cooled, the thermal
energy transferred to the surroundings reduced the kinetic and potential energies of the particles.
Describe liquid
particles’ positions
and motion:
 Moving faster
 Farther apart
Describe solid
particles’ positions
and motion:
 Moving slowly
 Close together
Solid water
Liquid water
Melting = _0_°C
Describe gaseous
particles’ positions
and motion:
 Moving quickly
 Far apart
Water vapor
Boiling = _100_°C
Average KE or Temperature
Translating to a Heating or Cooling Curve
∆KE
∆PE
100°C
∆KE
0°C
∆PE
∆KE
Why does the temperature remain constant when
energy is added during a phase change? Average
KE remains the same as energy is added because
the energy is being used to increase PE by
overcoming attractive forces between particles and
move them farther apart.
Time or Energy added
Label where PE and KE are changing on the heating curve above.
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Cooling Curve for Water
Temperature
Temperature
Heating Curve for Water
100°C
0°C
100°C
0°C
Heat added
Heat removed
Does the total internal energy of 10.g of water vapor change as it becomes ice? Why?
The total internal energy decreases as the water vapor condenses and freezes to form ice. As a
gas, the particles have high kinetic and potential energies because they are moving quickly and
relatively far apart from each other. Solid particles are closer together and moving slower. The
energy that the water loses is given off to the surroundings.
How would a heating or cooling curve for a mixture look different than a pure substance?
The mixture would not be as “crisp” as that for the pure substance since the different
components have phase changes at different points. A line is shown in blue on the cooling curve
above to represent a theoretical mixture.
Vapor Pressure and Boiling Water
Houston
Mt. Everest
8850 m above
sea level
When does boiling occur?
Boiling occurs when the vapor
pressure equals the external
pressure.
Mt. Everest
1 atm
bp=100°C
0.33 atm
bp=71°C
Houston
0 m above sea level
Why can water boil at a lower temperature on Mt. Everest than in Houston?
The atmosphere at Mt. Everest has less air particles resisting the water particles going from liquid to gas
(the air particles are further apart on Mt. Everest) therefore the water particles do not need as much
energy to move to the gas phase. There is less external pressure, so a lower temperature will be able to
create a vapor pressure equal to the atmospheric pressure.
Where would boiling water in Houston and
Mt. Everest be shown in the diagram?
How do the external pressure and
temperature affect the phase of matter?
High pressure and low temperature favor
solid phase of matter;
External Pressure (atm)
Phase Diagram for Water
Liquid
Solid
Houston
Mt. Everest
Triple Point
Gas
Low pressure and high temperature favor the
gas phase
Temperature (oC or K)
8/31/15
SCIE_CHEM_PCHNG_MAT_STUDYBUDDYTE_AL
copyright © 2015 CFISD
8/31/15
SCIE_CHEM_PCHNG_MAT_STUDYBUDDYTE_AL
copyright © 2015 CFISD