Section 12.4 Phase Changes
• Explain how the addition and removal of energy
can cause a phase change.
• Interpret a phase diagram.
Matter changes phase when energy is added
or removed
Energy Changes Accompanying Phase Changes
Gas
Condensation
Energy of system
Vaporization
Sublimation
Liquid
Melting
Freezing
Solid
Brown, LeMay, Bursten, Chemistry 2000, page 405
Deposition
Section 12.4
Phase Changes That Require Energy
• Melting occurs when heat flows into a solid object.
• The process by which a solid changes into a
liquid.
• The melting point of a crystalline solid is the
temperature at which the forces holding the crystal
lattice together are broken and it becomes a liquid.
• Add energy to overcome intermolecular forces.
Section 12.4 Phase Changes That Require Energy
(cont.)
• Vaporization is the process by which a liquid
changes to a gas or vapor.
-Vaporization is an endothermic process it requires heat.
-Energy is required to overcome intermolecular
forces
-Responsible for:
-cool earth
-Why we sweat
• Two types of vaporization seen are, evaporation
and boiling.
Section 12.4 Phase Changes That Require Energy
• Evaporation is vaporization only at the
surface of a liquid.
• To evaporate, molecules must have
sufficient energy to break IM forces.
• Molecules at the surface break away
and become gas.
• Only those with enough KE escape.
• Breaking IM forces requires energy.
• The process of evaporation is
endothermic.
endothermic
• Evaporation is a cooling process.
• It requires heat.
Section 12.4
Phase Changes That Require Energy (cont.)
• In a closed container, the pressure exerted by a
vapor over a liquid is called vapor pressure.
Section 12.4
Phase Changes That Require Energy (cont.)
• The boiling point is the temperature at which the
vapor pressure of a liquid equals the atmospheric
pressure.
Section 12.4
Phase Changes That Require Energy (cont.)
• Sublimation is the process by which a solid
changes into a gas without becoming a liquid.
Energy Changes Accompanying Phase Changes
Gas
Condensation
Energy of system
Vaporization
Sublimation
Liquid
Melting
Freezing
Solid
Brown, LeMay, Bursten, Chemistry 2000, page 405
Deposition
Section 12.4
Phase Changes That Release Energy
• Freezing is the process by which a liquid
changes into a solid.
• As heat flows from water to the surroundings, the
particles lose energy.
• The freezing point is the temperature at
which a liquid is converted into a crystalline
solid.
Section 12.4 Phase Changes That Release Energy
(cont.)
• The process by which a gas or vapor
becomes a liquid is called condensation.
•
Achieves a dynamic equilibrium with
vaporization in a closed system.
What is a closed system?
A closed system means
matter can’t go in or out.
(put a cork in it)
What the heck is a
“dynamic equilibrium?”
Dynamic Equilibrium
When first sealed, the molecules
gradually escape the surface of the
liquid.
As the molecules build up above the
liquid - some condense back to a
liquid.
The rate at which the molecules
evaporate and condense are equal.
Dynamic Equilibrium
As time goes by the rate of vaporization
remains constant but the rate of
condensation increases because there
are more molecules to condense.
Equilibrium is reached when:
Rate of Vaporization = Rate of Condensation
Molecules are constantly changing phase “dynamic”
The total amount of liquid and vapor remains constant
“equilibrium”
Section 12.4 Phase Changes That Release Energy
(cont.)
• Deposition is the process by which a gas
or vapor changes directly to a solid, and is
the reverse of sublimation.
Heating Curves
Gas- KE
140
120
Temperature (oC)
100
80
Liquid- KE
Vaporization- PE , KE remains the same
60
40
20
0
-20
-40
-60
-80
-100
Melting- PE , KE remains the same
Solid- KE
Time
Section 12.4
Phase Diagrams
• A phase diagram is a graph of pressure versus
temperature that shows in which phase a
substance will exist under different conditions of
temperature and pressure.
• The triple point is the point on a phase diagram
that represents the temperature and pressure at
which all three phases of a substance can
coexist.
http://www.wwnorton.com/college/chemistry/gilbert/overview/ch9.htm#intermolecular_force
Section 12.4 Phase Diagrams (cont.)
• The phase diagram for different substances
are different from water.