Heat in Changes of State

1
Heat in Changes of State
2
Energy Changes Accompanying Phase Changes
Gas
Condensation
Vaporization
Sublimation
Deposition
Energy of system
Liquid
Melting
Freezing
Solid
Brown, LeMay, Bursten, Chemistry 2000, page 405
3
Heating Curve for Water
E
gas
D
100
C
liquid
Temperature (oC)
B
0
A
solid
Heat added
LeMay Jr, Beall, Robblee, Brower, Chemistry
Connections to Our Changing World , 1996, page
487
4
Heating Curve for Water
vaporization
E
gas
D
100
condensation
C
liquid
melting
Temperature (oC)
B
0
A
freezing
solid
Heat added
LeMay Jr, Beall, Robblee, Brower, Chemistry
Connections to Our Changing World , 1996, page
487
5
Heating Curves

• Phase Change
• change in PE (molecular arrangement)
• temp remains constant

• Heat of Fusion (?Hfus)
• energy required to melt a specific amount of a
  substance at its melting point

 
 

• Heat of Vaporization (?Hvap)
• energy required to vaporize a specific amount of
  a substance at its boiling point

 
 
6
Heating Curves
140
120
100
Endothermic
80
60
40
Temperature (oC)
20
0
-20
Exothermic
-40
-60
-80
-100
Time
7
Heating Curves

• Temperature Change
• change in KE (molecular motion)
• depends on heat capacity

• Heat Capacity
• energy required to raise the temp of 1 gram of a
  substance by 1C
• water has a very high heat capacity

• The quantity of a substance, the amount of heat
  transferred, its heat capacity, and the
  temperature change are related in two ways
• q nCp?T where n number of moles of
  substance
• q mCs?T where m mass of substance in
  grams

Courtesy Christy Johannesson www.nisd.net/communic
ationsarts/pages/chem
8
Calculating Energy Changes - Heating Curve for
Water

140
DH mol x DHvap

DH mol x DHfus
120
100
80
Heat mass x Dt x Cp, gas
60
40
Temperature (oC)
20
Heat mass x Dt x Cp, liquid
0
-20
-40
-60
Heat mass x Dt x Cp, solid
-80
-100
Time
9
Phase Diagrams

• Show the phases of a substance at different temps
  and pressures.

Courtesy Christy Johannesson www.nisd.net/communic
ationsarts/pages/chem