Thermodynamics
& Phase Change
Heat can only do one
thing at a time: either
change the temperature
OR
change the state!
Changes of state
–
seen
in phase change diagram
temperature
heat added
Diagonal regions:
where
thermal energy is added and
particles increase in motion
(temperature changes)
Flat regions:
where changes in
state occur. All added thermal
energy goes to overcome the
forces holding the particles
together.
Heat of fusion
–
The amount
of energy required to
melt
one
kilogram of a substance
Heat of vaporization
–
the
amount of energy required to
vaporize
one kilogram of a
substance
Heat of Fusion &
Vaporization for Water
Heat of fusion for water:
•
H
f
= 3.34x10
5
J/kg
•
positive when melting, negative when freezing
Heat of vaporization for water:
•
H
v
= 2.26x10
6
J/kg
•
positive when evaporating, negative when
condensing
Heat required to melt:
Q =
mH
f
*use for melting & freezing!
Heat required to vaporize:
Q =
mH
v
*use for evaporating & condensing!
Specific Heat Values
C
ice
= 2060 J/kg
·˚
C
C
water
= 4180
J/kg
·˚
C
C
steam
= 2020
J/kg
·˚
C
Use these values for temps
in Celsius or Kelvin!
Example #1
You are asked to melt 0.100
kg of ice at its melting point
and warm the resulting water
to 20.0 ºC. How much heat
is needed?
Calculate heat needed to melt the ice.
Q
1
= mH
f
Calculate heat needed to raise water
temperature.
Q
2
= mC
Δ
T
Add all heats together to get the total heat
needed.
Q = Q
1
+ Q
2
Example #1 Strategy:
Example #1 Solution:
Q
1
=
33400 J
Q
2
=
8360 J
Q =
41,760 J
Example #2
A 40.0

g sample of
chloroform is condensed
from a vapor at 61.6 ºC. It
liberates 9,870 J of heat.
What is the heat of
vaporization of chloroform?
Example #2 Solution:
Q = mH
v

9870 = (.040)(
H
v
)
H
v
=

246,750 J/kg
(negative because it’s cooling)
Law of Conservation
of Energy
Energy cannot be destroyed
or created only transferred.
1
st
Law of Thermodynamics
The total increase in the
thermal energy of a system is
the sum of the heat added to it
and the work done on it.
Heat engines are devices which
convert thermal energy to
mechanical energy continuously
along with some waste heat.
2
nd
Law of Thermodynamics
Entropy of the universe is
always maintained or
increased. All things will
become more and more
disordered unless some
action is taken.
Entropy is a measure of the
disorder in a system.
Entropy increases when heat
is added and decreases
when heat is removed.
2
nd
Law of Thermodynamics
Thermal Engine efficiency
Called Carnot efficiency, or
the ideal efficiency of a heat
engine
(T
hot

T
cold
)/T
hot
= Ideal efficiency
Temperatures must be in
Kelvin.
Result is a percentage.
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