Thermodynamics & Phase Change

mistaureolinΜηχανική

27 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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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.