phase changes surface tension and hydrogen bonding
TRANSCRIPT
Phase ChangesSurface Tension and Hydrogen Bonding
Time (min)
Tem
pera
ture
(C)
0 5 10 15 20 25 30 35 40 45 50
110
100
90
80
70
60
50
40
30
20
10
0
-10
Phase Change of Water
Ice MeltingT = Constant
Water BoilingT = Constant
Water HeatingTemp = Rising
Temperature is constant during a phase change. Normally adding heat energy to
something will increase its temperature as the molecules move faster. During a phase change the energy is used not to make the
molecules move faster but to break the chemical bonds holding them together.
Heat Energy Added (J)
Tem
pera
ture
(C)
110
100
90
80
70
60
50
40
30
20
10
0
-10
Energy vs Temperature
Solid Melting
Liquid Boiling
Liquid Warming
Why are steam burns worse than water burns?
Gas
Solid Warming
In order for liquid water to turn into a gas (steam) you have to add a lot of extra energy to it. When hot steam hits your skin all the energy from the steam is released onto your skin as the steam cools to the same temperature of your skin (thermal equilibrium). Remember heat always transfers from hot to cold. All this extra heat is called latent heat.
Turning a liquid into steam takes a LOT of energy and the opposite is true. Going from steam to a liquid (condensation) releases all this energy.
Heat Energy Added (J)
Tem
pera
ture
(C)
110
100
90
80
70
60
50
40
30
20
10
0
-10
Energy vs Temperature
Energy Released by water!
Energy Absorbed by water
Draw a Phase Change graph of Heat (y-axis) vs Temperature (x-axis).
• During Condensation energy is_______ by water?• During evaporation energy is ________ by water?• During melting energy is __________ by water?• During freezing energy is __________ by water?
• Why is evaporation called a cooling process?• What is latent heat of vaporization?• What is latent heat of fusion?
Why does water have such a high latent heat? Why is so much
energy released or absorbed?
Water has a + side and a – side!
Water is A Polar Molecule!
Electrical Charges are like tiny magnets.• Opposite Charges Attract ( + and - ) • Like Charges Repel ( + and + or - and - )
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N
S
N
SOpposites Poles Attract
N
S
N
S
Like Poles Repulse
+ -
+ +
- -
O
H H
O
H H+
-
Water Molecules are attracted to
one another!
O
H
HYDROGEN BONDS Require energy to break!
During a phase change the energy is used not to move the molecules but first to break the chemical (hydrogen) bonds holding the water molecules together!
Water as a Polar MoleculePhet Water Simulation Perform Polarity mini-Lab
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Surface Tension Demo and Paper towels.Watch Closely.
A water strider uses the high surface tension of water to walk on its surface.
• “A molecule within the bulk of a liquid experiences attractions to neighboring molecules in all directions, but since these average out to zero, there is no net force on the molecule. For a molecule that finds itself at the surface, the situation is quite different; it experiences forces only sideways and downward, and this is what creates the stretched-membrane effect.”
Via Chemwiki
SURFACE TENSION: Hydrogen bonding is stronger at the surface.
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Violating Kinetic Theory: Polarity of water
• What happens if you put a can of soda in the freezer and leave it there?
• Shouldn’t it contract (get smaller) as it cools?
• Why does it expand and explode?
12
ICE CUBE
Did Mr. Sapone Lie?
• Ice is colder than water so shouldn’t it be more dense?
• Does Ice Sink in water?
• Density Decreases as Temperature Increases
• Density Increases as Temperature Decreases
• Substances Expand when Heated
• Substances Contract when Cooled
T↑ V↑ D↓ T↓ V↓ D↑
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• Water at 4°C will EXPAND when cooled until 0°C at which point it becomes ice.
• Likewise, as Ice at 0°C melts it CONTRACTS as it turns into a liquid.
100°C
4°C
0°C
-10°C
Water behaves different but only in this range! (32F-40F)
Water expands when heatedWater contracts when cooled
Water expands when heatedWater contracts when cooled
This is important for life as we know it on earth and explains why soda explodes in the freezer!
Naked Science VideoPHET SIM
Water as a Polar MoleculePhet Water Simulation Perform Polarity mini-Lab
14
Surface Tension Demo and Paper towels.Watch Closely.
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Penny Drop DataStudent # Drops Student # Drops Student # Drops Student # Drops
J. Medina 73?
Defenitions
• Cohesion• Adhesion• Surface tension • Capillary action (link)• Hydrogen Bond• Intermolecular forces• Why does a meniscus form in a graduated cylinder?