Transcript 13.2 The Nature of Liquids

13.2 The Nature of Liquids>
Chapter 13
States of Matter
13.1 The Nature of Gases
13.2 The Nature of Liquids
13.3 The Nature of Solids
13.4 Changes of State
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13.2 The Nature of Liquids>
CHEMISTRY
& YOU
How hot should water be when you
make coffee?
Ground coffee beans
contain many different oils,
which contribute to the
flavor and aroma of the
brew. If the water used for
brewing the coffee is too
hot, some of these oils
vaporize, leaving the coffee
less rich and flavorful.
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13.2 The Nature of Liquids> A Model for Liquids
A Model for Liquids
What factors determine the physical
properties of a liquid?
3
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13.2 The Nature of Liquids> A Model for Liquids
Substances that can flow are referred to
as fluids.
• Both liquids and gases can flow.
– The ability of
gases and
liquids to flow
allows them to
conform to the
shape of their
containers.
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13.2 The Nature of Liquids> A Model for Liquids
Gases and liquids have a key difference
between them.
• According to kinetic theory, there are no
attractions between the particles in a gas.
• The particles in a liquid are attracted to
each other.
– These intermolecular attractions keep the
particles in a liquid close together, which is
why liquids have a definite volume.
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13.2 The Nature of Liquids> A Model for Liquids
The interplay between the disruptive
motions of particles in a liquid and the
attractions among the particles
determines the physical properties of
liquids.
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13.2 The Nature of Liquids> A Model for Liquids
Liquids are much more dense than gases.
• Increasing the pressure on a liquid has
hardly any effect on its volume.
• The same is true for solids.
– Liquids and solids are known as
condensed states of matter.
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13.2 The Nature of Liquids>
Describe one way in which liquids and
gases are similar, and one way in
which they are different.
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13.2 The Nature of Liquids>
Describe one way in which liquids and
gases are similar, and one way in
which they are different.
Both liquids and gases can flow, so they can
take the shape of their container. The
molecules in a liquid have intermolecular
attractions that are not present in gases.
Therefore, liquids have a definite volume and
will not simply fill their container.
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13.2 The Nature of Liquids> Evaporation
Evaporation
What is the relationship between
evaporation and kinetic energy?
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13.2 The Nature of Liquids> Evaporation
The conversion of a liquid to a gas or
vapor is called vaporization.
• When this conversion occurs at the
surface of a liquid that is not boiling, the
process is called evaporation.
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13.2 The Nature of Liquids> Evaporation
The process of evaporation has a different
outcome in an open system, such as a lake or an
open container, than in a closed system, such as
a sealed container.
• In an open
system,
molecules
that
evaporate
can escape
from the
system.
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• In a closed
system, the
molecules
collect as a
vapor above
the liquid.
Some
condense back
into a liquid.
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13.2 The Nature of Liquids> Evaporation
During evaporation, only those
molecules with a certain minimum
kinetic energy can escape from the
surface of the liquid.
• Even some of the particles that do
escape collide with molecules in the
air and rebound back into the liquid.
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13.2 The Nature of Liquids> Evaporation
A liquid evaporates faster when heated.
• Heating the liquid increases the average
kinetic energy of its particles.
• The added energy enables more particles
to overcome the attractive forces keeping
them in the liquid state.
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13.2 The Nature of Liquids> Evaporation
As evaporation occurs, the particles with the
highest kinetic energy tend to escape first.
• The particles left in the liquid have a lower
average kinetic energy than the particles that
have escaped.
• As evaporation takes place, the liquid’s
temperature decreases.
– Evaporation is a cooling process.
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13.2 The Nature of Liquids> Evaporation
You can observe the effects of evaporative
cooling on hot days.
• When you perspire, water molecules in your
perspiration absorb heat from your body and
evaporate from the skin’s surface.
• This evaporation leaves the remaining
perspiration cooler.
– The perspiration that remains cools you
further by absorbing more body heat.
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13.2 The Nature of Liquids>
Explain why heating a liquid causes
evaporation to occur faster.
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13.2 The Nature of Liquids>
Explain why heating a liquid causes
evaporation to occur faster.
Evaporation occurs when particles have enough
kinetic energy to overcome the forces holding the
particles together in a liquid. Heating a liquid
increases the average kinetic energy of its
particles. Increasing the average kinetic energy
increases the number of particles with enough
energy to escape the liquid by evaporating.
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13.2 The Nature of Liquids> Vapor Pressure
Vapor Pressure
When can a dynamic equilibrium exist
between a liquid and its vapor?
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13.2 The Nature of Liquids> Vapor Pressure
The evaporation of a liquid in a closed
system differs from evaporation in an
open system.
• When a partially filled container of liquid is
sealed, some of the particles at the surface
of the liquid vaporize.
• These particles collide with the walls of the
sealed container, producing pressure.
– A measure of the force exerted by a gas
above a liquid is called vapor pressure.
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13.2 The Nature of Liquids> Vapor Pressure
Over time, the number of particles
entering the vapor increases and some
of the particles condense and return to
the liquid state.
Liquid
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evaporation
condensation
Vapor (gas)
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13.2 The Nature of Liquids> Vapor Pressure
Over time, the number of particles
entering the vapor increases and some
of the particles condense and return to
the liquid state.
Liquid
evaporation
condensation
Vapor (gas)
• Eventually, the number of particles
condensing will equal the number of particles
vaporizing.
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13.2 The Nature of Liquids> Vapor Pressure
In a system at constant vapor pressure,
a dynamic equilibrium exists between
the vapor and the liquid. The system is
in equilibrium because the rate of
evaporation of liquid equals the rate of
condensation of vapor.
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13.2 The Nature of Liquids> Vapor Pressure
In a system at constant vapor pressure,
a dynamic equilibrium exists between
the vapor and the liquid. The system is
in equilibrium because the rate of
evaporation of liquid equals the rate of
condensation of vapor.
• At equilibrium, the particles in the system
continue to evaporate and condense, but
no net change occurs in the number of
particles in the liquid or vapor.
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13.2 The Nature of Liquids>
CHEMISTRY
& YOU
To make the best-tasting coffee, many people
grind the coffee beans just prior to brewing
the coffee. Also, they are careful not to grind
the coffee beans too much. Explain how both
of these methods help prevent the natural oils
in coffee beans from vaporizing.
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13.2 The Nature of Liquids>
CHEMISTRY
& YOU
To make the best-tasting coffee, many people
grind the coffee beans just prior to brewing
the coffee. Also, they are careful not to grind
the coffee beans too much. Explain how both
of these methods help prevent the natural oils
in coffee beans from vaporizing.
The natural oils can vaporize only from the
surface of the coffee bean, so waiting to grind the
beans keeps the surface area small as long as
possible, preventing vaporization. Also, not
grinding the beans too much keeps the surface
area lower and gives less area for vaporization.
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13.2 The Nature of Liquids> Vapor Pressure
Vapor Pressure and Temperature Change
An increase in the temperature of a
contained liquid increases the vapor
pressure.
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13.2 The Nature of Liquids> Vapor Pressure
Vapor Pressure and Temperature Change
An increase in the temperature of a
contained liquid increases the vapor
pressure.
• This happens because the particles in the
warmed liquid have increased kinetic energy.
– More of the particles will reach the
minimum kinetic energy necessary to
escape the surface of the liquid.
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13.2 The Nature of Liquids>
Interpret Data
The vapor pressure data indicates how
volatile a given liquid is, or how easily it
evaporates.
Vapor Pressure (in kPa) of Three Substances
at Different Temperatures
Substance
0°C
20°C
40°C
60°C
Water
0.61
2.33
7.37
19.92
Ethanol
1.63
5.85
18.04
24.70
58.96
Diethyl ether
80°C
100°
C
47.34 101.33
47.02 108.34 225.75
122.80 230.65 399.11
647.87
• Of the three liquids shown, diethyl ether is
the most volatile and water is the least
volatile.
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13.2 The Nature of Liquids> Vapor Pressure
Vapor Pressure Measurements
The vapor pressure of a liquid can be
determined with a device called a manometer.
12.2 mm Hg
or 1.63 kPa
Mercury
Air
Air at standard temperature
and pressure
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Mercury
Ethanol
Ethanol at 0°C
43.9 mm Hg
or 5.85 kPa
Mercury
Ethanol
Ethanol at room
temperature (20°C)
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13.2 The Nature of Liquids> Vapor Pressure
Vapor Pressure Measurements
The vapor pressure is equal to the difference in
height of the mercury in the two arms of the U-tube.
12.2 mm Hg
or 1.63 kPa
Mercury
Air
Air at standard temperature
and pressure
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Mercury
Ethanol
Ethanol at 0°C
43.9 mm Hg
or 5.85 kPa
Mercury
Ethanol
Ethanol at room
temperature (20°C)
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13.2 The Nature of Liquids>
In a sealed gas-liquid system at a
constant temperature, eventually
A. there will be no more evaporation.
B. the rate of condensation decreases to
zero.
C. the rate of condensation exceeds the rate
of evaporation.
D. the rate of evaporation equals the rate of
condensation.
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13.2 The Nature of Liquids>
In a sealed gas-liquid system at a
constant temperature, eventually
A. there will be no more evaporation.
B. the rate of condensation decreases to
zero.
C. the rate of condensation exceeds the rate
of evaporation.
D. the rate of evaporation equals the rate
of condensation.
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13.2 The Nature of Liquids> Boiling Point
Boiling Point
Under what conditions does boiling
occur?
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13.2 The Nature of Liquids> Boiling Point
When a liquid is heated to a temperature
at which particles throughout the liquid
have enough kinetic energy to vaporize,
the liquid begins to boil.
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13.2 The Nature of Liquids> Boiling Point
When a liquid is heated to a temperature
at which particles throughout the liquid
have enough kinetic energy to vaporize,
the liquid begins to boil.
• Bubbles of vapor form throughout the liquid,
rise to the surface, and escape into the air.
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13.2 The Nature of Liquids> Boiling Point
When a liquid is heated to a temperature
at which particles throughout the liquid
have enough kinetic energy to vaporize,
the liquid begins to boil.
• Bubbles of vapor form throughout the liquid,
rise to the surface, and escape into the air.
• The boiling point (bp) is the temperature at
which the vapor pressure of the liquid is just
equal to the external pressure on the liquid.
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13.2 The Nature of Liquids> Boiling Point
Boiling Point and Pressure Changes
Because a liquid boils when its vapor
pressure is equal to the external pressure,
liquids don’t always boil at the same
temperature.
• Because atmospheric pressure is lower at
higher altitudes, boiling points decrease at
higher altitudes.
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13.2 The Nature of Liquids> Boiling Point
Boiling Point and Pressure Changes
101.3 kPa
101.3 kPa
100°C
70°
C
Sea Level
Atmospheric pressure at
the surface of water at
70°C is greater than its
vapor pressure. Bubbles
of vapor cannot form in
the water, and it does
not boil.
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34 kPa
70°
C
Sea Level
At the boiling point, the
vapor pressure is
equal to the
atmospheric pressure.
Bubbles of vapor form
in the water, and it
boils.
Atop Mount Everest
At higher altitudes, the
atmospheric pressure
is lower than it is at
sea level. Thus, the
water boils at a lower
temperature.
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13.2 The Nature of Liquids>
Interpret Graphs
You can use this graph to show how the
boiling point of a liquid is related to vapor
pressure.
• At a lower external pressure, the boiling point
decreases.
• At a higher
external
pressure, the
boiling point
increases.
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13.2 The Nature of Liquids> Boiling Point
Boiling is a cooling process, similar to
evaporation.
• The vapor produced is at the same temperature
as that of the boiling liquid.
– Although the vapor has the same
average kinetic energy as the liquid,
its potential (or stored) energy is
much higher.
– Thus, a burn from steam is more
severe than one from an equal mass
of boiling water, even though they
are both at the same temperature.
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13.2 The Nature of Liquids>
Interpret Data
Normal Boiling Point
The normal
boiling point
is defined as
the boiling
point of a liquid
at a pressure
of 101.3 kPa.
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Normal Boiling Points of Several Substances
Substance
Boiling Point (°C)
Carbon disulfide (CS2)
46.0
Chloroform (CHCl3)
61.7
Methanol (CH4O)
64.7
Carbon tetrachloride (CCl4)
76.8
Ethanol (C2H6O)
78.5
Water (H2O)
100.0
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13.2 The Nature of Liquids>
Is the boiling point of water at the top
of Mount McKinley (the highest point
in North America) higher or lower than
it is in Death Valley (the lowest point in
North America)?
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13.2 The Nature of Liquids>
Is the boiling point of water at the top
of Mount McKinley (the highest point
in North America) higher or lower than
it is in Death Valley (the lowest point in
North America)?
The boiling point of water decreases as altitude
increases. Therefore, the boiling point of water
is lower atop Mount McKinley than it is in Death
Valley.
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13.2 The Nature of Liquids> Key Concepts
The interplay between the disruptive
motions of particles in a liquid and the
attractions among the particles determines
the physical properties of liquids.
During evaporation, only those molecules
with a certain minimum kinetic energy can
escape from the surface of the liquid.
45
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13.2 The Nature of Liquids> Key Concepts
In a system at constant vapor pressure, a
dynamic equilibrium exists between the
vapor and the liquid. The rates of
evaporation and condensation are equal.
At a temperature at which particles
throughout a liquid have enough kinetic
energy to vaporize, the liquid begins to
boil.
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13.2 The Nature of Liquids> Glossary Terms
• vaporization: the conversion of a liquid to a
gas or a vapor
• evaporation: vaporization that occurs at the
surface of a liquid that is not boiling
• vapor pressure: a measure of the force
exerted by a gas above a liquid in a sealed
container; a dynamic equilibrium exists
between the vapor and the liquid
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13.2 The Nature of Liquids> Glossary Terms
• boiling point: the temperature at which the
vapor pressure of a liquid is just equal to the
external pressure on the liquid
• normal boiling point: the boiling point of a
liquid at a pressure of 101.3 kPa or 1 atm
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13.2 The Nature of Liquids>
END OF 13.2
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