Transcript HW: KMT Notes (4/21)

The Kinetic Molecular Theory
Monday, April 25th, 2016
Review

Matter can exist in several states:
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Solids have a definite shape and
volume, and are not compressible.
Liquids have a definite volume but
not a definite shape. They are not
compressible.
Gases do not have a definite shape
or volume. Gases are compressible.
The Kinetic Molecular Theory

Kinetic molecular theory (KMT):
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Explains the observed properties of matter
in its different states.
Particles are in constant, rapid, random
motion.
The speed of this motion depends on
temperature.
Collisions between particles are elastic.
(No kinetic energy lost).
Particles in Solids, Liquids, and Gases
The motion of gas molecules is rapid, constant, and random.
The Kinetic Molecular Theory
In a perfectly elastic
collision, the kinetic
energy of each
molecule might
change, but the total
kinetic energy stays
the same.
Collisions between particles are elastic.
States of Matter

The state (solid, liquid, or gas) of a substance
depends on three things:
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Chemical identity, Temperature and Pressure
Chemical identity matters because:
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Different substances have different degrees of
intermolecular forces.
The molecules of O2 are small and non-polar.
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Attracted to each other only by London forces.
O2 has a very low boiling point and is a gas at room temperature.
NaCl is a salt with very strong ionic bonds between ions.
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NaCl has a very high melting point and is solid at room temperature.
States of Matter

In solids, attractive forces outweigh kinetic
energy of particles.

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In liquids, attractive forces are balanced by
kinetic energy.
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Particles stick together in rigid position.
Particles stick together but can move past each
other.
In gases, kinetic energy outweighs attractive
forces.

Particles fly rapidly past each other as if there
are no attractive forces between them.
Pressure

Pressure = force / area
Pressure
One gas molecule
exerts a tiny force
against the side of a
balloon.
Pressure
When you have a
huge number of gas
molecules colliding
against the sides of a
balloon, the force
adds up. Force
spread out over the
inner surface of the
balloon is pressure.
Units of Pressure

SI Unit: Pascals (Pa)
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1 Pa = 1 Newton / square meter
Other units:
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atmospheres (atm)
millimeters of mercury (mmHg)
torr
pounds per square inch (psi)
Standard Pressure
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Standard Pressure = 1 atm
Standard pressure is the normal air
pressure at sea level.
Pressure Conversion Factors
1 atm = 760 mmHg
1 atm = 101.3 kPa
1 atm = 14.7 psi
1 atm = 760 torr
Atmospheric Pressure
As you move upward through the
atmosphere, the density decreases.
This is because most air molecules
are held close to Earth’s surface by
gravity. As the density decreases,
there are fewer molecules colliding
with surfaces; hence, less pressure.
Temperature

Temperature – measure of the
average kinetic energy of a
substance’s molecules.

The molecules in a hot object are moving
faster than the molecules in a cold object
on average!
Temperature

Absolute temp. is measured in Kelvins (K)
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To convert:
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No degree symbol!
from °C to K. K = °C + 273
from °C to °F °F = 9/5 (°C) + 32
from °F to °C °C = 5/9 (°F – 32)
EXAMPLE:
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Water boils at 100°C, or 373 K.
Water freezes at 0°C, or 32°F.
Avg. human body temp. is 97.8°F, or 36.5°C.
Standard Temperature
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Standard Temperature = 0°C.
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Also, 273 K.
Absolute Zero = -273°C = 0 K.
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Lowest temperature.
All molecular motion stops.
Kinetic Molecular Theory
Postulates of the Kinetic Molecular Theory of Gases
1.
Gases consist of tiny particles (atoms or molecules)
2.
These particles are so small, compared with the distances between
them, that the volume (size) of the individual particles can be assumed
to be negligible (zero).
3.
The particles are in constant random motion, colliding with the walls of
the container. These collisions with the walls cause the pressure exerted
by the gas.
4.
The particles are assumed not to attract or to repel each other.
5.
The average kinetic energy of the gas particles is directly proportional
to the Kelvin temperature of the gas
Kinetic Molecular Theory
(KMT)
 explains why gases behave as they do
 deals w/“ideal” gas particles…
1.
…are so small that they are assumed to have zero volume
2. …are in constant, straight-line motion
3. …experience elastic collisions in which no energy is lost
4. …have no attractive or repulsive forces toward each other
5. …have an average kinetic energy (KE) that is proportional
to the absolute temp. of gas (i.e., Kelvin temp.)
AS TEMP.
, KE
Characteristics of Gases
Gases expand to fill any container.
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random motion, no attraction
Gases are fluids (like liquids).
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no attraction
Gases have very low densities.
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no volume = lots of empty space
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Characteristics of Gases
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Gases can be compressed.
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no volume = lots of empty space
Gases undergo diffusion & effusion.
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random motion
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem