The Kinetic Molecular Theory
By
the end of the lecture, you should be
able to:
Describe the assumptions (rules) of
the “kinetic molecular theory” as it
applies to the states of matter.
The Kinetic Molecular Theory
Scary
Name, but it’s not so scary.
Kinetic Energy means motion!
Particles in Solids, Liquids, and Gases
Super important with the phases of matter!
Particles in Solids
Solids:
Made up of a bunch of particles (atoms or molecules)
Packed tight = high or low kinetic energy?
Is there much room to move?
Low kinetic energy
Particles in Liquids
Liquids:
Also made up of atoms or molecules
Still locked on to the neighbors, but more
room to move.
More or less Kinetic Energy?
More kinetic energy than
solids
Particles in Gases
Gases:
Tons of room!
So a lot of kinetic energy.
Average speed of particles in this container is approximately
1000 mph!
The Kinetic Molecular Theory
Right-click on the circle
and select “Play” to see
the animation.
The motion of gas molecules is rapid, constant, and random.
States of Matter
The
state (solid, liquid, or gas) of a substance depends
on three things:
Chemical
identity
Temperature
Pressure
Chemical
Different
identity matters because:
substances have different melting, freezing, and boiling
points.
 O2
has a very low boiling point and is a gas at room temperature.
NaCl has a very high melting point and is solid at room temperature.
Why?
Kinetic Molecular Theory
In
order to investigate these phases, we’re
going to have to establish some rules.
These especially to the gases.
Write down the red definitions, and take
notes as needed.
Kinetic Molecular Theory
Rules:
1. Gases consist of small particles that
are far apart, relative to their size.
Gas particles are teeeny tiiiny. The particles that you’ve
seen on the screen are huge. If gas particles were the size
of a marble, the container that’s been shown would be the
size of a football stadium
Kinetic Molecular Theory
2. Gas particles are in constant, random
motion. The moving particles are
constantly colliding with each other and
with the walls of the container.
Like bouncy balls that never loses their energy!
Or three year olds at a birthday party.
Kinetic Molecular Theory
3. Collisions between other gas particles
and the container are elastic collisions.
Kinetic Molecular Theory
3. Collisions between other gas particles and
the container are elastic collisions.
Did it bounce?
Not elastic!
The energy isn’t wasted, it just changes
direction.
Gas Particles are elastic.
Did it bounce?
Elastic!
Kinetic Molecular Theory
4. There are no forces of attraction or
repulsion between gas particles.
Water likes to bond to each other. Remember
surface tension? Water likes to bead up and
bond.
Gas Particles
Don’t Care
Kinetic Molecular Theory
MOST
IMPORTANT RULE OF ALL!
5. The Average Kinetic Energy depends on
the temperature!
The hotter the material, the faster the molecules are moving!
Temperature
Temperature
Temperature
Temperature
Slower
Faster
Temperature
Absolute
temp. is measured in Kelvins (K)
No
degree symbol!
To
convert:
°C to K, add 273.
from K to °C, subtract 273.
from
Temperature
Practice!
Gallium is a metal that can melt in your
hand at 302.93 K. What is the
temperature in °C?
A basket of chicken strips that you order
from Raising Cane’s is 77 °C, what is the
temperature in K?
Standard Temperature
Standard
Also,
Temperature = 0°C.
273 K.
Absolute
Lowest
Zero = -273°C = 0 K.
temperature.
All molecular motion stops.
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.
Standard Pressure
Standard
Also,
Pressure = 1 atm
760 mmHg or 101.325 kPa
Standard
pressure is the normal air
pressure at sea level.
QUESTION: Why does air pressure
decrease as you climb a mountain or
ascend in an airplane?
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.
Phases and Phase Changes
Phase
= state of matter
Check Yourself!
Gas
melting
Solid
Liquid
freezing
Phases and Phase Changes
Endothermic
or exothermic? What’s the
difference?
Exo: exit…goes out.
Endo: in..takes in
Endothermic
Endothermic
Solid: How
Liquid: How
Gas: How
much energy?
much energy?
much energy?
Gain or lose
Gain or lose
energy to change
energy to
to liquid?
change to gas?
Phases and Phase Changes
What
about the reverse?
Think about the movement of the
particles
Exothermic
Exothermic
Solid: How
Liquid: How
much energy?Gain or losemuch energy?Gain or lose
energy to
change to solid?
Gas: How
much energy?
energy to change
to liquid?
Do you understand?
Which
kinetic theory rule is the most
important? Explain.
Which state of matter keeps the volume,
but changes to the shape of the
container?
Your friend filled up their car early in the
morning. They worked a full shift during
a hot summer day. They came out and
gas was on the ground next to their car.
What happened?
Phase Diagrams
Phase
Diagram - Shows the phases of
a substance at various combinations of
temperature and pressure.
Phase Diagrams
Need to know Four Vocab words!
• Triple Point – The point where is substance is both solid, liquid,
and gas.
• Melting Point – The temp & pressure where something melts.
• Boiling Point – The temp & pressure where something boils.
• Critical Point – Liquid and gas are the same
thing…indistinguishable .
Phase diagram of water
Phase diagram of CO2
Where are the melting, vaporizing,
and triple points?
Melting
Triple
Vaporizing
/Boiling
Heating and Cooling Diagrams
What is heat?
How is heat different from temperature?
Heat is a measure of kinetic energy
In chemistry heat is measure in Joules (J)
Heat is dependent on mass, temperature change and specific
heat (or ability of a substance to absorb heat)
Temperature is a measure of warmth or coldness.
Temperature is measured in Kelvin (K) or Celsius (C)
What is the heating curve?
The heating curve is a graph which represents how a
sample changes phases. As heat is added over time, the
sample changes temperature and phase accordingly. Thus
heating curve.
How does the heating curve look?
Is this substance warming up or cooling down?
Sketch this!
What are the parts of the graph?
Liquid
Gas
Solid
All of the diagonal lines are a STATE
What are the parts of the graph?
Melting
Vaporizing
All of the flat lines are a phase change!
What are the parts of the graph?
Hv
Hf
• The melting, between B&C is called the Heat
of fusion (Hf).
• The vaporizing, between D&E, is called Heat
of vaporization (Hv)
What is heat of fusion?
What is heat of vaporization?
Hf is the amount of
energy needed to
completely make a solid
into a liquid
Hv is the amount of
energy needed to
completely make a
liquid into a gas
How do we use this diagram?
We need to know how to use TWO Formulas.
• Q=mHf or v
• Q=mCΔT
Let’s break it down!
Q is the amount of energy, usually in Joules.
m is the mass of the given
Hf is the heat of fusion of the substance (how much energy it needs to melt)
Hv is the heat of vaporization of the substance (how much energy it
needs to vaporize)
C is the specific heat of the substance
ΔT is the change in temperature.
How do we use this diagram?
Q=mHv
Q=mCΔT
Q=mHf
Q=mCΔT
We use the equations in different places.
We use Q=mHf or Hv during the phase changes or the flat parts.
We use Q=mCΔT in the sloping parts or in the states.
Makes sense, because ΔT is all about temperature chages!
Q=mCΔT
How do we use this diagram?
Specific Heat is the amount of heat required to
raise the temperature of 1gram of a substance
by 1 °C
Let’s try an example!
It takes 487.5 J to heat 25 grams of copper from 25 °C to 75 °C. What is the specific heat
in Joules/g·°C?
Write down what you have!
Q = 487.5J
m= 25g
ΔT = (75-25)=50
Which equation should we use
Q = mHf or v or
Q =mC ΔT
Because it asks for specific heat, and
specific heat is represented by C
How do we use this diagram?
Let’s try an example!
It takes 487.5 J to heat 25 grams of copper from 25 °C to 75 °C. What is the specific heat in
Joules/g·°C?
Write down what you have!
Q = 487.5J
m= 25g
ΔT = (75-25)=50
Q = mC ΔT
Plug and Chug!
487.5J = (25g)(C)(50)  so multiply 25x50
487.5J = 1250(C)  divide both sides by 1250 to get (c) alone!
0.39J/g = C
So, it takes 0.39 Joules for every gram of copper to increase by 1 degree celsius.
More practice
What is the heat in Joules required to convert 25 grams of water
into steam? The heat of vaporization (Hv) of water = 2257 J/g
Write down all of the information given.
m=25g
Hv= 2257J/g
Which equation should we use?
Q = mHf or v or Q =mC ΔT
More Practice
What is the heat in Joules required to convert 25 grams of
water into steam? The heat of vaporization (Hv) of water =
2257 J/g
Write down all of the information given.
m=25g
Hv= 2257J/g
Plug and Chug!
Q=mHv
Q=(25g)(2257j/g)
Q = 56425J
It takes 56,425 Joules to convert 25g of water into steam!