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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 moleculesStill 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
The motion of gas molecules is rapid, constant, and random.
Right-click on the circle and select “Play” to see the animation.
States of MatterThe state (solid, liquid, or gas) of a substance depends on three things:Chemical identityTemperaturePressureChemical identity matters because:Different 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 Theory3. Collisions between other gas particles and the container are elastic collisions.
The energy isn’t wasted, it just changes direction.Gas Particles are elastic.
Did it bounce?Not 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
FasterSlower
Temperature
Absolute temp. is measured in Kelvins (K)No degree symbol!To convert:from °C to K, add 273.from K to °C, subtract 273.
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 Temperature = 0°C.Also, 273 K.Absolute Zero = -273°C = 0 K.Lowest 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 Pressure = 1 atmAlso, 760 mmHg or 101.325 kPaStandard 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
Solid Liquid
Gas
sublim
ation
deposit
ion
vaporizationcondensation
melting
freezing
Check Yourself!
Phases and Phase Changes
Endothermic or exothermic? What’s the difference?Exo: exit…goes out.Endo: in..takes in
Solid: How much energy?
Liquid: How much energy?
Gas: How much energy?
Gain or lose energy to change to liquid?
Gain or lose energy to change to gas?
Endothermic Endothermic
Phases and Phase Changes
Solid: How much energy?
Liquid: How much energy?
Gas: How much energy?Gain or lose
energy to change to solid?
Gain or lose energy to change to liquid?
What about the reverse?Think about the movement of the particles
ExothermicExothermic
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?
Sketch this!
Is this substance warming up or cooling down?
What are the parts of the graph?
Solid
Liquid Gas
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?
• The melting, between B&C is called the Heat of fusion (Hf).
• The vaporizing, between D&E, is called Heat of vaporization (Hv)
Hf
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=mCΔTLet’s break it down!
Q is the amount of energy, usually in Joules.m is the mass of the givenHf 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?
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=mHf
Q=mHv
Q=mCΔT
Q=mCΔT
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 = 487.5Jm= 25gΔT = (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
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 = 487.5Jm= 25gΔT = (75-25)=50
Q = mC ΔT
Plug and Chug!
487.5J = (25g)(C)(50) so multiply 25x50487.5J = 1250(C) divide both sides by 1250 to get (c) alone!0.39J/g = CSo, it takes 0.39 Joules for every gram of copper to increase by 1 degree celsius.
How do we use this diagram?
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=25gHv= 2257J/g
Which equation should we use?Q = mHf or v or Q =mC ΔT
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=25gHv= 2257J/g
More Practice
Plug and Chug!Q=mHv
Q=(25g)(2257j/g)Q = 56425JIt takes 56,425 Joules to convert 25g of water into steam!
