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Kinetic Molecular Theory
A law is a summary of observations, and a theory is an explanation of those observations. The
individual gas laws give us a set of mathematical tools to help predict the behavior of gases under
specific conditions of pressure, temperature, volume and number of moles of gas. These conditions are
called STP (standard temperature 0°C, 273K and pressure 1 atm, 760 mm Hg, 101.3 kPa, 14.7 psi,760
Torr : Stadard 4d). The volume of an ideal or a nearly ideal gas at a fixed temperature and pressure is
proportional to the number of moles. The number of moles of a gas can be calculated from its volume by
using the relationship that at STP one mole of gas occupies a volume of 22.4 liters. (3d)
They do not, however, explain why gases behave the way they do. Kinetic molecular theory is an attempt
to explain some of the bulk properties of matter by describing how particles interact with one another.
Kinetic molecular theory can help us understand how and why the gas laws work and to predict when the
gas laws won’t work.
Daniel Bernoulli started kinetic molecular theory in 1738 when he proposed a thought model consistent
with Boyle’s Law in an attempt to explain how gases exert pressure. Clausius refined the theory in the
mid-1800s.
The Kinetic Molecular Theory explains the forces between molecules and the energy that they possess.
This theory has 4 basic assumptions.
1.
The average kinetic energy depends on temperature, the higher the temperature, the higher the
kinetic energy and the faster the particles are moving, greater velocity, and higher diffusion.
2. When the molecules collide with each other, or with the walls of a container, there is no loss of
energy. When particles collide with one another (or the walls of the container) they bounce
rather than stick. These collisions are elastic; if one particle gains kinetic energy another loses
kinetic energy so that the average remains constant. Collisions with the wall are used to
measure pressure. More collisions higher pressure.
3. Matter is composed of small particles (molecules). The measure of space that the molecules
occupy (volume) is derived from the space in between the molecules and not the space the
molecules contain themselves. The individual particles are neither attracted to one another nor
do they repel one another.
4. The molecules are in constant motion. This motion is different for the 3 states of matter.
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Solid - Molecules are held close to each other by their attractions of charge.
They will bend and/or vibrate, but will stay in close proximity.
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Liquid - Molecules will flow or glide over one another, but stay toward the bottom of the
container. Motion is a bit more random than that of a solid.
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Gas - Molecules are in continual straight line motion. A gas is composed of
particles in constant motion. The kinetic energy of the molecule is greater than the
attractive force between them (intermolecular forces), thus they are much farther
apart and move freely of each other. Compared to the space through which
they travel, the particles that make up the gas are so small that their volume
can be ignored.
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1) Temperature and Thermal Energy Key Ideas
Temperature is a measure of the average kinetic energy of each particle within an object.
Three temperature scales are Fahrenheit, Celsius, and Kelvin
Thermal energy is the total energy of the particles that make up an object
a) Temperature – The measure of the average kinetic energy of the particles in a substance.
Fahrenheit scale – The temperature scale on which 32°F (water freezes) and 212°F (water boils).
Celsius scale - The temperature scale on which 0°C (water freezes) and 100°C (water boils).
Kelvin scale - The temperature scale on which zero is the temperature at which no more energy can
be removed from matter. Also known as Absolute zero. Standard 4f There is no temperature lower
than 0 Kelvin.
Standard Practice 4e: Convert between the Celsius and Kelvin temperature scales (Temp and Its Measurement
2) The Nature of Heat Key Ideas
 Heat is a transfer of thermal energy from an object at a higher temperature to an object at a lower
temperature.
 Heat is transferred by conduction, convection, and radiation.
 A conductor transfers heat well whereas an insulator does not.
 The amount of heat necessary to raise a given mass of a substance by a specific unit of temperature
is called the specific heat.
Key Terms
 Conduction – The transfer of heat between particles within a substance.
 Convection – The transfer of heat by the movement of currents within a fluid.
 Convection current – The transfer of heat by the movement of currents within a fluid.
 Radiation – The transfer of energy by electromagnetic waves.
 Specific heat –is the amount of heat per unit mass required to raise
the temperature by one degree Celsius. The relationship between
heat and temperature change is usually expressed in the form shown
where c is the specific heat. The relationship does not apply if a phase change is encountered, because
the heat added or removed during a phase change does not change the temperature.
3) Thermal Energy and States of Matter Key Ideas
Matter can exist in three states: solid, liquid, or gas. Matter can undergo a change of state then thermal
energy is added or removed.
When a substance is changing state, the temperature of the substance remains constant even though its
thermal energy is changing.
In general, matter expands when it is heated and contracts when it is cooled. (slide 13)
Energy
Effects
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Key Terms State – The three forms (solid, liquid, and gas) in which matter exists.
Solid
Liquids
Gases
Brownian Motion and Diffusion (Standard 4b random motion of molecules) Diffusion vs Effusion
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Change of state – The physical change of matter from one state to another.
Melting – The change from a solid to the liquid form of matter.
Melting point – The temperature at which a substance melts.
Freezing – The change from the liquid to the solid form of matter.
Freezing point – The temperature at which a substance freezes.
Vaporization - The change from the liquid to the gaseous form of matter.
Evaporation – Vaporization that occurs at the surface of a liquid.
Boiling – Vaporization that occurs below the surface of a liquid.
Boiling point – The temperature at which a liquid substance boils.
Condensation – The change from the gaseous to the liquid form of matter.
Thermal expansion – The expansion of matter when it is heated.
Thermostat – A device that regulates temperature.
Bimetallic strip – A strip made of two different metals that expand at different rates.
Freezing and Boiling Point Graph, Vapor Pressure and Boiling, Phase Diagram
http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg
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Random Motion of molecules collide with a surface creating observable pressure on that surface (4a)
Fluids – liquid and gas molecules move in random directions. A substance that can easily change shape.
Water is a fluid, air is a fluid, gas is a fluid, and even glass is a fluid.
Intermolecular forces
Liquids
Gases
When fluids collide with walls of the container a force is created.
Pressure – Forces in Fluids (liquids and gases) Key Ideas
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Pressure is the force per unit area on a surface.
 newton is not a unit of pressure…it is a unit of force.
 A pascal is a unit of pressure.
 N/m2 and Pa are unit of pressure
 Fluid pressure results from the motion of the atoms or molecules that make up the fluid.
 Pressure is equal to force divided by area, or P = F/A.
a) Pressure has many units of measures: atm, mmHg, Pascals, Torrs, lb/in2 Practice Conversions:
b) Pressure values are higher in liquids than gases because liquids have greater density (g/cm3 =
mass/volume) because liquid atoms/molecules are closer to one another.
c) Pressure at a given level in a fluid is the same in all directions. Air pressure increases as elevation
increases.
 The higher you go up a tall mountain, the less air pressure there is.
 At higher elevations, air pressure is less because there is less air above.
 Air pressure, or atmospheric pressure, is the pressure exerted by the gas in the Earth’s
atmosphere.
 Water pressure increases as you go deeper and deeper.
Diagrams showing effects of gravity.
Problem Solving Lab Graphing Effects of Depth on Air Pressure
Pressure Relationship Notes
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We can connect these assumptions with the four variables from the individual gas laws at STP
http://www.chm.davidson.edu/vce/GasLaws/index.html
Boyles Law
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Charles Law
Combined Gas Law
Ideal Gas Law
Pressure is force per unit area. What we observe as the pressure of a gas is the force of
collisions as the particles strike the walls of the container. If these collisions occur frequently,
the gas pressure is high. If the collisions don’t occur very often, the pressure is low. Any change
in the conditions that results in more frequent collisions will increase the pressure.
What we observe as the volume of a gas is the empty space the particles travel through. The
larger the volume, the greater the distance between particles. Any change in the conditions that
results in a longer distance between particles is due to an increase in volume.
What we observe as n, or number of moles, is the number of particles.
What we observe as temperature of a gas is the average speed of the particles. The hotter the
gas, the faster the particles are moving.
The KMT and Non-Ideal Behavior:
As experimental apparatus improved in the late 1800s, chemists recognized that gas laws were only approximate. The best experimental
agreement with the mathematical predictions occurs when the gas is under relatively low pressure and high temperature. Some gases obey the
laws better than others, even under the same sets of conditions. Gases with smaller molar masses and ones that are relatively inert obey better
than larger, more reactive gases. Whether by nature or by conditions such as pressure, gases that do not obey the gas laws very well are called
“real” gases and those that do obey are called “ideal” gases. If an ideal gas is one who obeys the assumptions of kinetic molecular theory, a
real gas must be one that violates one or more of these assumptions.
High Molar Mass and Reactivity:
One assumption of the kinetic molecular theory is that the gas particles are neither attracted to nor repelled by one another. When the
particles of a gas are very large, they have higher induced dipole attractions, so they are more attracted to one another. When the gas particles
collide, they stick together and the average kinetic energy drops. These sticky gas particles hit the walls of the container less frequently and
with less force than ideal gas particles do. Polar molecules have greater intermolecular attractions, too, so a molecule like water vapor is
much less ideal than one like helium. The upshot is that some substances make better gases than others. Under the same conditions of
temperature and pressure, helium is an ideal gas and water vapor is a real gas.
High Pressure:
One assumption of the kinetic molecular theory is that the volume of the container is large enough that the volume of the particles is
negligible. When a gas is under very high pressure (and/or the volume for that mass of gas is very low), the volume of the particles
themselves can no longer be ignored, and calculated volumes are lower than real volumes. To make matters worse, as the distance between
the particles drops, the attractions between particles increase. As particles stick together, they are less likely to strike the wall of the
container, so calculated pressures are higher than real pressures.
Low Temperatures:
Even very small, very inert gases like helium have induced dipole attraction for one another. Hot gas particles have a lot of kinetic energy to
overcome these weak attractions. When a gas is very cold, the average molecular speeds and kinetic energy are low. The kinetic energy is no
longer able to supply the energy needed to overcome the attractions between particles. Particles stick together and thus average kinetic energy
drops. The upshot is that really cold gases are so non-ideal that they become liquids or even solids and the particles are not free to move
throughout the container.
The van der Waals Equation
Atoms and molecules are never truly ideal because they all interact with other gas particles; weak attractions between separate gas particles
are known as intermolecular attractions or van der Waals forces after the chemist who proposed a correction to the ideal gas law to calculate
pressure of a real gas.
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Transmitting pressure in a fluid Key Ideas
According to Pascal’s principles, an increase in pressure on a confined fluid is transmitted equally to all
part of the fluid.
 A force pump, such as the human heart, works because of Pascal’s principle.
 Fluids exert pressure because they are made up of tiny moving molecules that exert force.
 A device that increases force that works because of Pascal’s principle, such as the braking
system of a car, is called a hydraulic device
 A hydraulic device works by transmitting an increase in pressure from one part of a
confined fluid to another. A small force exerted over a small area at one place
results in a large force exerted by a larger area at another place.
Buoyant Force Key Ideas
1) The upward force on an object submerged in a fluid is called the buoyant force.
2) A buoyant force always works in an upward direction.
3) The buoyant force on an object is equal to the weight of the fluid displaced by the object. This is called
Archimedes’ principle.
4) An object will sink, rise to the surface, or stay where it is in a fluid depending on whether its density is
less than, greater than, or equal to the density of the fluid.
 Ice will float because the buoyant force acting on it must be greater than the force of gravity
pulling it down. Ice must be less dense than water
 The density of a substance is it mass per unit volume.
 A helium balloon will rise for the same reason. The density of the must be less than that of air.
 To float at a constant level, it must be the same density as air. To rise, it must be less dense
than air.
 An object under water feels lighter that it does in the air because of buoyancy.
 Bubbles rise in water because they are less dense than water.
 The density of an object would be changed by changing the object’s volume or mass.
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