The Kinetic Molecular Theory

Basic Assumptions

Particle Size

• Gas particles have no volume (pin point particles)

• The space between particles is extremely large compared to the volume of the particles. Due to this distance, there is no significant attractive or repulsive force acting on the particles.

The Kinetic Molecular Theory

Basic Assumption

Particle Motion

• Gas particles are in constant random motion.

• Collisions between particles are elastic (Energy can be transferred from one particle to another during a collision, but no energy is lost when particles collide)

Basic Assumptions

Basic Assumptions

Particle Energy

• The mass and velocity of a particle determine the kinetic energy of a particle

• Temperature is a measure of the average kinetic energy of particles in a sample.

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The Kinetic Molecular Theory

Mass/Velocity Relationship Questions

Condition #1:

Two particles (one heavy and one light) traveling at the same velocity.

Which exhibits the greatest kinetic energy?

Condition #2:

Two particles of the same size traveling at different velocities (fast and slow).

Which exhibits the greatest kinetic energy?

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Explaining the Behavior of Gases

Properties

• Low Density (pinpoint mass/volume of empty space)

• Random Motion

Behaviors

CompressionGases can be compressed due to the large space

that exists between particlesGases expand to fill their containers due to

constant random motion

Explaining the Behavior of Gases

Properties

• No attractive or repulsive forces acting on particles

• Particles exhibit constant random motion

Behaviors

Particles can flow easily past each other in a process called diffusion. The rate of diffusion is dependent on the mass of the particles.

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B

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Question: Based on this equation which particles diffuse faster, heavy or light particles?

Explaining the Behavior of Gases

Property

• Particles exhibit constant random motion

Behavior

Effusion (similar to diffusion, where particles escape through a tiny opening)

Graham’s Law of Effusion

Rate of effusion

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Gas PressureGas PressureAn important An important property of any gas is its pressure

A linear momentum of the gas moleculesA linear momentum of the gas molecules

Pressure is defined as force per unit areaPressure is defined as force per unit area

Gas particles exert pressure when they Gas particles exert pressure when they collide with the walls of their containercollide with the walls of their container

The less mass, the less pressure it exertsThe less mass, the less pressure it exerts

Gas Pressure Gas Pressure (Continued)(Continued)

Temperature, volume and number of moles Temperature, volume and number of moles affect the pressure that a gas exertsaffect the pressure that a gas exerts

The Earth is surrounded by an atmosphere The Earth is surrounded by an atmosphere that extends into space for hundreds of that extends into space for hundreds of kilometers. kilometers.

The particles in air move in every direction The particles in air move in every direction which exerts pressure in all of these which exerts pressure in all of these directions; this is known as directions; this is known as atmospheric atmospheric pressurepressure

Air Pressure varies at different points on earthAir Pressure varies at different points on earth

Measuring Gas Measuring Gas PressurePressure

Evangelista Torricelli (1608-1647) was Evangelista Torricelli (1608-1647) was the first to demonstrate that air exertedthe first to demonstrate that air exerted

He designed experiments in order to He designed experiments in order to test his theories about liquid and test his theories about liquid and densitydensity

He invented a He invented a barometerbarometer: an instrument : an instrument used to measure atmospheric pressureused to measure atmospheric pressure

Units of PressureUnits of PressureThe SI unit of pressure is called the pascal (Pa)The SI unit of pressure is called the pascal (Pa)

The pressures measured by barometers and The pressures measured by barometers and monometers can be reported in:monometers can be reported in:

Millimeters of mercury (mm Hg)Millimeters of mercury (mm Hg)

Also the Torr, which is named after TorricelliAlso the Torr, which is named after Torricelli

Air Pressure is often reported in a unit called an Air Pressure is often reported in a unit called an atmosphere (atm)atmosphere (atm)

1 atm= 760 mm Hg1 atm= 760 mm Hg

760 torr760 torr

101.3 kPa (kilopascals)101.3 kPa (kilopascals)

14.7 psi (pounds per square inch)14.7 psi (pounds per square inch)

Dalton’s Law of Dalton’s Law of Partial PressurePartial Pressure

Dalton’s Law explains that the totally Dalton’s Law explains that the totally pressure is equal to the sum of all the pressure is equal to the sum of all the pressures of the partspressures of the parts

This is only true for ideal gases, but the error is This is only true for ideal gases, but the error is small for real gasessmall for real gases

PressurePressureTotal Total = Pressure= Pressure11 + Pressure + Pressure22 + + PressurePressurenn

The portion of the total pressure contributed The portion of the total pressure contributed by a single gas is called its by a single gas is called its partial pressure partial pressure

Partial pressure of a gas depends on Partial pressure of a gas depends on Number of moles of gasNumber of moles of gasSize of the containerSize of the containertemperaturetemperature

Forceholds all matter together. The force in a molecule that is between atoms is a chemical or intramolecular force. The force between molecules is called a physical or intermolecular force.

Dipole-dipole Forces

are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together.

Figure: two arrangements of polar iodine monochloride (ICl) molecules that give

rise to dipole-dipole attractions.

Hydrogen Bonds

special type of dipole-dipole attraction

Definition: a dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom with at least one lone electron pair.

only form to fluorine, oxygen, or nitrogen atoms.

REASON: they are electronegative enough to cause a large partial positive charge on the hydrogen atom, but small enough that the lone pairs of electrons can come close to hydrogen atoms.

Hydrogen Bonds in a water molecule:

Water has two hydrogen atoms and one oxygen atom. Two molecules of water can form a hydrogen bond between them. The oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with hydrogen on two other water molecules.

Density & Compression

Density- Density is a measurement of the mass per unit volume

Compression- This unit of measure expresses the degree of firmness or softness

Liquids can be compressed but the change in volume for liquids is much smaller because liquid particles are already tightly packed

Liquids are much denser than gases

FluidityFluidity- The ability to flow

A liquid can diffuse through another liquid

Liquids diffuse more slowly than gases even at the same temp

Liquids are less fluid than gases

ViscosityViscosity- The measure of the resistance of a liquid to flow

Viscosity of a liquid is determined by the type of intermolecular forces, the shape of the particles, and temperature

The stronger the forces, the higher the viscosity

Viscosity decreases with temperature

The added energy from a higher temperature makes it easier for the molecules to overcome the intermolecular forces that keep the molecules from flowing

Higher viscosity with higher molecular mass

Surface Tension

Surface Tension- The measure of the inward pull by particles in the interior if the liquid

For the surface area to increase particles from the interior must move to the surface

The stronger the attractions between particles, the greater the surface tension

Surfactants- Compounds that lower the surface tension of water, also referred to as surface agents

Water has a high surface tension because its molecules can form multiple hydrogen bonds

Capillary Action

Capillary Action- The movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid (adhesion)

Molecules of water are naturally attracted to each other and form temporary hydrogen bonds with each other (cohesion); their attraction for each other on the surface of a liquid, for example, gives rise to surface tension

Results in the elevation or depression of liquids in capillaries (small narrow tubes)

Density of SolidsThere is about a 10% difference in density between the solid and liquid states of most substances

In general, the particles in solids are more closely packed than those in liquids

Most solids are more dense than most liquids

Except waterthe water molecules in ice are less closely packed together than in liquid water

Crystalline Solids

Crystals -- The individual pieces of a crystalline solid

Unit Cell -- the smallest arrangement of connected points that can be repeated in 3 dimensions to form the lattice

The shape of a crystalline solid is determined by the type of unit cell from which its lattice is built

A solid whose atoms, ions, or molecules are arranged in an orderly, geometric, three-dimensional structure

• Crystal shapes differ because the surfaces of unit cells do not always meet at right angles and the edges of the faces vary in length

Molecular Solids

Most are not solid at room temperature

Molecular compounds are solids at room temperature because of their large molar mass

Because they have no ions, molecular solids are poor conductors of heat and electricity

Fairly soft and have low to moderately high melting points

• The molecules are held together by dispersion forces, dipole - dipole forces or hydrogen bonds

ex: table sugar

Covalent Network of Solids

Atoms that can form multiple covalent bonds are able to form covalent network solids

Very high melting points

Usually have poor conductivity

ex: quartz and diamond

Ionic SolidsEach ion is surrounded by ions of opposite charge

The type of ions and the ratio of ions determine the structure of the lattice and the shape of the crystal

There is a network of attractions that extends throughout an ionic crystal

This gives these compounds their high melting points and hardness

Ionic crystals are strong but brittle

ex: table salt

Metallic SolidsConsists of positive metal ions surrounded mobile electrons

Strength varies by the physical property

Mobile electrons make metals malleable and ductile

They also make metals good conductors of electricity and heat

When force is applied to a metal, electrons shift and keep the metal ions bonded in their new positions

All metallic elements can form a metallic solid

Amorphous Solids

Has no shape

Any liquid can be made into an amorphous solid if given a sufficiently high cooling rate

Cooling reduces molecular mobility

An amorphous solid often forms when a molten material cools too quickly to allow enough crystals to form

Not all particles are arranged in specific order

A solid in which there is no long range order of the positions of the atoms

ex: glass and rubber

Endothermic Processes

Endothermic- A chemical reaction in which a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in the product molecules

http://www.youtube.com/watch?v=GmiZ0huvZzs

MeltingThe amount of energy required to melt one mole of a solid depends on the strength of the forces keeping the particles together in the solid

The melting point of a crystalline solid is the temperature at which the forces holding its crystal lattice together are broken and it becomes a liquid

Vaporization vs. Evaporation

Vaporization is the process by which a liquid changes to a gas or a vapor

Evaporation is when vaporization occurs only at the surface of a liquid

BoilingThe temperature at which the vapor pressure of a liquid equals the external or atmospheric pressure is called the boiling point

Process of BoilingMolecules throughout the liquid have energy to vaporize

Bubbles of vapor collect below the surface of the liquid and rise to the surface

SublimationSublimation is the process by which a solid changes directly to a gas without first becoming a liquid

Exothermic Changes

Exothermic- A chemical reaction in which more energy is released than is required to break bonds in the initial reaction

http://www.youtube.com/watch?v=80Q3GgeelVM&feature=related

CondensationThe energy-releasing process by which a gas or vapor becomes a liquid

Condensation is the reverse of vaporization

Different causes of the condensation of water vapor in which all involve a transfer of energy

CondensationThe vapor molecules can come in contact with a cold surface such as the outside of a glass containing ice water

Heat from the vapor molecules is transferred to the glass as the water vapor condenses

The water vapor that condenses on blades of grass or the car forms liquid droplets called dew

Condensation When a layer of air near the ground cools, water vapor in the air condenses and forms fog

Clouds form when layers of air high above the surface of Earth cool

DepositionThe energy-releasing process by which a substance changes from a gas or vapor to a solid without first becoming a liquid

Deposition is the reverse of sublimation

When water vapor comes in contact with a cold window in winter, it forms a solid deposit on the window called frost

FreezingFreezing is the reverse of melting

The freezing point is the temperature at which a liquid is converted into a crystalline solid

Phase DiagramsTwo variables control the phase of a substance: temperature and pressure

Phase Diagram- a graph of pressure versus temperature that shows which phase substance exists under different conditions of temperature and pressure

Phase DiagramsTriple Point- the point on a phase diagram representing the temperature and pressure at which three phases of a substance (solid, liquid, and gas) can coexist

Critical Point- indicates the critical pressure and critical temperature above which water can not exist as a liquid

Additional Example of a Phase Diagram