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8/10/2019 The Kinetic Molecular Theory0
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States of MatterGases, Liquids and Solids
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The Kinetic Molecular Theory
Literal interpretation:
The theory of moving molecules
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The Kinetic Molecular Theory
Observations to support the theory:
Diffusion in gases and liquids
Movement of substances from an area of highconcentration to one of lower concentration
Ability of a gas to spread out and fill a container
Brownian MovementThe observable movement of particles due to collisions
with moving molecules.
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The Kinetic Molecular Theory
The theory explains these observations
The theory describes the differences between gas,liquids and solids
The theory explains the gas laws
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The Kinetic Molecular Theory
Major points: Supports the concept of an ideal gas
An ideal gas is one that perfectly fits all the assumptions ofthe kinetic-molecular theory.
Do not actually existin theory this is how they wouldbehave:
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The Kinetic Molecular Theory
1. Gases are made of tiny particles far apart relative to
their size:
Volume occupied by the molecules is inconsequential
Volume is mostly space
Explains why gases are compressible
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The Kinetic Molecular Theory
2. Gas particles are in continuous, rapid, random
motion
As a result there are collisions with othermolecules or with the wall of the container
Creates pressure
Increase in temperature increases themovement of the molecules and thus thepressure exerted by the gas
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The Kinetic Molecular Theory
3. There are no attractive forces between molecules
under normal conditions of temperature and
pressure
Gas molecules are moving too fast
Gas molecules are too far apart
Intermolecular forces are too weak
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The Kinetic Molecular Theory
4. Collisions between gas particles and betweenparticles and container walls are elastic collisions.
Collisions in which there is no net loss of total kineticenergy
Kinetic energy can be transferred between two particles
during collisions
Total kinetic energy remains the same as long astemperature remains the same
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The Kinetic Molecular Theory
5. All gases at the same temperature have the same average
kinetic energy. The energy is proportional to the
absolute temperature.
Absolute temperature = Kelvin temp scale
Ke= mv2
Ke = the kinetic energy
m = mass
v = the velocity
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The Kinetic Molecular Theory
1. Gases are made of tiny particles far apart relative to their size
2. Gas particles are in continuous, rapid, random motion
3. There are no attractive forces between molecules
under normal conditions of temperature and pressure
4. Collisions between gas particles and between particles andcontainer walls are elastic collisions.
5. All gases at the same temperature have the same averagekinetic energy. The energy is proportional to the
absolute temperature.
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The Kinetic Molecular Theory
Applies only to idealgases
Most gases behave likean ideal gas undernormal conditions
Gases with littleattraction betweenmoleculesHe/H2/N2
Real gases
Deviate from idealbehavior
Due to intermolecular
interaction (H2O, NH3) High pressure
Low temperature
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The Kinetic Theory and
Changes of State
GasesAttractions areinsignificant
LiquidsAttractions aremore important leadingto a more ordered state
Solids Attractions aremost important with anordered state
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Kinetic Molecular Theory and Changes of State
Attractions between particles in strength:
Least London dispersion forces
Dipole-dipole interaction
Hydrogen bonding
Greatest Metallic, Ionic and Covalent network
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Kinetic Molecular Theory and Changes of State
Changes of state occurwith a change intemperature or pressure
Particles of a substanceovercome (or succumb)
to intermolecularattraction
Involvesenergy
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Kinetic Molecular Theory and Changes of State
Solids, liquids and gasescan undergo variouschanges in processes thatare either endothermic
or exothermic
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Kinetic Molecular Theory and Changes of State
Consider the evaporationof a liquid:
Temperature= theaverage kinetic energy
Some molecules havemore kinetic energy thanothers
These molecules escapeand become gasmolecules
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Evaporation will occur inclosed container alsoexcept
As the liquid evaporatesthe space above starts tofill with gas molecules
until it can hold no more
Gas will start tocondense.
Kinetic Molecular Theory and Changes of State
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Kinetic Molecular Theory and Changes of State
Eventually the rate ofevaporation will equalthe rate of condensation
Two processes will occursimultaneously with no
net change
State of Equilibrium
Vapor molecules abovethe liquid will collidewith each other and thecontainerand exert a pressure.
Equilibrium vaporpressure!!!
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Kinetic Molecular Theory and Changes of State
Every liquid has a specificvapor pressure at a giventemperature.
Reflection of the strengthof the intermolecularbonding between
molecules
Vapor pressure alsoincreases with temperature
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Kinetic Molecular Theory and Changes of State Equilibrium vapor
pressure is (EVP) used todefine boiling point, BPt
Boiling point is thetemperature at which the
equilibrium vaporpressure equalsatmospheric pressure
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Kinetic Molecular Theory and Changes of State
Boiling point of water is100 oC only at 760mm Hg
When atmosphericpressure is > 760 mm Hgthe boiling pt is > 100.
When atom0sphericpressure is
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Kinetic Molecular Theory and Changes of State
Boiling requires acontinuous supply ofenergy..
Water boils at 100oC andthe temperature does notchange.even though
there is a continuoussupply of energy..
Where does the energy go?
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Kinetic Molecular Theory and Changes of State
Same is true when icemelts
It melts (or freezes) at 00
C
At this temperature thereis a state of equilibrium
Temp will not change ifboth phases are present
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Kinetic Molecular Theory and Changes of State
Energy is can be addedcontinuously, but thetemperature does not
change
Energy is used to change
the physical statethisrequires a lot of energy!!
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Kinetic Molecular Theory and Changes of State
The amount of heatenergy required to meltone mole of a solid at the
solids melting point isthe solids molarenthalpy of fusion.
DHf
Energy absorbedrepresents potentialenergy
For water it is 6.009kJ/mol
Xj/g =6.009kJ/M x 1M/18g x 1000J/1kJ
= 333.8 j/g
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Kinetic Molecular Theory and Changes of State
The amount of heatenergy required tovaporize one mole of a
liquid at the liquidsboiling point is theliquids molar enthalpyof vaporization.
D
Hv Energy absorbed
represents potentialenergy
For water it is 40.79kJ/mol
Xj/g =40.79J/M x 1M/18g x 1000J/1kJ
= 2266 j/g
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Kinetic Molecular Theory and Changes of State
Compared to othersubstances these valuesare very high.
Water has very strongintermolecular bonding
Hydrogen bondsbetween highly polarmolecules
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Kinetic Molecular Theory and Changes of State
Unique properties ofwater is related to thehydrogen bond
4-8 molecular groups inliquid water
Hexagonal arrangementin solid
--> Dipole w/ partial +/-
High boiling pt of
water
Solid is less dense..Ice
floats
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Changes of State are Shown in Phase Diagrams
Changes of phase aredepicted in phasediagrams
Show the relationshipbetween state of matter,
temperature andpressure
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Changes of State Shown in Phase Diagrams
Phase diagrams define:
Triple point=the T/P conditionsat which all three phases coexist
Critical point = Critical tempand press
Critical temp = temp abovewhich the substance cannot
exist as a liquid
Critical press= lowest pressure atwhich the substance can exist asa liquid at the criticaltemperature
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Phase Diagram of Water
Interesting points ADIce and vapor in
equilibrium
ACLiquid and vapor in
equilibrium ABIce and liquid in
equilibrium. Notean increase in pressurelowers melting point
nbp=normal boiling pt mp =melting point
Critial temp =373.99
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Phase Diagram of Carbon Dioxide
Note the following:
Very different temp andpressure compared towaters diagram
Liquid is only possible at
high pressureAt normal room
conditions CO2 onlyexists as a gas
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Phase Change vs Temperature change in a single phase Melting/Fusion
Molar heat of fusion
6.009 kJ/mol
Vaporizing
Molar hear of vaporization
40.79kJ/mol
Raising the temperature of ahomogeneous material
Specific heat
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Phase Change How much energy is absorbed when 47g of ice melts?
(at STP)
Energy =47g x1 mol x 6.009kJ
18g 1 mol
= 15.7 kJ
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Phase Change How much energy is absorbed when 47g of water
vaporizes? (at STP)
Energy =47g x1 mol x 40.79kJ
18g 1 mol
= 106 kJ (vs 15.7 kJgases have ahigher energy content)
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Phase ChangeWhat mass of steam is required to release 4.97 x 105kJ
of energy when it condenses?
grams =4.97 x 105kJ x 1mol x 18g
40.79kJ 1 mol
= 2.19 x 105g
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Temperature change in a single phase Quantity of energy
transferred as heat whilea temperature change
occurs depends on The nature of the
substance
The mass of the material
The size of thetemperature change.
Water has a high specificheat
Metals have low specificheat
Units = J/(g x oC)
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Temperature change in a single phase Specific heat of water (l) = 4.18 J/goC
Specific heat of water (s) = 2.06
Specific heat of water (g) = 1.87 Specific heat of ethanol (g) = 1.42
Specific heat of ethanol (l) = 2.44
Specific heat of mercury (l) = 0.140
Specific heat of copper (s) = 0.385
Specific heat of lead (s) = 0.129
Specific heat of aluminum (s) = 0.897
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Solids and the Kinetic Molecular theory Properties: Dominated by the fact that
Closely packed particles
Relatively fixed positions
Highest intermolecular or interatomic attractions
Properties are
Definite shape and volume Definite melting point
High density and incompressibility
Low rate of diffusion
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Solid structureSolids may be crystalline Solids may be amorphous
Crystals in which particles arearranged in a regular
repeating pattern
Particles are randomly arranged
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Solid structureCrystals
Total 3-D arrangement ofparticles is the crystal
structure CUBIC
BODY CENTERED CUBIC
TETRGONAL
HEXAGONAL TRIGONAL
MONO
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4-Classes of Crytsalline Solids
Ionic --IonsHard and Britle
Covalent Network
Network of molecules
Quartz (SiO)
Diamond
Metallic Crystals
Free moving e-
Covalent Molecular Crystals
Weak.
Water, dry ice
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Amorphous solidsWithout shape No regular pattern
Glasses Plastics
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