intermolecular forces
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Intermolecular ForcesIntermolecular Forces
Keith WarneKeith Warnewww.www.sciencecafesciencecafe.org.za.org.za
OO+
-HH
HH
OO+
-
HH
HHInterIntermolecular
molecular
IntraIntramolecu
lar
molecular
Intermolecular ForcesIntermolecular Forces
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SolubilityInvestigate the solubility's of the following solids in each of the liquids that follow. Use small quantities of each and
mix them together in a testube. Note down whether they are soluble, insoluble or slightly soluble.
SOLUBILITYWaterPolar
HexaneNon Polar
EthanolP & N.P
I2
Non Polar
Insoluble Soluble soluble
KMnO4
Ionic Soluble Insoluble/v
slightly soluble
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• Two different types of bonds occur in substances.
• Intramolecular– Between hydrogen atoms
and oxygen atoms inside the molecules Covalent bonds
• Intermolecular– Between two different water
molecules Hydrogen bonds
Intermolecular forces
OO+
-HH
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OO+ -HH
HHInterIntermolecular
molecular
IntraIntramolecular
molecular
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Bond Polarity in WaterThe oxygen atom has greater electronegativity than the hydrogen atoms so oxygen attracts the bonding electrons (shared pairs) closer to itself.
-+ OOHH
HH
The water molecule is therefore a DIPOLEDIPOLE - it has two oppositely charged “poles”. We say water is a polar compound.
Electrons Electrons attracted closer attracted closer
to oxygento oxygen
Lewis diagramLewis diagram Space filled modelSpace filled model
= small charge = small charge created by created by
unequally shared unequally shared electronselectrons
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• The positive and negative atoms on each of the water molecules attract each other.
• This electrostatic attraction is called Hydrogen bonding.
• It is the strongest form of intermolecular attraction.
• Hydrogen bonding exists only between molecules in which hydrogen is bonded to a very electronegative atom H-O-X, H-NX2 or H-F. (X = any atoms)
• These bonds result in abnormally high boiling points.
• USE THE DOTTED LINES BELOW TO SHOW WHERE HYDROGEN BONDS WOULD BE FOUND IN THE DIAGRAM (7 Bonds)
Hydrogen Bonds
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-HH
HHOO+ -
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HH
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Example:Example:Water
Other Other Examples:Examples:HF, NHHF, NH3 3 any any alcohol CHalcohol CH33OH OH etc.etc.
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Hydrogen BondingWhere would you expect the next Where would you expect the next boiling points to be? (Hboiling points to be? (H22O, HF, NHO, HF, NH33))
TRENDTREND
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This work has been released into the public domain by its author, Materialscientist at the wikipedia project. This applies worldwide.In case this is not legally possible:Materialscientist grants anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.
Hydrogen Bonding in Ice
Label the Label the oxgen and oxgen and hydrogen hydrogen atoms in the atoms in the diagram and diagram and indicate the indicate the hydrogen hydrogen bonds.bonds.
http://commons.wikimedia.org/wiki/File:Hex_ice.GIF licence behind imagehttp://commons.wikimedia.org/wiki/File:Hex_ice.GIF licence behind image
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Ion - dipole forces- • arise from the electrostatic
attraction between an ion and the oppositely charged pole of a polar molecule
• Shown in diagram by dotted lines.
• What type of substances (solutes & solvents) would display this type of forces?).
Ion-Dipole Forces
OO+
-HH
HH
OO+
-
HH
HHExample:Example:
????????
OO +-HH
HH
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Ion-induced Dipole• Attractive force between ions and
weak dipoles which are induced (caused) in non polar molecules
• This happens when an ion comes close to a non polar molecule. (Polarisation)
• It occurs between ionic substances and non polar solvents.
• SHOW THE DIRECTION THE ELECTRONS HAVE MOVED USING ALSO INDICATE THE RESULTING POLARITY ON THE MOLECULE
Example:Example:????????
Cl-Non-polarNon-polarMoleculeMolecule
IonIon
Induced dipole? Induced dipole?
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Dipole - Dipole Forces
• Attractive forces that exist between molecules that have permanent dipoles.
• These exist in any polar substance.
• In order to create a dipole or polar molecule, the molecule must have:
– …………………..– …………………..
• Determine which of the following would exhibit Dipole Dipole forces
H2S, CO2, C2H4 give a reason in each case.
Exampes:Exampes:????????
HH ClCl+ - HH ClCl
+ -
dipoledipole dipoledipole
Attractive force
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London Forces • A temporary dipole is induced
in a non polar molecule due to electron movements.
• These INDUCE similar but opposite forces in neighboring molecules which cause weak momentary attractions.
• These are the WEAKEST
attractive forces that exist between molecules.
• How would the strength of London forces be affected by the size of the molecule??
Example:Example:…………………………....
Electrons Electrons repelledrepelled
Temporary Temporary dipoledipole
Induced dipoleInduced dipole
Electrons Electrons movingmoving
Weak short lived Weak short lived attractionattraction
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BO
ND
STR
EN
GTH
BO
ND
STR
EN
GTH
Melting/Boiling Points INCREASEMelting/Boiling Points INCREASE
Intermolecular ForcesIntermolecular Forces
Hydrogen Hydrogen BondsBonds
Ion-dipoleIon-dipole
Ion-induced Ion-induced DipoleDipole
Van der WaalsVan der Waals ForcesForces
Dipole-dipoleDipole-dipole
Dipole-induced Dipole-induced DipoleDipole
Momentary Momentary Dipole (London)Dipole (London)
Visco
sity IN
CR
EA
SE
Visco
sity IN
CR
EA
SE
CONSTRUCT A FLOW CONSTRUCT A FLOW CHART WHICH CHART WHICH
CONNECTS ALL THE CONNECTS ALL THE INTERMOLECULAR INTERMOLECULAR
FORCES AND SHOWS FORCES AND SHOWS THEIR RELATIONSHIPSTHEIR RELATIONSHIPS
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Dissolution (dissolving)Salt (NaCl) dissolves in water
spontaneously.
Since water is a .................. molecule it can be represented as a dipole (two oppositely charged poles)
--++
ClCl--NaNa++--++
--++The The charged endscharged ends of the dipole would of the dipole would be be attractedattracted to the to the ................ ................ charge on the ionic solid.charge on the ionic solid.
O
H
H
`̀
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The dissolution processSolvent molecules (water)
are attracted to the solute particles as they have ......................... forces of attraction between them.
Solvent: water -....................... bonding (electrostatic forces - strong)
ClCl-- --++
--
++--++
NaNa++
--++
--++
--
++
ClCl--NaNa++
SoluteSolute: Sodium Chloride - : Sodium Chloride - ................. bonds................. bonds strong strong electrostatic attraction of oppositely charged ions.electrostatic attraction of oppositely charged ions.
Since the Since the forces in the SOLVENT are similarforces in the SOLVENT are similar to those in to those in the the SOLUTESOLUTE the solvent particles are able to substitute the solvent particles are able to substitute for and break up the forcesfor and break up the forces in the in the solutesolute material - which material - which is then literally is then literally ripped apart!ripped apart!
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Molecular solids
Iodine - I2
Strong covalent bonds
Weaker intermolecular bonds – London forces (between molecules)
Covalently bonded molecules held together by weaker intermolecular
bonding.
PROPERTIES
• ............. melting points
• .......................
• Soluble in ................. solvents
• (............. – conducting)
http://commons.wikimedia.org/wiki/File:Iodine-unit-http://commons.wikimedia.org/wiki/File:Iodine-unit-cell-3D-balls-B.pngcell-3D-balls-B.png
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Solubility
Water Parrafin Ethanol
I2
KMnO4
Explain the solubility's you have observed i.t.o intermolecular bonding.
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Vapour Pressure• The vapor pressure of a liquid is the equilibrium ............................... of a
vapor above its liquid (or solid.
• Liquids boil when their vapour pressure ................ the atmospheric pressure.
• There is a relationship between intermolecular force strength and vapour pressure.
substancevapor
pressure at 25oC
diethyl ether 0.7 atm
bromine 0.3 atm
ethyl alcohol 0.08 atm
water 0.03 atm
Low VpLow Vp Higher VpHigher Vp
Vapour pressure is inversely proportional to intermolecular bond strength:
STRONGER IMF = LOWER VpSTRONGER IMF = LOWER Vp
Stronger IMFStronger IMF Weaker IMFWeaker IMF
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Density Density = mass/volume (g.cm-3)
............. density – solids............. density – solids
............ particles per cm............ particles per cm3 3
............ IMF............ IMF
.......... density – gases.......... density – gases
............ particles per cm............ particles per cm33
............. IMF............. IMF
1cm x 1cm x 1cm 1cm x 1cm x 1cm = 1cm= 1cm33
http://commons.wikimedia.org/wiki/http://commons.wikimedia.org/wiki/File:Carbon-dioxide-crystal-3D-vdW.pngFile:Carbon-dioxide-crystal-3D-vdW.png
http://commons.wikimedia.org/wiki/http://commons.wikimedia.org/wiki/File:Kinetic_theory_of_gases.svgFile:Kinetic_theory_of_gases.svg
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Permission requested: Permission requested: http://www.reologie.ro/http://www.reologie.ro/
Viscosity• Viscosity is a measure of how Viscosity is a measure of how
thick (viscous) and sticky a thick (viscous) and sticky a liquid is. liquid is.
• Viscosity reduces the ability Viscosity reduces the ability of a liquid to flow. of a liquid to flow.
• Liquids that flow readily Liquids that flow readily (water) have a low viscosity. (water) have a low viscosity.
• Viscosity is a function of Viscosity is a function of (depends on) the attractive (depends on) the attractive forces of the molecules of the forces of the molecules of the liquid.liquid.
• Strong forces – high Strong forces – high viscosityviscosity
• Temperature also greatly Temperature also greatly affects viscosity: as affects viscosity: as temperature increases, temperature increases, viscosity decreases.viscosity decreases.
Kinetic energy enables particles to overcome forces.Kinetic energy enables particles to overcome forces.
Permission needed from: smartsynthetics.comPermission needed from: smartsynthetics.com
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Iodine (solid) dissolves in hexane• I2 and hexane both have london forces between their
molecules• These forces are similar in strength and iodine
molecules can substitute hexane molecules in the solution
• Dissolution can take place.
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Iodine + WaterIodine is only sparingly soluble in water?
• Water contains hydrogen bonds, • iodine contains london forces.• The bonds have very different strengths.• Iodine molecules can not substitute for water
molecules in the solution - there is very weak attraction between the solvent particles (H2O) and solute particles (I2).
• Dipole - induced dipole forces exist between water and iodine molecules.
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Ethanol + IodineIodine is soluble in Ethanol (CIodine is soluble in Ethanol (C22HH55OH)OH)INTERMOLECULAR FORCESINTERMOLECULAR FORCES
Ethanol + KMnO4Potassium permanganate is soluble in Ethanol (CPotassium permanganate is soluble in Ethanol (C22HH55OH). OH).
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Solubility Rules• PolarPolar solutes will be soluble in solutes will be soluble in ………………………………solvents.solvents.
• Non-polarNon-polar solutes will be soluble in solutes will be soluble in ………………………… solvents. solvents.
• Non-polarNon-polar solutes will NOT be soluble in solutes will NOT be soluble in ………... ………... solvents.solvents.
• Polar solutesPolar solutes will NOT be soluble in will NOT be soluble in ……………….………………. solvents.solvents.
…………………………………………………………………………..
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Hi -
This is a SAMPLE presentation only.
My FULL presentations, which contain loads more slides and other resources, are freely available on my resource sharing website:
www.sciencecafe.org.za
(paste into your browser if link above does not work)Have a look and enjoy!
Keith Warne
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