Intermolecular Forces

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Intermolecular ForcesIntermolecular Forces

Keith WarneKeith Warnewww.www.sciencecafesciencecafe.org.za.org.za

HHInterIntermolecular

molecular

IntraIntramolecu

molecular

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

Non Polar

Insoluble Soluble soluble

Ionic Soluble Insoluble/v

slightly soluble

• 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

HHInterIntermolecular

molecular

IntraIntramolecular

molecular

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

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

• 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

HHOO+ -

Example:Example:Water

Other Other Examples:Examples:HF, NHHF, NH3 3 any any alcohol CHalcohol CH33OH OH etc.etc.

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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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

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

HHExample:Example:

????????

OO +-HH

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?

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

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

Melting/Boiling Points INCREASEMelting/Boiling Points INCREASE

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)

sity IN

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

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.

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!

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

Solubility

Water Parrafin Ethanol

Explain the solubility's you have observed i.t.o intermolecular bonding.

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

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

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.

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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.

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.

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).

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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Keith Warne

Intermolecular Forces

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