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Keith Warnewww.TeachBomb.com

Intermolecular Forces


O+

-H

H

O+

-

H

HIntermolecular

Intramolecu

lar


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Dissolution of a salt


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.

SOLUBILITY Water Hexane Ethanol

I2

KMnO4

• Work through this presentation on intermolecular forces and the one on Bonding & Structure and try to find explanations for the above observations!

• Try and work out some simple solubility rules consistent with your investigation.



• 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

• USE ARROWS TO INDICATE THE TWO TYPES OF BONDS IN THE DIAGRAM

Intermolecular forces

O+

-H

H

O+ -H

H

Intermolecular ??

Intramolecular ??


Bond Polarity in WaterThe oxygen atom has greater electronegativity than the hydrogen atoms so oxygen attracts the bonding electrons (shared pairs) closer to itself.

-+ OH

H

The water molecule is therefore a DIPOLE - it has two oppositely charged “poles”. We say water is a polar compound.

Electrons attracted closer

to oxygen

Lewis diagram Space filled model

= small charge created by

unequally shared electrons


• 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 DOTTED LINES TO SHOW WHERE HYDROGEN BONDS WOULD BE FOUND IN THE DIAGRAM

Hydrogen Bonds

O+

-H

HO+ -

H

H

O+

-

H

H

Example:Water

Other Examples??



Hydrogen Bonding


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 oxgen and hydrogen atoms in the diagram and indicate the hydrogen bonds.

http://commons.wikimedia.org/wiki/File:Hex_ice.GIF licence behind image

http://commons.wikimedia.org/wiki/File:Hex_ice.GIF


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

O+

-H

H

O+

-

H

HExample:

????

O +-H

H



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:????

Cl-Non-polarMolecule

Ion

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:????

H Cl+ - H Cl

+ -

dipole dipole

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:……………..

+ d -d +d d-

Electrons repelled

Temporary dipole

Induced dipole

Electrons moving

Weak short lived attraction


BO

ND

STR

EN

GTH

Melting/Boiling Points INCREASE


Hydrogen Bonds

Ion-dipole

Ion-induced Dipole

Van der Waals Forces

Dipole-dipole

Dipole-induced Dipole

Momentary Dipole (London)

Visco

sity IN

CR

EA

SE

CONSTRUCT A FLOW CHART WHICH

CONNECTS ALL THE INTERMOLECULAR

FORCES AND SHOWS THEIR 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)

-+

Cl-Na+-+ -

+The charged ends of the dipole would be attracted to the ................ charge on the ionic solid.

O

H

H

`



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)

Cl- - +

-

+-+

Na+

-+

-+

-

+

Cl-Na+

Solute: Sodium Chloride - ................. bonds strong electrostatic attraction of oppositely charged ions.

Since the forces in the SOLVENT are similar to those in the SOLUTE the solvent particles are able to substitute for and break up the forces in the solute material - which is then literally 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-cell-3D-balls-B.png


Solubility

Water Parrafin Ethanol

I2

KMnO4

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 Vp Higher Vp

Vapour pressure is inversely proportional to intermolecular bond strength:

STRONGER IMF = LOWER Vp

Stronger IMF Weaker IMF



Density Density = mass/volume (g.cm-3)

............. density – solids

............ particles per cm3

............ IMF

.......... density – gases

............ particles per cm3

............. IMF

1cm x 1cm x 1cm = 1cm3

http://commons.wikimedia.org/wiki/File:Carbon-dioxide-crystal-3D-vdW.png

http://commons.wikimedia.org/wiki/File:Kinetic_theory_of_gases.svg



Permission requested: http://www.reologie.ro/

Viscosity• Viscosity is a measure of how

thick (viscous) and sticky a liquid is.

• Viscosity reduces the ability of a liquid to flow.

• Liquids that flow readily (water) have a low viscosity.

• Viscosity is a function of (depends on) the attractive forces of the molecules of the liquid.

• Strong forces – high viscosity• Temperature also greatly

affects viscosity: as temperature increases, viscosity decreases.

Kinetic energy enables particles to overcome forces.

Permission needed from: smartsynthetics.com



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.



KMnO4 + WaterPotassium permanganate is very soluble in water.• KMnO4 is an ionic substance.• Water contains hydrogen bonds, • ION - DIPOLE FORCES EXSIST between

these two substances.• These are relatively strong intermolecular

forces and that accounts for the solubility of ionic substances in polar solvents.



KMnO4 + HexanePotassium permanganate is insoluble in hexane.

• KMnO4 is an ionic substance• Hexane is non-polar - v.d. Waals (london) forces• Ion - induced dipole forces would exist between solvent

molecules - these are VERY weak interactions• The solubility is VERY LOW.



Ethanol + IodineIodine is soluble in Ethanol (C2H5OH)INTERMOLECULAR FORCES

Ethanol + KMnO4Potassium permanganate is soluble in Ethanol (C2H5OH).



Ethanol + IodineIodine is soluble in Ethanol (C2H5OH)INTERMOLECULAR FORCES• Iodine - v.d. Waals (London) forces• Ethanol - hydrogen bonds and london forces• Dipole - induced dipole forces would exist between solvent

and solute as well as v.d.Waals (London) forces• The london forces between ethanol molecules could be

substituted for london forces in the iodine and so iodine is soluble in ethanol.

Ethanol + KMnO4Potassium permanganate is soluble in Ethanol (C2H5OH). The

ion-dipole interactions between KMnO4 and ethanol molecules are strong enough to cause dissolution of the ionic salt.



Solubility Rules• Polar solutes will be soluble in ………………solvents.

• Non-polar solutes will be soluble in …………… solvents.

• Non-polar solutes will NOT be soluble in ………... solvents.

• Polar solutes will NOT be soluble in ………………. solvents.

…………………………………….
