Intermolecular forces of attractionLearning ObjectivesDifferentiate between polar and non polar moleculesState the names of different types of intermolecular forces of attraction and differentiate between themExplain the terms dipole dipole , instantaneous dipole induced dipole attractions and hydrogen bondingDescribe the effects of attractive forces on the physical properties of substances( This presentation is to enhance your understanding of the topic . This is only part of the syllabus)

PolaritySeparation of charge

An asymmetrical difference in electronegativity along a bond or in a molecule

The polar molecules with d+ and d- ends and non polar molecules without dipolesd-d+d-d-d+d+

Van der Waals Forces Small, weak interactions between molecules

Van der Waals Forces Intermolecular: between molecules (not a bond)Intramolecular: bonds within molecules (stronger)

What is being attracted?d+ attracted to d- electrostatic attraction

e- s of one atom are attracted by another atoms nucleus

e-e-++

Evidence of VDW Forces?Non-polar molecules can exist as gases, liquids and solids. Example: CO2

3 Types of Van der Waals Forces1) dipole-dipole2) permanent dipole - induced dipole3) Instantaneous dipole induced dipole

Dipole-Dipole

Two polar molecules align so that d+ end of one molecule and d- end of another molecule are next to each other giving rise to electrostatic attraction between the two molecules. Lets look at:

fluromethane (CH3F) and ethane (C2H6) on the next slide

Fluoromethane (CH3F) boiling point = 194.7 KHHHCFHCFHHpolar or non-polar?Ethane (C2H6) boiling point = 184.5 Kpolar or non-polar?

Permanent Dipole - Induced DipoleA permanent dipole can induce (cause) a temporary dipole in a Neighbouring non-polar molecule

The molecules then line up So that d+ end of one molecule and d- end of another molecule attract each other.

ExampleAre-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-A Permanent DIPOLE ( polar molecule)INDUCEDDIPOLE non- polar molecule

d-d+Permanent Dipole Induced Dipole (weak and short-lived)

Dispersion Forces or instantaneous dipole induced dipole forcesA temporary dipole forms in a non-polar moleculewhich leads toa temporary dipole to form in ANOTHER non-polar molecule

Dispersion is the ONLY intermolecular attraction that occurs between non-polar molecules

Dispersion ForcesCl-Cle-e-e-e-e-e-e-e-e- e-e-e-e-e-e-e-e-e-Non-polar

INDUCEDDIPOLE TEMPORARY DIPOLE

Non-polarCl-Cle- e-e-e-e-e-e-e- e-e-e-e- e-e-e-e-e-Dispersion (weakest and very short-lived)

Review Intermolecular forcesDipole Dipolebetween two polar moleculesDipole Induced Dipolebetween a polar & a non-polar molecule Instantaneous dipole induced dipole or Dispersionbetween two non-polar molecules

Hydrogen BondingIs the STRONGEST Intermolecular Force!!Occurs only if hydrogen in a molecule is bonded to a F, O or a N atomIceLiquid

Hydrogen Bonding Hydrogen bonds keep water in the liquid state over a wide range of temperatures than is found for any other molecule of its SizeHydrogen bonded to either of F, O or N, (the 3 most electronegative elements inthe periodic table); is highly deficient inelectrons and so is easily attracted by the lone pair of electrons on F, O or N of another molecule

Hydrogen bonds account for the high boiling point of water

Expansion of IceIce expands when water freezes compared to most substances that contract on freezingThis is due to the presence of hydrogen bonding between water moleculesIce as a result gets an open structure, which makes it less dense than liquid water.

Ponds FreezingSolid water (ice) has a lower density than liquid water

Why can liquid water change to vapor at room temperature?High energy molecules escape leaving the low energy molecules behindEvaporation occurs at all temperatures# particleslow KEave KEhigh KE

Volatile SubstancesEasily evaporateHave weak attractive forces between the particles Have low boiling point Have high vapor pressure

Non-volatile substancesDo not easily evaporateHave strong attractive forces between the particlesHave high boiling pointHave Low vapor pressure

BoilingBoiling occurs when Vapor Pressure = Atmospheric Pressure

When Vapor Pressure = 760 mmHg, Boiling Point of water = 100oC

Boiling and Evaporation What is the difference between evaporation and boiling?Evaporation: occurs at any temperature; high energy molecules escapeBoiling: occurs when vapor pressure = atmospheric pressure

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