Intermolecular Forces of Attraction as Level
-
Upload
chai-kah-chun -
Category
Documents
-
view
28 -
download
0
description
Transcript of Intermolecular Forces of Attraction as Level
-
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
***********************