intermolecular forces... explaining the properties of liquids
TRANSCRIPT
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Intermolecular Forces
. . . explaining the properties of liquids
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Let’s get one thing straight . . .
Intra*molecular forces include:
• Non-polar covalent bonds
• Polar covalent bonds
• Ionic bonds (not technically within a molecule, but anyway . . .)
• Metallic Bonding
*intra = within/between (pick one)
within
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Intermolecular forces exist
between molecules
Intermolecular forces are the glue that holds molecules together
Consider the following data:
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Compound Formula Molar mass (g)
mp (oC) bp (oC)
methane CH4 16 -183 -164
ammonia NH3 17 -77.7 -33.3
water HOH 18 0 100
neon Ne 20 -249 -246
methanol CH3OH 32 -93.9 64.9
hydrogen sulfide
HSH 34 -85.5 -60.7
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Why is the boiling point of water (and its mp) so much higher than that of other small molecules?
Due to the high strength of intermolecular forces.
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Types of Intermolecular Forces
1. van der Waals Forces include:Dipole-dipole
Ion-dipole
Induced Intermolecular Forces• Ion-induced dipole• Dipole-induced dipole
Dispersion (London) Forces
2. Hydrogen Bonding
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Let’s look more closely at each type …
Dipole-Dipole Forces
• exist between polar molecules
--or represet as--
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• polar molecules will have a stronger attraction for one another than similarly-sized non-polar molecules
• this results in ____________ boiling points for polar molecules compared to similarly-sized non-polar molecules
(higher)
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Ion-Dipole Forces
exist between a polar solvent and an ionic solute
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eg. NaCl dissolved in water
Remember: Ion-dipole attractions only occur with soluble ions.
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Induced Intermolecular Forces
What does induced mean?
1. To lead or move, as to a course of action, by influence or persuasion.
Synonym: ______________.
2. To bring about or stimulate the occurrence of; cause: a drug used to induce labour. (thanks to dictionary.com)
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Ion-induced dipole
• Occurs when an ion induces a nearby non-polar molecule to become polar
• ie. The ion distorts the electron density of the nearby non-polar molecule
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eg of Ion-induced dipole
Bonding between Fe2+ ion and O2 in hemoglobin
Vital for O2
transport in
the body.
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dipole-induced dipole
Similar to
ion-induced dipole.
A polar molecule
induces a nearby
molecule to become
polar by distorting its
electron cloud.
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example of dipole-induced dipole
Explains the—limited, but significant—solubility of O2 in water.
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Dispersion (London) Forces
Seems like nonpolar molecules have no way of attracting each other.
BUT: nonpolar gases (O2, N2, CH4) can be liquefied.
This indicates that if kinetic energy of molecules reduced, some type of attractive force can predominate.
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Fritz London (1930) suggested that the motion of electrons within an atom or non-polar molecule can result in a transient dipole moment.
(transient = short-lived)
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A Model To Explain London Dispersion Forces:
Consider two Helium atoms (each with 2 electrons)
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Explanation of Dispersion Forces
At a given instant, theremay be an uneven distribution of e-s aroundan atom or molecule.
Think electron “pile-up”.
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How does the strength of dispersion forces correlate to molecular size?
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• as the size of a non-polar molecule increases, so does its electron cloud
• this will increase the strength of the temporarily induced dipole
• ie. as non-polar molecules get larger, their bp increases
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Consider the following bp data
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What’s with the unusually high bp of
• HF
• NH3
• H2O ?
What does this suggest about the strength of intermolecular bonding in these compounds?
Very strong . . .
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Hydrogen Bonding
• HF, HOH, NH3 all have significantly higher bp than predicted from the trend in each family.
(bp of H2O ca. 200oC greater than expected)
• due to the very polar
H-F, H-O, H-N bonds
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Simple molecules which exhibit H-bonding:
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For Hydrogen Bonding to occur . . .
Hydrogen must be attached to one of the most electronegative elements (N,O,F)
very polar bonds
dipole vectors don’t cancel
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In these v polar bonds H acquires a significant amount of positive charge.
Remember: O has 2 lone pairs of e-s
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The H must be bonded to either N, O, F.
These atoms each have at least one "active" lone pair.
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“Active” lone pairs in second E level are contained in a relatively small volume results in high density of negative charge.
(Lone pairs at higher levels are more diffuse and not so attractive to positive charges.)
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Consider five water molecules close together
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Review of Intermolecular Bonding
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Another Review
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HomeworkFrom Chemistry 12 (Mustoe et al)This is posted as “intermolecular forces explanation”From this handout: P 208 SR 1, 2, 3abc
(omit “network”) 4, 5 (omit “network”), 6, 7 Also: Illustrate the intermolecular forces that
exist in NH3(aq). Draw a pseudo 3-D picture of NH3, decide on molecular polarity and intermolecular forces.