Between. Intra- › strong forces that hold the atoms in a molecule together takes 464 kJ/mol to...
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Inter molecular Forces Topic 4.4 between
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Transcript of Between. Intra- › strong forces that hold the atoms in a molecule together takes 464 kJ/mol to...
Intermolecular ForcesTopic 4.4between
Intra vs. Inter molecular forces Intra-
› strong forces that hold the atoms in a molecule together takes 464 kJ/mol to break the H-O bonds within a water molecule
› responsible for chemical properties
Inter-› weak forces that holds molecules to
one another takes only 19 kJ/mol to break the bonds between water molecules
› the strength of the intermolecular forces determines the physical properties of the substance melting, boiling, solubility, conductivity, volatility
3 main “types” of intermolecular forces1. London forces (also called
dispersion forces or instantaneous induced dipole forces
2. dipole-dipole forces (polar molecules)
3. a stronger type of dipole-dipole bonding called hydrogen bonding
* Van der Waals and London forces are often used interchangeably– more later on this.
strength increases
1. London forces
van der Waals’ YouTube (:20) occurs in non-polar molecules the distribution of electrons
around an individual atom, at a given instant in time, may not be perfectly symmetrical› this can produce
temporary/instantaneous dipole (polar molecule)
› this can then induce a nearby molecule to be polar and therefore a very weak attraction between the two molecules
› magnitude of the force depends on…1. number of electrons and size of
the electron cloud with more electrons, valence electrons
are farther away from the nucleus and can be polarized more easily
2. shape of molecules molecules with shapes that have more
contact area have greater forces between them than those don’t
boiling point increases
this flat shape allows it to stick to one another
better
these round shapes do NOT allow
them to stick to one
another
Dipole-Dipole Forces attractive forces between the
positive end of one polar molecule and the negative end of another polar molecule
must be in close proximity for the dipole-dipole forces to be significant
the more polar the molecule, the greater the dipole-dipole force
stronger than London forces
Hydrogen Bonding
YouTube Hydrogen Bonding (1:40) YouTube Hydrogen Bonding Video (:
58) a specific type of dipole-dipole type
interactions stronger than other dipole-dipole
and London forces the hydrogen (H) in a molecule is
intermolecularly bonded to a small, highly electronegative element (usually an N, O or F atom) on another molecule
H-NOF
Van der Walls vs. London Forces
the term London forces is used for instantaneous induced dipole – induced dipole force in non-polar molecules
Van der Waals is a more inclusive term, for all intermolecular attractions
Physical Properties Related to Bonding Type
Melting point (mp) - solid to liquid Boiling point (bp) - liquid to gas Volatility - how easily it is converted
to gas Conductivity (conducts electricity)
› depends on whether the substance contains freely moving charged particles
Solubility - solute’s ability to dissolve in solvent
Intermolecular forces affect on boiling point
the greater polarity of a molecule, the higher the boiling point› In HF, H is 2.1 and F is 4.0, difference
of 1.9› In HCl, H is 2.1 and Cl is 3.0,
difference of 0.9BP is 20°C BP is -85°C
› H2O vs. H2S? In H2O, H is 2.1 and O is 3.5, difference of 1.4 water molecules can hydrogen bond (and London forces) to each other
BP is 100°C
In H2S, H is 2.1 and S is 2.5, only a difference of 0.4 H2S can dipole-dipole bond (and London forces) to each other
BP is -60°C
H-NOF ?
H-NOF ?yes
H-NOF ?no
CH3OCH3 vs. CH3CH2OH ?
-24°C 78°C dipole – dipole has hydrogen bonding
H-NOF ?
NH3 vs. PH3 ?
-33°C -87°C has hydrogen bonding dipole – dipole
CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?
only London forces (VDW) low BP
VDW & dipole-dipole medium BP
VDW, D-D, & hydrogen bonding
highest BP
