Transcript Intermolecular Forces


Two things affect intermolecular forces:
› Charge
› Distance
Bigger distance equals LESS attraction!
 More charge equals MORE attraction!
 Coulomb’s Law

› F ~ charge/distance2
› Distance has more effect since it is squared.

Bonds are permanent attractions, IMF’s
are temporary

Covalent
› Sharing of electrons in permanent tug of
war.

Ionic
› Stealing electrons and then being attracted
by opposite charges
Ion Dipole
 Hydrogen Bonding
 Dipole-Dipole
 London Dispersion forces


Between an ion and a dipole!
› Ion = permanently charged particle
› Dipole = a polar molecule

Almost exclusively seen in aqueous
solutions.

Between two dipoles!
› Dipoles are polar molecules
that have no net charge, but
the charge is distributed
unequally.
› Positive end of one attracts
negative end of the other.

ALL covalent compounds experience
this force.
› It is between the electrons in one atom and
the nucleus of another atom.
› More mass and/or more atoms means more
dispersion forces.
›
More mass doesn’t cause an increase in LDF!
›
More mass infers that there are more P and N,
which MEANS there are more electrons.
It is an increase in the number of e- which
causes a larger e- density which leads to a
stronger temporary dipole.
You can use molar mass to figure it out since a
larger molar mass implies a bigger density but
you can't use the words molar mass in your
justification in a Free Response Question
›
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
Occurs in molecules in which hydrogen is
having FON!
› The most electronegative atoms are F, O
and N. With a hydrogen, there is excessive
polarization.
› This excessive polarization attracts the lone
pair of electrons.

Strongest:
Ion-Dipole
(very strong)
Hydrogen Bonding (strong)
Dipole-Dipole
Dispersion Forces (very weak)

More IMF’s or stronger IMF’s affect the
following properties:
› Boiling point
› Melting Point
› Viscosity (thickness)
› Vapor Pressure
› Surface tension

Stronger IMF
› Higher boiling point

Weaker IMF
› Lower boiling point

Compare CS2 to CO2
Both are linear
Both are nonpolar covalent
Both have LDF
CS2 has stronger LDF – larger molecule
means larger e- density

Stronger IMF
› Higher melting point

Weaker IMF
› Lower melting point
Think of red rover…
 Compare HBr to Cl2

Both are liner
 HBr has LD, Dipole
 Cl2 has LD, nonpolar


HBr has stronger IMF – higher melting
point

Stronger IMF
› Higher viscosity

Weaker IMF
› Lower viscosity

More attached to each other, the hard
to flow, like solids vs. liquids.

Compare CH3OCH3 to CH3CH2OH
CH3OCH3 LD, Dipole
CH3CH2OH LD, HB
Therefore CH3CH2OH has stronger IMF and
a higher viscosity

Stronger IMF
› Lower vapor pressure

Weaker IMF
› Higher vapor pressure

Again…think red rover.

Compare H2S to H2O2
H2S LD, Dipole
 H2O2 LD, HB


H2O2 has stronger IMF therefore lower
vapor pressure

Resistance of a liquid to increase in its
surface area

Stronger IMF
› Higher surface tension

Weaker IMF
› Lower surface tension

Compare H2CO to H2O
H2CO
 H2 O


LD, Dipole
LD, HB
Hydrogen bonding is a stronger IMF than
Dipole, therefore H2O has a stronger
surface tension

Stronger IMF
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
Higher boiling point
Higher melting point
Higher viscosity
Higher surface tension
Lower vapor pressure
Weaker IMF
›
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Lower boiling point
Lower melting point
Lower viscosity
Lower surface tension
Higher vapor pressure