Problems du Jour
Identify the types of intermolecular forces present in the
following substances and select the substance in each pair
that has the highest boiling point:
C6H14 or C8H18
C3H8 or CH3OCH3
HOOH or HSSH
NH2NH2 or CH3CH3
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Hydrogen bonding
In general, boiling point increases with increasing MW;
why?
Dispersion forces increase with increasing MW
Note that the boiling points of the hydrides of O, N, F are
abnormally high relative to their MW:
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What does this suggest about the intermolecular forces in
these compounds?
These strong intermolecular interactions result from
hydrogen bonding
H bonding: attraction that exists between H atom in a polar
covalent bond (e.g., H-F, H-O, H-N) and an unshared e- pair
on a nearby electronegative atom (e.g., F, O, N)
F, N, O are more electronegative than H; H-F, H-O, H-N
bonds are very polar
e- on H shifts toward N, O, F, leaving proton exposed & able
to interact with (-) charges on nearby atoms
H"+" is very small & can approach electronegative atoms
very closely- makes the interaction very strong!
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H bonding interactions are much stronger than dipoledipole or dispersion forces
e.g., which would have the higher b.p., C2H5Cl or
C2H5OH?
How does H-bonding explain the observation that ice
floats on water?
3. Ion-dipole forces
Notice: liquids are typically composed of molecular
substances (e.g., NH3; H2O)
Why does an ionic substance (e.g., NaCl) dissolve in
H2O?
How does an ion interact with a polar substance?
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The ion-dipole force is the only case of a force between a
polar substance and a permanent charge (an ion)
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Comparing Intermolecular Forces
We identify the intermolecular forces which operate within
a substance by examining composition and structure
NOTE: dispersion forces operate between all molecules,
polar or nonpolar!
For polar species, dipole-dipole forces are also present
but these often make a smaller contribution to the total
intermolecular attraction that the dispersion forces
For small, highly polar molecules with hydrogen bonding,
H-bonding leads to the strongest interactions
Take a look at some data…
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Contribution of the dispersion force to the total
intermolecular interaction energy
Molecule pair
% of the total energy of interaction
H2O-H2O
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NH3-NH3
57
CH3Cl-CH3Cl
68
HCl-HCl
86
H2O-CH4
87
HBr-HBr
96
HCl-HI
96
HI-HI
99
CH4-CH4
100
Ne-Ne
100
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When comparing relative strengths of intermolecular
forces, consider:
For molecules of  same weight and shape,
dispersion forces are approximately equal; we
then look at the effects of polarity on
intermolecular forces
For molecules of similar polarity, we look at the
effects of molecular weight and therefore
dispersion forces on intermolecular forces
Note that the effects of intermolecular attractions are
additive…
E.g. Acetic acid (CH3COOH, MW = 60 amu) boils at 391 K
but propanol (CH3CH2CH2OH, MW = 60 amu) boils at 370 K.
Why?
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Properties of Liquids
Viscosity & surface tension
Viscosity: resistance of a liquid to flow
Related to the ease with which individual molecules
can move relative to each other
Depends upon the magnitude of the intermolecular
attractive forces and on shape
For a series of related compounds (e.g. alkanes),
viscosity increases with increasing MW
Decreases with increasing T; why?
Surface tension
Notice that H2O 'beads up' on a waxy surface (e.g., a
waxed car); why?
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Surface tension is a measure of the net “inward” forces
which must be overcome to expand the surface area of a
liquid
It is the amount of energy required to increase the surface
area of a liquid by a certain amount (usually 1 m2)
E.g. water has a surface tension (at 20oC) of 7.29 x 10-2
J/m2
Cohesive vs adhesive forces
Phase Changes
Energy changes
When change of state involves a transition to a less
ordered system, energy is required to overcome
intermolecular attractive forces
Process which requires energy: endothermic
Process which releases energy: exothermic
As strengths of intermolecular forces increase, amounts
of energy required to cause a phase change increase
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There is an enthalpy change H associated with each of
these processes, e.g., heat of fusion, heat of vaporization,
heat of sublimation
e.g., H2O heating curve: start with H2O(s, 1 atm, -25oC) and
heat to H2O(g, 1 atm, 125oC)
When a solid, liquid or gas is heated, T increases
Notice that T does not change during a phase transition as
heat is added!! (why?)
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Critical pressure and temperature
A gas will form a liquid under two conditions:
Increase pressure at constant T
Decrease T at constant P
In terms of the relationship between kinetic energy,
temperature, and intermolecular forces, does this make
sense?
There is, for any gas, a critical temperature Tc– this is the
highest temperature at which a gas will form a distinct
liquid phase
The critical pressure Pc is the pressure required for
liquefaction at Tc
E.g., for water, Tc=374oC, Pc=218 atm
Above Tc: supercritical fluid
In general, nonpolar, low MW substances tend to have
lower Tc and Pc than do polar or high MW compounds
Why is this so?
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Vapor pressure
Defined as partial pressure of vapor when it is
equilibrium with the liquid phase
Molecules continuously move from the liquid phase to
the vapor phase and vice versa
This is a dynamic equilibrium - opposing
processes occurring at equal rates
Liquid & vapor are at equilibrium when
evaporation & condensation occur at equal rates
Liquids boil when vapor pressure = external pressure
acting on surface of liquid
E.g. water boils at 100oC at sea level (1 atm
pressure)
How is the vapor pressure of a pure substance related to
intermolecular forces and volatility?
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Problems du Jour
The critical temperatures (K) and pressures (atm) for a
series of halogenated methanes are given below:
Compound CCl3F
CCl2F2
CClF3
CF4
Tc
471
385
302
227
Pc
43.5
40.6
38.2
37.0
List the intermolecular forces that occur for each
compound. Predict the order of increasing IMF for this
series from least to most.
Using the trends in this table, predict the critical
temperature and pressure for CCl4.
Use the vapor pressure curve for water (Fig 11.24) to
estimate the boiling temperature at a pressure of 710 torr.
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