Chapter 11 Intermolecular Forces. A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases. Solid phase - ice. Liquid phase - water. 11.1.
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A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.
Solid phase - ice
Liquid phase - water
Generally, intermolecular forces are much weaker than intramolecular forces.
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
“Measure” of intermolecular force
Intermolecular forces are feeble; but without them, life as we know it would
be impossible. Water would not condense from vapor into solid or liquid
forms if its molecules didn't attract each other. Intermolecular forces are
responsible for many properties of molecular compounds, including crystal
structures (e. g. the shapes of snowflakes), melting points, boiling points,
heats of fusion and vaporization, surface tension, and densities.
Intermolecular forces pin gigantic molecules like enzymes, proteins, and
DNA into the shapes required for biological activity.
Occur between every compound and arise from the net attractive forces
amount molecules which is produced from induced charge imbalances
Figure 10-8 Olmsted Williams
The magnitude of the Dispersion Forces
is dependent upon how easily it
is to distort the electron cloud.
The larger the molecule the greater
it’s Dispersion Forces are.
Figure 10-9 Olmsted Williams
The boiling point of alkanes increase with the length of the carbon
chain. Long-chain alkanes have larger dispersion forces because
of the increased polarizability of their larger electron cloud.
Olmsted Williams Fig 10-10 Pg 437
How molecular shape affects the strength of the dispersion forces
The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons.
Long thin molecules can also lie closer together - these attractions are at their most effective if the molecules are really close.
For example, the hydrocarbon molecules butane and 2-methylpropane both have a molecular formula C4H10, but the atoms are arranged differently. In butane the carbon atoms are arranged in a single chain, but 2-methylpropane is a shorter chain with a branch.
Butane has a higher boiling point because the dispersion forces are greater. The molecules are longer (and so set up bigger temporary dipoles) and can lie closer together than the shorter, fatter 2-methylpropane molecules.
Dispersion forces usually increase with molar mass.
the ease with which the electron distribution in the atom or molecule can be distorted.
Is the Molecule Polar?
We have already talked about diatomic molecules. The more
Electronegative atom will pull the electron density of the bond
Closer to itself giving it a partial negative charge leaving the other
Atom with a partially positive charge. Thus giving the molecule
A dipole moment.
But what about molecules made up of more than two molecules?
Molecules with 3 Atoms
Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO2 is non-polar.
The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar
SO2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle
Molecules with 4 Atoms
CCl4 is non-polar
CHCl3 is polar
How to Determine if a Molecule
Which of the following molecules are polar (have a
dipole moment)?H2O, CO2, SO2, and CH4
no dipole moment
no dipole moment
Orientation of Polar Molecules in a Solid
Attractive forces between polar molecules
Dipole Forces occur between molecules containing a dipole moment.
The positive end of the dipole moment on one mole is attracted to the
Negative end of the dipole moment on a nearby molecule.
Consider 2-methyl propane
(left) and acetone (right)
Both compounds are about
Equal in size and shape therby
Having similar dispersion forces,
But Acetone contains an
Oxygen (red) and causes the
Molecule to have a dipole
Moment allowing it to have
Dipole forces and thus a
Higher boiling point
Attractive forces between an ion and a polar molecule
The larger the charge the stronger the force
A molecular picture showing the ion-dipole
Interaction that helps a solid ionic crystal dissolve
in water. The arrows indicate ion-dipole interactions.
What type(s) of intermolecular forces exist between each of the following molecules?
HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4 is nonpolar: dispersion forces.
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.
A & B are N, O, or F
contain pairs of
together head to
then stack in
planes which are
Held together by
Courtesy Stephen Frisch
These forces affect how molecules will interact with each other and
As a general rule as the strength of the force increases the boiling
Point of the compound increases
Liquids and Surface Tension
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Strong intermolecular forces
High surface tension
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
T2 > T1
H2O (l) H2O (g)
The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation
A substance with a high
Vapor pressure is considered
To be volitile therefore, the lower
The boiling point the higher the
Vapor pressure and the weaker
The intermolecular forces
The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure.
The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.
The critical temperature(Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure.
The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature.
H2O (s) H2O (l)
The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium
H2O (s) H2O (g)
Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid.
DHsub = DHfus + DHvap
( Hess’s Law)
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.
Phase Diagram of Water