Download Chemistry 11AP Notes Fall 2013 Ms Butler Intermolecular Forces

Chemistry 11AP Notes Fall 2013 Ms Butler
Intermolecular Forces
Intermolecular forces- forces of attraction and repulsion between molecules that hold
molecules, ions, and atoms together. These depend on the distance between the
atoms/molecules – they will grow weaker as the distance between particles increases, and
stronger the closer the particles are to one another
Intramolecular - forces of chemical bonds within a molecule
Observable Properties
Solids keep their shape and volume
and are virtually incompressible.They
often have crystalline shapes.
Liquids retain their volume when
placed into a container, but conform
to the shape of the container. They
are fluid and are able to flow. Liquids
are nearly incompressible.
Gases are easily compressed, but
expand spontaneously to fit whatever
container they are in.
Molecules are tightly packed and
highly ordered. Very strong
attractions between molecules hold
them in place so they are effectively
locked in position.
Molecules lightly packed but with
little order. They are able to move
past each other with little difficulty.
Intermolecular attractive forces are
relatively strong.
Widely spaced molecules with much
empty space between them. Motion
is random and with very weak
attractions between the molecules.
Boiling Point and Electronegativity
Boiling Point
- the temperature at which the liquid form of a compound vaporizes into a gas.
- the attractive forces in the liquid must be overcome to become a gas.
- boiling point of 0 = no attractive force
Trends:
Increasing
attractive
forces
Compound
NaCl
H2O
BrCl
Ar
Boiling Point
1413oC
100oC
5oC
-186oC
Increasing boiling point
Stronger intermolecular forces result in higher boiling points. The dipole moments
increase with the polarity of a bond (the greater the difference in EN between the atoms, the
stronger the dipole-dipole attractions.) The strength of London forces increases with the number
of electrons.
Chemistry 11AP Notes Fall 2013 Ms Butler
Electronegativity:
- The attractive force that an atom exerts on an electron
- Closer e- the nucleus = more electronegative
- More protons, more positive, more attractive to eElectronegative Difference:
- Ionic Bonds >1.7
- Polar Covalent Bonds 0.5-1.7
- Slightly polar covalent above 0 to 0.5
- Non-polar or pure covalent – 0
- Electrostatic Attraction (common in every non-covalent molecular force) = when opposite
charges with permanent dipoles attract bonding
IONIC
Ionic Bond (strongest bond)
 a complete transfer of one or more e- occurs when one interacting atom is much more
electronegative than the other.
o one gives up e- (cation) and one takes e- (anion) making ions that are
electrostatically attracted.
 It takes a lot of energy to break ionic bonds and to turn the molecule into
a gas
Cl
Na
EN
0.9
3.0
∆EN = 2.1  Ionic Bond
Strength: strongest bond
Trend: The greater the charge difference, the stronger the attraction
DIPOLE-DIPOLE
-
sharing of electrons in a bond is covalent but not completely even (Polar Covalent)
∆EN = 3.0-2.8 = 0.2
Slightly Polar
Br
Cl
Cl
Br
Chemistry 11AP Notes Fall 2013 Ms Butler
Trends:
 Electronegative Difference. Polarity is amplified when electronegativity difference is large
and minimized when the difference is small.
 Bond Length (primarily atomic radii of the bonding partners)
 Increasing bond length decreases dipole
HYDROGEN BONDING




A dipole-dipole interaction where the H atom is non-covalently attracted to an
electronegative atom
The H must have a large positive delta and the other atom must have a source of
electrons to be attracted to H
Usually negatively charged F, O, N is attracted to the positively charged H
Very important biologically
Strength:
 a H bond is a weak bond and is dynamic
 In solid phase H-bond is static
 H-bonds between molecules continually form and break in liquid.
 Every water molecule forms 3.4 H bonds with other water molecules
Trend:
 The more electronegative the atom attracted to H, the stronger the bond
LONDON DISPERSION FORCES
London Dispersion Forces/ Van der Waals Forces
 The attraction between instantaneous electric dipoles on adjacent molecules
 Present in all molecules because everything has an electron cloud
Strength:
 Weak because temporary
Trends:
 Strength increases with # of electrons in molecule
 strength depends on the polarizability [ease with which their electron clouds can be
distorted]
 Molecules with lesser EN and larger radii have higher polarizability.
 Higher polarizability = stronger attraction.
Chemistry 11AP Notes Fall 2013 Ms Butler
Summary of Intermolecular Attractions
Type of Substance that Exhibit
Intermolecular Attraction
Attraction
Dipole-Dipole attractions
Hydrogen bonding
London dispersion forces
Ion-dipole attractions
Ion-induced dipole attractions
Occurs between molecules that have
permanent dipoles (i.e. polar
molecules)
Occurs when molecules contain N-H,
O-H or F-H bonds
All atoms, molecules, and ions
experience this type of attraction.
They are present in all substances
(and are the only type of force in
non-polar molecules).
Occurs when ions interact with polar
molecules
Occurs when an ion creates a dipole
in a neighbouring particle, which may
be another molecule or ion.
Relative Strength to a
Covalent Bond
1-5%
5-10%
Depends on the sizes and
shapes of molecules. For large
molecules, the cumulative
effect of many weak
attractions leads to a stronger
net attraction.
≈ 10% depending on the ion
charge and polarity of the
molecule
Variable, depends on the
charge on the ion and the
polarizability of its neighbour
Physical Properties Affected by Intermolecular Forces
-
-
Viscosity: is the resistance of a liquid to flow. Viscosity increases with stronger
intermolecular forces, and decreases with increasing temperature.
Surface Tension: Water will bead up on a waxed surface. This is due to an “imbalance”
of intermolecular forces at the surface of the liquid. In the interior of the liquid, the
molecules are surrounded by other, similar molecules, but at the surface they are only
attracted to the sides and inward. This works to reduce the surface area, attempting to
create the shape with the minimal surface area, a sphere. The surface tension is the
amount of energy required to stretch or increase the surface area. Molecules with high
IMF also have high surface tensions
Vapor Pressure: At any temperature, some molecules in a liquid have enough energy
to escape (become a gas/vapour).
As the temperature rises, the
fraction of molecules that have
enough energy to escape
increases.
Chemistry 11AP Notes Fall 2013 Ms Butler
As more molecules escape the liquid, the
pressure they exert increases. The liquid and the
vapour reach a state of dynamic equilibrium:
liquid molecules evaporate and vapour molecules
condense at the same rate.
•
The boiling point of a liquid is the
temperature at which its vapor
pressure equals atmospheric
pressure.
•
The normal boiling point is the
temperature at which its vapor
pressure is 760 torr.
•
Vapor Pressure is not affected by
surface area.
•
If the intermolecular forces are strong,
the vapor pressure will be relatively
low. If the forces are low, the vapor
pressure will be relatively high.
ethyl ether (C4H10O)
Pvapor (25oC) = 520 torr
ethyl alcohol (C2H6O)
Pvapor (25oC) = 75 torr
The relatively weak dipole-dipole forces and
London dispersion forces between molecules
results in a much higher vapor pressure compared
to ethyl alcohol.
Although dipole-dipole forces and London
dispersion forces also exist between ethyl alcohol
molecules, the strong hydrogen bonding
interactions are responsible for the much lower
vapor pressure compared to ethyl ether.
-
Compressibility: is a measure of a substance’s ability to be forced into a smaller
volume. In a liquid and solid, there is very little space in which to crowd other molecules.