Chemical Bonding Unit 2
Section 6: Intermolecular Forces
All intermolecular forces depend on the attractions
Intramolecular vs. Intermolecular Forces
Intramolecular Forces are bonds.
Intermolecular Forces are forces of attraction
between molecules.
O
H H
O H
Intermolecular forces are much weaker than intramolecular forces.
e.g.) The intramolecular forces (Bonds) within HCl molecules are 96% stronger than the
intermolecular forces between neighboring HCl molecules.
Intermolecular forces are, however, responsible for many of the physical characteristics of a
Ion-Dipole
The forces of attraction between an ion and a polar molecule. + + + + + -+ -+ -+ -Positive Ion Negative Ion
Negative Side of
This is the strongest type of intermolecular force.
It occurs when a soluble ionic compound is mixed in a polar liquid such as water.
The ions break apart, as they have a higher
attraction for the water molecules than they do for one another.
The negative ends of a polar molecule, such as water, will line up with the positive ions to form ion-dipole intermolecular forces.
Ions and polar molecules stick together.
The ions drag the water molecules around with them in solution.
Na+
(aq) and Cl-(aq)
𝞭
-Na+
𝞭
-Cl
-𝜹+
Here we can see that the negative ends of the water molecules form ion-dipole
attractions with the positive ions (Na+), and
the positive ends of the water molecules form ion-dipole attractions with the
Dipole - Dipole
The attraction forces between the negative end of one polar molecule and the positive end of another
polar molecule.
-This is the third strongest type of intermolecular force.
It occurs between polar molecules.
• The positive end of one molecule lines up
with the negative end of another molecule.
• These forces are relatively weak, as the
attractions are between partial charges.
• Collisions between molecules cause
Hydrogen Bonds: A type of Dipole - Dipole
Occurs between a Hydrogen that is covalently bonded to Fluorine, Oxygen, or Nitrogen and another F, O, or N with at least one lone pair
A1 - H .………….. : A2
-F, O, or N
This is the second strongest type of intermolecular force.
e.g.) A1 could be oxygen and A2 could be
Hydrogen Bonds
Why are H-Bonds so strong?
• F - H, O - H, and N - H bonds are very polar.
• These atoms are very small, so the partial charges
caused by the difference in electronegativity is highly concentrated.
• The lone pair(s) on F, O, or N increases the already
partially negative charge on these atoms, thereby
creating a stronger attraction for the slightly positive
1) Because of extreme polarity of these bonds, the
atoms carry a relatively large amount of positive or negative charge.
2) The additional electron density around N, O, or F is very concentrated, as it is distributed over a small volume. In hydrogen, the reduced electron density increases the exposure of its proton.
3) The lone pairs make F, N, and O even more
negative, thereby increasing their attractive forces for the partially exposed proton of the hydrogen
Hydrogen Bonds Between Water Molecules
O
H H
…
…
..
.
O
The hydrogen is bonded to the oxygen in H2O.
• Hydrogen atoms form Hydrogen Bonds with
the oxygen atoms from other molecules, the oxygens have lone pairs.
• These strong bonds account for the
Hydrogen Bonds Between Water and Methanol
Both form
Alcohol dissolves in water. When you add alcohol to water the H-Bonds change.
Initially, all waters formed H-bonds with other waters and methanols formed H-bonds with other methanols.
After mixing, H-bonds form between the waters and the methanols.
Substances with like intermolecular forces are soluble in one another.
Hydrogen Bonds in Acetamide Forms two H-Bonds with
itself
C
C C C
Because this structure can form two
H-bonds with other like molecules, its boiling point is higher than that of water.
Ethanol does not form Hydrogen Bonds
This structure does not form H-bonds.
Hydrogen is bonded to Carbon.
Hydrogen needs to bond with F, O, or N to form an H-bond.
H - C = O
C
H
Here we are examining the boiling points trends for different molecules that form covalent bonds with hydrogen.
When moving down a group, the molar mass of
the overall molecule increases, since the molar mass of the central atom increases.
The boiling point increases slightly as mass
increases, because it takes more energy to push it into the gas phase.
Also, larger and more polarized as we will see shortly
The only exceptions to this trend, seen in Group 5 and Group 6, are water and ammonia.
Water and ammonia have much higher boiling points because they are the only molecules in this graph that forms
H-Bonds.
Ion Induced Dipole
Cation Non-polar
molecule
Positive Charge of ion attracts
An ion will induce a dipole in a non-polar species.
Electrons are attracted to the positive ion. This will cause a temporary increase in the
electron density on the side of the molecule that is facing the positive ion.
The extra electron density will only remain on the side of the molecule that points toward the ion for a very brief moment of time, as the
electrons continue to move.
This type of attractive force produces a series of small tugs.
● These small tugs sum up to an overall net force of attraction.
● However, the attraction is small, as it is constantly being turned on and off.
This occurs during the absorption of oxygen gas in your blood.
● Fe2+ (from hemoglobin) induces a dipole in the
This is similar to the ion-induced-dipole, except that it is a polar
molecule that is inducing the dipole in a non-polar molecule.
As polar molecules only have partial
These attractions only last for a brief moment, as the electrons continue to move.
Thus, the increase in electron density on one side of the molecule is temporary but constantly recurring.
This is the type of force involved
when oxygen gas and nitrogen gas (or other non-polar gases) dissolve in
London Dispersion Forces
These forces exist between all species: atoms, ions, non-polar and polar molecules.
Contribute the the overall force of attraction between all particles.
London Dispersion Forces are the only
The previous two examples showed how ions and polar species can polarize otherwise non-polar
species.
London dispersion forces are the only forces that cause non-polar species to attract each other.
London Dispersion Forces
+
-+ - + - + - +
-+
- - + - + - + - +
Time = 0.1 sec
Atom, molecules, or ions that are in close
proximity to one another will induce dipoles on one another.
This causes them to synchronize the movements of their electron densities with one another.
These are the only forces that keep oils in the liquid state.
They also make a fairly major contribution to the forces that keep other types of species together.
This trend is due to the fact that species with larger molar masses have more
electrons.
Larger species with more electrons are more polarizable
They have a weaker hold on their outer electrons.
When moving down a group, or constructing molecules with more atoms, the resulting species has more
electrons and is larger
The more electrons a species has, the more polarizable it is.
This is not always true when moving from left to right across a period, as atomic radius
Atomic radius decreases when moving from left to right across a period.
This tends to decrease the polarizability of the element, as the nucleus has a greater force of
attraction on the electrons.
However, due to the increase in molar mass, it often takes more energy to push the elements into the gas phase.
When moving down a group or building larger molecules, the size of the species increases and more electrons are added.
This is a general trend about the relative strength of intermolecular forces.
Very large molecules may have London dispersion forces that are greater than the dipole-dipole forces of molecules with weak polarity.
This sequence is, however, a general guide that will help you predict which species will
Ex1)Which species have the higher boiling point? Identify the intermolecular forces acting on each.
C2H6 or C3H8
Both are Non-Polar
The larger molecule has the higher BP, as larger molecules are more polarizable. Only London dispersion forces act on these
molecules
Ex2)Which species have the higher boiling point? Identify the intermolecular forces acting on each.
Polar Non-Polar
H-Bonds
London Dispersion London Dispersion
C3H8
BPC3H8 = - 42o C
BPCH3CH4OH = + 78o C
H - C - C - O - H H H
H H
:
Ex3)Which species have the higher boiling point? Identify the intermolecular forces acting on each.
Polar Polar
H-Bonds
London Dispersion
BPC3H8 O2= + 188o C
BPCH3OH = + 66o C
H - C - O - H H
H
H-Bonds x 2
London Dispersion Smaller/Less polarizable Larger/More polarizable
Ex4)Which species have the higher boiling point? Identify the intermolecular forces acting on each.
Polar Non-Polar
London Dispersion
MPCl4= + 171o C
MPCH4 = - 183o C
London Dispersion Smaller/Less polarizable Larger/More polarizable
Ex4)Which species have the higher boiling point? Identify the intermolecular forces acting on each.
Ionic Polar
Ionic Bonds
BPH2O= + 100o C
BPNaCl = + 1413o C
Hydrogen Bonds London Dispersion