TOPIC: 3.
1 INTERMOLECULAR FORCES
ENDURING UNDERSTANDING:
SAP-5 Intermolecular forces can explain the physical properties of a material.
LEARNING OBJECTIVE:
SAP-5.A Explain the relationship between the chemical structures of molecules and the relative strength of their
intermolecular forces when:
a. The molecules are of the same chemical species.
b. The molecules are of two different chemical species
ESSENTIAL KNOWLEDGE:
SAP-5.A.1 London dispersion forces are a result of the Coulombic interactions between temporary, fluctuating dipoles.
London dispersion forces are often the strongest net intermolecular force between large molecules.
a. Dispersion forces increase with increasing contact area between molecules and with increasing
polarizability of the molecules.
b. The polarizability of a molecule increases with an increasing number of electrons in the molecule; and the
size of the electron cloud. It is enhanced by the presence of pi bonding.
c The e m London di e ion fo ce ho ld no be ed non mo l i h he e m an de Waal
fo ce
SAP-5.A.2 The dipole moment of a polar molecule leads to additional interactions with other chemical species.
a. Dipole-induced dipole interactions are present between a polar and nonpolar molecule. These forces are
always attractive. The strength of these forces increases with the magnitude of the dipole of the polar
molecule and with the polarizability of the nonpolar molecule.
b. Dipole-dipole interactions are present between polar molecules. The interaction strength depends on the
magnitudes of the dipoles and their relative orientation. Interactions between polar molecules are typically
greater than those between nonpolar molecules of comparable size because these interactions act in addition
to London dispersion forces.
c. Ion-dipole forces of attraction are present between ions and polar molecules. These tend to be stronger than
dipole-dipole forces.
SAP-5.A.3 The relative strength and orientation dependence of dipole-dipole and ion-dipole forces can be understood
qualitatively by considering the sign of the partial charges responsible for the molecular dipole moment, and
how these partial charges interact with an ion or with an adjacent dipole.
SAP-5.A.4 Hydrogen bonding is a strong type of intermolecular interaction that exists when hydrogen atoms covalently
bonded to the highly electronegative atoms (N, O, and F) are attracted to the negative end of a dipole formed
by the electronegative atom (N, O, and F) in a different molecule, or a different part of the same molecule.
SAP-5.A.5 In large biomolecules, noncovalent interactions may occur between different molecules or between different
regions of the same large biomolecule.
EQUATION(S):
N/A
NOTES:
Intermolecular forces, IMF, are the attractions BETWEEN
atoms, ions or molecules that hold them together. There
are six different types of IMF. They differ in the
arrangement of the electrons. Intermolecular forces
explain why carbon dioxide, CO2, is a gas, water, H2O is a
liquid and silicon dioxide, SiO2, is a solid, even though they
are all only made from three atoms.
To determine which type of IMF a substance will exhibit
you need to look at the types of atoms that are present in
the sample. Start by identifying the types of atoms in the
sample as either a metal or a nonmetal.
Weakest to strongest (general trend) All strong, difficult to compare The strongest
(These are collectively called van der Waal fo ce with each other
Dipole-Dipole
London Hydrogen Ionic Metallic Covalent Network
dispersion bonding
(A strong type of
dipole-dipole)
All Polar Occurs in Metals with Only metal Specifically these:
substances molecules polar nonmetals. atoms C(diamond)
The only Asymmetrical molecules C(graphite)
force in molecules that have C60
nonpolar with only hydrogen SiO2
molecules non-metals directly SiC
or single bonded to
nonmetal N, O or F.
atoms.
Ne, H2, CO2 HCl, CH2O, HF, H2O, NaCl, KBr, Au, Al,
CH3Cl CH3OH MgO ZnCu
NON-METALS ONLY
LONDON DISPERSION FORCES (LDF)
London dispersion forces occur in all substances so they should always be considered. They are the only type of
IMF present in nonpolar molecules.
London dispersion forces are caused by the motion of electrons within an atom or molecule. As the electrons move
around with the atom there is a chance that they are unequally distributed. When this happens it causes the atom
or molecule to have a temporary dipole due to there being an excess of negative charge (partially negative, -)
in one place and an area that is deficient in electrons and therefore is more positive (partially positive, +). The
temporary dipole in one atom or molecule causes the neighboring atoms/molecules to change; the negative end of
the temporary dipole repels electrons while the positive end of the dipole attracts electrons. When the
neighboring species electrons move this forms an induced dipole which is electrostatically attracted to the first
atom/molecule. This is usually a short lived occurrence and will return to being evenly distributed, which is why
LDF is a weak force.
Far apart (not polarized)
http://www.chem.ucla.edu/~harding/IGOC/L/london_force.html
The ability of an atom to form a temporary or induced dipole is known as its polarizability.
London dispersion forces are stronger when an atom or molecule has more electrons. This is shown in the
halogens. Moving down the periodic table, F2, Cl2, Br2, I2, they change in state from being gases to liquid to solid.
The increase in LDF due to the increase in electrons is the explanation for this phenomenon. Large molecules will
have stronger LDF because the increase in electrons increases their polarizability.
Another factor that increases the strength of the LDF is the shape of the molecule. Long, thin molecules have more
surface area and are more polarizable while short, round molecules have less surface area and are less polarizable.
The more polarizable the molecule, the stronger the LDF will be.
It is also enhanced by pi bonding, the presence of double or triple bonds.
DIPOLE-DIPOLE
Dipole-dipole interactions occur between molecules that have a permanent dipole due to them being polar
molecules. Molecules are polar when their electron distribution is asymmetrical. It is always a good idea to sketch
the molecule to determine if the electrons are symmetrical or not.
Dipole-dipole interactions are similar to London Dispersion Forces, except that the dipoles are permanent and so
the attractions are generally greater. The partially positive end of one molecule is attracted to the partially negative
end of a different molecule, and so on throughout the system. Dipoles can be measured by the dipole moment of
the molecule.
https://www.emedicalprep.com/study-material/chemistry/states-of-matter/intermolecular-forces/
Dipole-dipole interactions are stronger when the substance involved is more polar, meaning that their electrons
are more unequally distributed. Be sure to consider the electronegativity values of the elements involved.
HYDROGEN BONDING
Hydrogen bonding is a confusing name for this type of intermolecular force, because it is not a bond, it is an
attraction. It occurs in molecules that contain H-N, H-O or H-F bonds, but the attraction isn t the bond itself, rather
the relatively large difference in electronegativity between the N, O or F and the rest of the molecule causes the
molecule to have a large dipole. This dipole causes a strong attraction to the dipole on the molecules surrounding
it.
https://dashboard.dublinschools.net/lessons/?id=80c6b421d856dd18c9548981e367d9aa&v=3
Hydrogen-bonding IMF is strongest for H-F > H-O > H-N, because of the differences in electronegativity, and is
stronger when there are multiple sites for hydrogen bonding.
NAMING INTERACTIONS
Substance A Substance B Name of Notes
interaction
Nonpolar Nonpolar London dispersion The strength increases with the polarizability of the
only nonpolar molecules.
Polar Polar Dipole-dipole The strength depends on the magnitudes of the dipoles and
(and LDF) their relative orientation. Interactions between polar
molecules are typically greater than those between nonpolar
molecules of comparable size because these interactions act
in addition to London dispersion forces.
Polar with Polar with Hydrogen-bonding The strength of this attraction is typically greater than
Hydrogen- Hydrogen- (and dipole-dipole dipole-dipole.
bonding bonding and LDF)
capability capability
Nonpolar Polar Dipole-Induced The strength of these forces increases with the magnitude of
Dipole the dipole of the polar molecule and with the polarizability
(and LDF) of the nonpolar molecule.
Nonpolar Ion Ion-Induced dipole The strength increases with larger charged ions, smaller ion
(and LDF) size and more polarizable molecules.
Polar Ion Ion-Dipole These tend to be stronger than dipole-dipole forces. The
(and LDF) strength increases with larger charged ions, smaller ion size
and more polar molecules.
LARGE BIOMOLECULES
In large biological molecules the covalent bonds AND
intermolecular and intramolecular forces can determine their
shapes. For example, in DNA, the base pairs GC and AT link
together via hydrogen bonds, shown by the dotted lines in the
diagram. This phenomenon occurs in many biological molecules.
http://www1.lsbu.ac.uk/water/nucleic_acid_hydration.html
NOT TECHNICALLY IMF BUT THEY DESERVE A MENTION HERE!
IONIC
Ionic interactions occur between metal and nonmetal atoms when they lose or gain electrons to form ions.
This type of attraction is a coulombic or electrostatic attraction.
The ionic bonds are stronger when the charges are larger and the ions are smaller, this is explained by
Coulomb La Electrostatic Force (attraction) ∝ (Charge 1) (Charge 2)
(Distance)2
METALLIC
Metallic bonding occurs between metal atoms. It can be for just one type of metal, a pure substance, or for
different types of metal, a mixture called an alloy.
The metallic attractions are due to multiple metallic cations being attracted to a delocalized sea of valence
electrons.
The IMF is stronger when there are smaller metallic cations and when there are more valence elections.
COVALENT NETWORK
Covalent network occurs between carbon atoms, silicon atoms, and carbon with oxygen or silicon, and silicon
with oxygen.
The attraction due to multiple covalent bonds forming a large macromolecule.
The IMF is stronger when there are more covalent bonds formed.
THIS IS NOT PART OF 3.1 BUT IT IS USEFUL TO KEEP IT TOGETHER
Properties as a result of IMF Effect of increasing IMF
Boiling/Melting/Freezing etc. Point Increases
Enthalpy of fusion/vaporization etc. Increases
Vapor Pressure Decreases
Viscosity Increase
Surface Tension Increases
Solubility Decreases
(This depends on the polarity of the solvent and solute)
Lattice Energy Increases
I DO:
Pentane, C5H12 and Octane C8H18, are both hydrocarbons.
a) Identify the type of intermolecular forces present.
Both molecules would exhibit London dispersionforces LDF
b) Predict the substance
only
Bothare madeupfrom c H and c c which havenegligable electronegativitydifferences
that would have a higher vapor pressure and justify your claim.
CsHr would have higher vapor pressure CsHr is less
polarizable than CSH as it has is fewer electrons meaning
that the IMF is weaker Weaker IMF means that it vaporizes more
WE DO:
The boiling point of HF is 293K whereas the boiling point of F2 is 85K.
a) Determine the type(s) of intermolecular forces present.
b) Explain the difference in boiling points based on their intermolecular forces.
YOU DO:
1) In each pair decide which would have a lower boiling point. Explain why. LowerBP weaker
both are Df
fewer electrons only
a) He or Ne
i less polarizable Imf
LDF
weaker
b) CH4 or CCl4
0 same as
CASH is LDF and dipole
D Cfc is LDFonly
c) CF or CF H
4 3
LDF is weaker than d d
d) NH3 or PH3 generally
ride
is LDF dipole dipole NHz can
PHz polar
bond dipole dialed LDF Since HBondingis
hydrogen
stronger NHz has a higher b p
2) 1-Butanol and diethyl ether are structural isomers, meaning they have the same formula but the atoms are
arranged differently.
Butanol Diethyl ether
Boiling Point : 117°C Boiling Point 34.5°C
https://commons.wikimedia.org/wiki/File:Buta https://en.m.wikibooks.org/wiki/File:Diethyl-ether-
nol_flat_structure.png 2D-flat.png
a) Identify the intermolecular forces present in each.
Butanol H Bonding dipole dipole LDF
b) Justify the difference in their boiling points.
Diethyl ether dipole dipole4
f
H Bonding is a strongerattractionbetween molecules
it requires more
energy to
3) Place the following in order of increasing boiling point.
overcome i 913P
C3H8, He, CH3OH, HOCH2OH, HCl
He C 3h8 HCl CHzOH HO CHIH
4) Which of the following would have the strongest ion-dipole interaction with water? Explain.
Ion Ionic Radius
Tl3+
Cd2+
95 pm
97 pm 9 Charge f coolombic attraction
Na+ 95 pm
5) Circle the molecules that are capable of hydrogen-bonding with another of the same molecule?
11 10 10
a) H2S b) HCl c) C5H12 d) CH3OH e) NCl3 f) CH3NH2
For all of the molecules that exhibit hydrogen bonding, draw the two molecules and use a dotted line to
indicate the hydrogen bond.
8
H
to
g g
H
in
H H
6) Pentane has 3 isomers:
Use the structures to place them in order of
increasing intermolecular forces.
7) Use intermolecular forces to explain why the vapor pressure of CH4 is greater than that of NH3. (both at
room temperature)
8) The amount of energy needed to melt a substance at its melting point is called the heat of fusion and for
water the value is 6.01 kJ/mol. The amount of energy needed to vaporize a substance at its boiling point is
called the heat of vaporization and for water the value is 40.7 kJ/mol. Use the relative distances in the
states of matter and the concepts intermolecular forces to explain the difference.
