BOND POLARITY AND
INTERMOLECULAR FORCES
OF ATTRACTION
Review
• We know how to draw Lewis structures for simple
molecules and polyatomic ions.
• We also know how to predict the 3-D geometry of
these molecules and ions, if we apply the VSEPR
Theory.
• Electronegativity (EN) is an atom's tendency to attract
electrons in chemical bonds.
• EN increases to the right and up on the periodic table,
excluding the noble gases.
BOND POLARITY
• When two nonmetal atoms bond, they
share electrons.
• They may or may not share electrons evenly.
• Consider the following molecules:
• F2
• HF
BOND POLARITY
• H vs. F
• The EN of H is 2.2.
• The EN of F is 4.0.
H
F
EN = 2.2
EN = 4.0
• In F2, both atoms pull with equal strength on
the bonding e-.
• The e- are shared evenly between them.
• In HF, the F atom pulls harder than the H atom.
• The e- are drawn more toward the F atom.
F
F
F
H
BOND POLARITY
F
F
• EN is same for both
atoms.
• e- density is spread
evenly around molecule.
• Highest e- density occurs
between atoms.
• Bond is nonpolar.
F H
• EN is much higher for F
than for H.
• e- density is drawn
toward F side.
• F atom acquires partial
negative charge.
• H atom acquires partial
positive charge.
• Bond is polar.
BOND POLARITY
•In general, a covalent bond is:
• polar if it occurs between two different
atoms.
• nonpolar if it occurs between two
identical atoms.
MOLECULAR POLARITY
• Dipole Moment - a measure of the polarity of a bond.
• Is often represented by a special arrow.
H
F
Arrow points toward
more EN atom.
MOLECULAR POLARITY
Polarity of Diatomic Molecules
• Diatomic Molecules - molecules made of only
two atoms.
• If atoms are the same, molecule is nonpolar.
• If atoms are diff., molecule is polar.
• NOTE: Polar does not mean charged.
• Is Cl2 polar or nonpolar?
• Is CO polar or nonpolar?
MOLECULAR POLARITY
Molecules With 3 or More Atoms
• A molecule with 3 or more atoms is:
• Polar if its central atom has lone pairs OR
• If the outer atoms are not all the same.
• Nonpolar if its central atom has no lone
pairs AND
• All the outer atoms are identical.
MOLECULAR POLARITY
CO2 vs. H2O
• Consider the Lewis structure of CO2:
This molecule is nonpolar.
MOLECULAR POLARITY
CO2 vs. H2O
• Consider the Lewis structure of H2O:
This molecule is polar.
MOLECULAR POLARITY
CH4 vs. CH3Cl
• Neither CH4 nor CH3Cl has any lone pairs on the
central carbon atom.
• Is CH4 polar or nonpolar?
• Is CH3Cl polar or nonpolar?
H
HCH
H
H
H C Cl
H
INTERMOLECULAR FORCES OF ATTRACTION
Molecular Comparison of Matter based on State
© Brown, T. et..al. 2012. Chemistry: The Central Science. 12th ed. USA: Pearson Prentice Hall. Pg. 426.
INTERMOLECULAR FORCES OF ATTRACTION
Intramolecular Forces
Attractive forces within
a molecule
vs
Intermolecular Forces
Attractive forces
between molecules
Intramolecular
Intermolecular
Affects molecular
Influence physical
shape, bond energies
properties of
and
chemical behavior
condensed phases
Stronger
INTERMOLECULAR FORCES OF ATTRACTION
Intermolecular Forces
Responsible for the bulk properties of matter
Weaker then intramolecular forces.
To break: 930 kJ
To break: 41 kJ
INTERMOLECULAR FORCES OF ATTRACTION
INTERMOLECULAR FORCES OF
ATTRACTION (IMFA)
LONDON
DISPERSION
DIPOLE-DIPOLE
HYDROGEN BONDING
ION DIPOLE
INTERMOLECULAR FORCES OF ATTRACTION
LONDON
DISPERSION
© Duke Physics
Fritz London
motion of electrons in an atom or
molecule can create an
instantaneous, or momentary, dipole
moment.
INTERMOLECULAR FORCES OF ATTRACTION
LONDON
DISPERSION
instantaneous attraction is called dispersion force;
significant only when molecules are very close
together
Strength depends on:
Polarizability
Molecular shape
INTERMOLECULAR FORCES OF ATTRACTION
LONDON
DISPERSION
Polarizability
ease with which e-’s can move in response to an
external charge
greater the polarizability, the more easily the electron
cloud can be distorted, larger the dispersion force
Large atoms with large electron clouds tend to have
stronger dispersion forces
Dispersion forces increase in strength with increasing
molecular weight and size.
INTERMOLECULAR FORCES OF ATTRACTION
LONDON
DISPERSION
C5H12, n-pentane
BP =309.4 K
Molecular shape
C5H12, neopentane
BP = 282.7 K
The greater the possible contact between non-polar molecules, the stronger
the LDF.
INTERMOLECULAR FORCES OF ATTRACTION
DIPOLE-DIPOLE
permanent dipole moment in polar molecules
Electrostatic interaction between δ+ end of one
molecule and δ- end of one molecule.
δ- δ+
δ- δ+
δ+ δ
-
δ+ δ-
INTERMOLECULAR FORCES OF ATTRACTION
DIPOLE-DIPOLE
C2H3N, acetonitrile
MM = 41.054 g/mol
MP = 355 K
C3H8, propane
MM = 44.094 g/mol
MP = 231 K
Acetonitrile is polar; propane is non-polar.
Images from: Wikimedia Commons
INTERMOLECULAR FORCES OF ATTRACTION
DIPOLE-DIPOLE
In solid state
In liquid state
overall effect is a net attraction strong enough to keep
the molecules from moving apart to form a gas
Image from: Wikimedia Commons
INTERMOLECULAR FORCES OF ATTRACTION
DIPOLE-DIPOLE
C3H8, propane
C2H6O, dimethyl ether
C3HO, acetaldehyde
C2H3N, acetonitrile
MW = 44 amu
MW = 46 amu
MW = 44 amu
MW = 41 amu
BP = 231 K
BP = 248 K
BP = 294 K
BP = 355 K
Increasing polarity
Increasing strength of dipole-dipole forces
For molecules of approximately equal mass and size, the
strength of IMFA increases with increasing polarity.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
Figure 3.1.1 Boiling points
of covalent hydrides of the
elements as a function of
molecular weight.
© Brown, T. et..al. 2012. Chemistry: The Central Science. 12th ed. USA: Pearson Prentice Hall. Pg. 431.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
Recall: Down a group, MM increases, polarizability
increases, LDF increases.
The BPs of the compounds containing group 4A
element increase down a group.
Groups 5A, 6A, and 7A follow the same trend except
for NH3, HF, and H2O which have high boiling points
than expected.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
Special type of dipole-dipole interaction present
between a hydrogen atom in a polar bond (H-F, H-O, HN) and non-bonding electron pair on a nearby small
electronegative ion (usually F, O, N) in another
molecule.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
H-bonds are much weaker than covalent bonds, but is
generally stronger than dipole-dipole or dispersion
forces.
H-bonds help stabilize the structures of proteins and
are responsible for the way the DNA is able to carry
genetic information.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
Figure 3.1.2 Effect of hydrogen bonding in the density of water. (Left – ice
in water; right – solid benzene in liquid benzene)
© Chang, Raymond. 2008. General Chemistry: The Essential Concepts. 5th ed. New York: McGraw-Hill Companies.
INTERMOLECULAR FORCES OF ATTRACTION
HYDROGEN BONDING
Figure 3.1.3 Three dimensional structure of ice.
© Chang, Raymond. 2008. General Chemistry: The Essential Concepts. 5th ed. New York: McGrawHill Companies. Pg 400
INTERMOLECULAR FORCES OF ATTRACTION
ION DIPOLE
Attraction between an ion and a dipole (polar
molecule)
Strength depends on:
+ ion
Charge
and
size
of
δ- δ+
Magnitude of dipole moment
Size of molecule
INTERMOLECULAR FORCES OF ATTRACTION
ION DIPOLE
δ+
δ-
δ+ δ-δ- δ+
Effect of the charge and size of ion
Na+
F
Mg2+
Stronger attraction – charge
in cations more
concentrated
Stronger attraction – smaller;
can interact more with the
polar molecule
INTERMOLECULAR FORCES OF ATTRACTION
ION DIPOLE
δ+
δ-
Na+
δ+
δ-
Ion dipole increases as
the ionic charge increases.
the magnitude of the dipole increases.
Mg2+
INTERMOLECULAR FORCES OF ATTRACTION
Comparing IMFAs
INTERACTING
MOLECULES
ARE POLAR
MOLECULES
INVOLVED?
NO
ARE IONS
INVOLVED?
YES
ARE POLAR
MOLECULES AND
IONS BOTH
INVOLVED?
YES
NO
ARE H ATOMS BONDED
TO N, O, OR F ATOMS?
NO
DISPERSION
FORCES ONLY
DIPOLEDIPOLE
FORCES
YES
NO
YES
HYDROGEN
BONDING
ION-DIPOLE
FORCES
IONIC
BONDING
INTERMOLECULAR FORCES OF ATTRACTION
Comparing IMFAs
Dispersion forces are found in all substances.
The strongest IMFA is hydrogen bonding.
The effects of these attractions are additive.
INTERMOLECULAR FORCES OF ATTRACTION
Generalizations
If MW and shape are comparable: LDF approximately
equal. Difference is due to differences in dipoledipole. IMFA gets stronger as molecule polarity
increases, with those capable of H-bonding having
the strongest interactions.
If the molecules differ widely in MW: Consider LDF. The
higher the MW, the stronger the IMFA.
SAMPLE PROBLEMS
SP3.1.1. The dipole moments of acetonitrile, CH3CN,
and methyl iodide, CH3I, are 3.9 D and 1.62 D,
respectively. a) Which has a greater dipole-dipole
attraction? b) Which has a greater LDF? c) The boiling
points of CH3CN and CH3I are 354..8 K and 315.6 K,
respectively. Which has greater overall attractive
forces?
Answer: a. CH3CN, b. CH3I, c. CH3CN
SAMPLE PROBLEMS
SP3.1.2. List the following substances in
order of increasing boiling point: BaCl2,
H2, CO, and HF.
Answer: H2 < CO < HF < BaCl2
SAMPLE PROBLEMS
SP3.1.3. Which member of the following pairs
stronger IMFA?
a) H2O or H2S
b) CO2 or CO
c) SiH4 or GeH4
Answer: a. H2O
b. CO2
c. GeH4