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Lecture

CHEM 1AA3 Lecture Notes - Intermolecular Force, Van Der Waals Force, Hydrogen Bond


Department
Chemistry
Course Code
CHEM 1AA3
Professor
Pippa Lock

Page:
of 11
Chapter 12: Intermolecular Forces: Liquids and Solids
Intermolecular Forces
Van der Waals Forces
o Several different types of intermolecular forces
Instantaneous and Induced Dipoles
o A possibility is that at some particular instance, electrons are concentrated in
one region of an atom or molecule
o This displacement of electrons causes a normally non-polar species to become
momentarily polar
o An instantaneous dipole is formed (molecule has an instantaneous dipole
moment)
o After this, electrons in a neighbouring atom or molecule may be displaced to also
produce a dipole
o This is the process of induction and a newly formed dipole is called an induced
dipole
o Commonly used names for this intermolecular force of attraction are dispersion
forces and London forces
o Polarizability is the term used to describe the relative tendency for a charge
distribution to distort from its normal shape in an atom or molecule
o The greater the tendency, the more polarizable an atom or molecule is said to be
o Polarizability increases with atomic or molecular size, which is defined by the
volume of the electron cloud around a substance
o Also, in large molecules, some electrons, being farther from atomic nuclei, are
less firmly held
o These electrons are more easily displaced and the polarizability of the molecule
increases
o Because dispersion forces become stronger (more attractive), as polarizability
increases and because polarizability generally increases with molecular mass, the
melting points and boiling points of molecular substances generally increase with
increasing molecular mass
o The strength if dispersion forces also depends on molecular shape
o Electrons in elongated molecules are more easily displaced than are those in
small, compact, symmetrical molecules (elongated molecules more polarizable)
Dipole-Dipole Interactions
o In a polar substance, molecules have permanent dipole moments so the
molecules tend to line up with the positive end of one dipole directed toward
the negative ends of neighbouring dipoles
o This additional partial ordering of molecules can cause a substance to persist as a
solid or liquid at temperatures higher than otherwise expected
o Considering N2, O2 and NO, if we only looked at dispersion forces, we would
expect the BP of NO to be between N2 and O2
The BP of NO is higher than both because of its additional permanent
dipole
Hydrogen Bonding
o The boiling points of NH3, H2O and HF are as high or higher than those of any
other hydride ion their group, not lowest as might be expected
o A special type of intermolecular force causes this behaviour and using HF, we see
that
The alignment of HF dipoles places an H atom between two F atoms
Because of the very small size of the H atom, the dipoles come
close together and produce strong dipole-dipole attractions
Although an H atom is covalently bonded to one F atom, it is also weakly
bonded to the F atom of a nearby HF molecule
This occurs through a lone pair of electrons on the F atom
Each H atom acts as a bridge between two F atoms
The bond angle between two F atoms bridged by an H atom is about 180o
o The type of intermolecular force just described is called a hydrogen bond
o In a hydrogen bond, an H atom is covalently bonded to a highly electronegative
atom, which attracts electron density away from the H nucleus
o This in turn allows the H nucleus, a proton, to be simultaneously attracted to a
lone pair of electrons on a highly electronegative atom in a neighbouring
molecule
o Hydrogen bonds are possible only with certain hydrogen-containing compounds
because all atoms other than H have inner-shell electrons to shield their nuclei
from attraction by lone-pair electrons of nearby atoms
o Only F, O and N meet the requirements for hydrogen-bond formation
o Weak hydrogen bonding is occasionally encountered between an H atom of one
molecule and a Cl or S atom in a neighbouring molecule
o Compared with other intermolecular forces, hydrogen bonds are relatively
strong
o By contrast, single covalent bonds (intramolecular bonds) are much stronger still
Hydrogen bonding in water
o The most common substance in which hydrogen bonding occurs
o One water molecule is held to four neighbours in a tetrahedral arrangement by
hydrogen bonds
o In ice, hydrogen bonds hold the water molecules in a rigid, but open structure
o As ice melts, only a fraction of the hydrogen bonds are broken
o The open structure of ice gives it a low density
o When ice melts, some of the H-bonds are broken
o This allows water molecules to be more compactly arranged, accounting for the
increase in density when ice melts
o As liquid water is heated above the melting point, hydrogen bonds continue to
break
o The molecules become even more closely packed and the density of liquid water
continues to increase
o Liquid water attains its maximum density at 3.98oC, above this temperature, the
water behaves in a normal fashion, density increases as temperature increases
o The unusual freezing-point behaviour of water explains why a freshwater lake
freezes from the top down
o When water temperature falls below 4oC, the denser water sinks to the bottom
of the lake and the colder surface water freezes
o The ice over the top of the lake then tends to insulate the water below from
further heat loos
o This allows fish to survive in the winter in a lake that has been frozen over
Other properties affected by hydrogen bonding
o In acetic acid, pairs of molecules tend to join together to form dimers (double
molecules), both in liquid and in vapour states
o Not all hydrogen bonds are disrupted when liquid acetic acid vaporizes and as a
result, the heat of vaporization is abnormally low
o Certain trends in viscosity can also be explained by hydrogen bonding
In alcohols, the H atom in a OH group in one molecule can form a
hydrogen bond to the O atom in a neighbouring alcohol molecule
An alcohol molecule with two OH groups (a diol) has more possibilities
for hydrogen bond formation than a comparable alcohol with a single
OH group
Having stronger intermolecular forces, the diol is more viscous than the
simple alcohol
When still more OH groups are present (polyols), we expect a further
increase in viscosity
Intermolecular and Intramolecular Hydrogen Bonding
o Examples of hydrogen bonding presented to this point have involved an
intermolecular force between two molecules and this is called an
intermolecular hydrogen bond
o Another possibility occurs in molecules with an H atom covalently bonded to one
highly electronegative atom and with another highly electronegative atom in the
same molecule
o This type of hydrogen bonding within the molecule is called intramolecular
hydrogen bonding
Hydrogen bonding in living matter
o Some chemical reactions in living matter involve complex structures such as
proteins and DNA and in these reactions certain bonds must be easily broken
and re-formed
o Hydrogen bonding is the only type of bonding with energies of the right
magnitude to allow this
Ionic Interactions