Project on
Intermolecular Forces
Members: Liu Wing Chiu (19)
Siu Nga Chiu (24)
Intermolecular Forces
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The origin of intermolecular forces
The classification of intermolecular forces
Van der Waal’s force
Hydrogen bonding
Explore an example in depth to show the
significance of existence of intermolecular
forces.
The Origin of Intermolecular Forces
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It is weak electrostatic force of
attraction that exist an area of
negative charge on one molecule and
an area of positive charge on a
second molecule.
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What causes intermolecular forces?
Molecules are made up of charged
particles: nuclei and electrons. When
one molecule approaches another,
there is a multitude of interactions
between the particles in the two
molecules. Each electron in one
molecule is subject to forces from all
the electrons and the nuclei in the
other molecule.
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Intermolecular force is weak compared to covalent
bond. It is relatively weak interactions that occur
between molecules.
There are 2 types of intermolecular forces (both of
them are electrostatic attraction between dipoles
formed by uncharged molecules.)
1. Van der Waals' force
2. Hydrogen bonding
Van der waals’ force is formed by dipoles. There are
3 types of dipoles:
1. Permanent dipoles
2. Instantaneous dipoles
3. Induced dipoles
Permanent Dipole
These molecules have a permanent separation of positive and negative charge.
A simple example is HCl
+
-
The pair of electrons in the covalent bond between hydroge and chlorine is
unequally shared due to the difference in electronegativity between hydrogen and
chlorine. Chlorine has a greater electronegativity compared to hydrogen and hence
Chlorine tends to attract the bonded electron pair to itself. chlorine becomes slightly
negatively charged (-), hydroge atom has a partial positive charged (+) .The
unsymmetrical distributed charge on the HCl molecule produces a permanent
dipole.
Instantaneous Dipole
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Instantaneous dipole is due to the fluctuation of
electron clouds on non-polar molecules, positive and
negative charges exist temporarily.
Induced Dipole
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Induced dipole exist when a permanent dipole or
instantaneous dipole comes close to a non-polar
molecule, the non-polar molecule will be induced to
form a dipole temporarily.
The Classification of Intermolecular
Force
There are 2 major types of intermolecular force:
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Van der Waals’ force
It can be divided into three categories:
1.
Dipole-dipole Interactions
2.
Instantaneous dipole-induced dipole Interactions
3.
Dipole-induced dipole Interactions
Hydrogen bond
Classification diagram of intermolecular force
Van der Waals’ Forces
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It is an attractive force which exist between all
molecules.
It is the weakest of intermolecular force.
The force can be divided into three categories:
1 Dipole-dipole Interactions
2 Dipole-induced dipole Interactions
3 Instantaneous dipole-induced dipole
Interactions
Dipole-Dipole Interactions
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Dipole-dipole interactions exist between molecules which are
permanent dipole. They tend to orientate themselves that the
attractive forces between molecules are maximized while
repulsive forces are minimized.
In the illustration :
the H end of HCl is permanently slightly positive charge. The
Cl end of HCl has a permanent slight negative charge, the "H"
in one molecule is attracted to the "Cl" in a neighbor.
Instantaneous Dipole-Induced Dipole
Interactions
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Also known as London forces or Dispersion Forces
Instantaneous dipole-induced dipole Interactions exist in non-polar
molecules. These forces result from temporary charge imbalances.
The temporary charges exist because the electrons in a molecule
or ion move randomly in the structure. The nucleus of one atom
attracts electrons form the neighboring atom. At the same time, the
electrons in one particle repel the electrons in the neighbor and
create a short lived charge imbalance.
These temporary charges in one molecule or atom attract opposite
charges in nearby molecules or atoms. A local slight positive
charge + in one molecule will be attracted to a temporary slight negative charge in a neighboring molecule.
Note: dispersion forces operate in all molecules whether they are polar or non-polar.
Dipole-Induced Dipole Interactions
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Also known as induction force.
When a polar molecule approaches a nonpolar molecule, the
permanent dipole on the polar molecule can distort the electron
cloud of the nonpolar molecule, forming an induced dipole.
Van der Waals Radius VS Covalent
Radius
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Van der Waals radius is one half
of the distance between the
nuclei of two atoms in adjacent
molecules.
Covalent radius is one half of
the distance between two atoms
in the same molecules.
Van der Waals’ radius of a nonmetal is always larger than the
corresponding covalent radius
because the covalent radius
because covalent bond is much
stronger than van der Waals’
forces.
The Strength of the Van der Waals’
Forces
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The strength of the van der Waals' forces depends on
size of electron cloud (how easily the electron cloud is
distorted or polarized).
For all molecules, the more number of electron (or
weaker attraction force between nucleus and electrons),
causing the higher in polarizability. The degree of uneven
distribution of electron cloud is higher and the strengths
induction force and dispersion force become stronger.
Thus the stronger van der Waals’ forces.
The strength of van der Waals' forces is also related to
the surface area (or shape) of the molecule.
For molecules with similar relative molecular masses or
size, the higher contact surface area, the stronger van der
Waals’ forces.
Hydrogen Bonding
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Hydrogen bond is a electrostatic force of attraction
existing between polar hydrogen(+) and
electronegative atom(-) of dipoles.
The hydrogen bond is weaker than the covalent bond,
but relatively strong compared to van der Waals’ force.
Hydrogen bonding is a unique type of intermolecular
molecular attraction. There are two requirements.
1. The first is a covalent bond between a H atom and
either F, O, or N (These are the three most
electronegative elements.)
2. The second is an interaction of the H atom in this
kind of polar bond with a lone pair of electrons on a
nearby atom of F, O, or N.
The presence of hydrogen bonding has an important effect on
the properties of various substances:
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The melting and boiling points of the period two hydrides NH3, H2O and
HF are much higher than are expected if only dipole-dipole forces were
acting between the molecules.
The solubility of molecular substances in water is greatly influenced by
their ability to form H-bonds with water molecules.
Water has several unusual properties which are related to H-bonding:
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High melting and boiling points given
High surface tension.
Expansion on freezing due to the formation of a regular ‘open-cage’
network of H-bonded water molecules.
Liquids with hydrogen bonds between molecules usually have higher
viscosity than comparable liquids that don't.
H-bonding can influence acidity. H-bonded hydrogen atoms are often less
likely to dissociate as H+ ions.
H-bonding also plays important roles in:
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The folding of proteins.
The structure of DNA.
The manner in which hydrated crystals cleave.
Hydrogen Bonding in Alcohols
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An alcohol is an organic molecule
containing an -O-H group.
Any molecule which has a
hydrogen atom attached directly to
an oxygen or a nitrogen is capable
of hydrogen bonding. Such
molecules will always have higher
boiling points than similarly sized
molecules which don't have an -OH or an -N-H group. The hydrogen
bonding makes the molecules
"stickier", and more heat is
necessary to separate them.
Ethanol, CH3CH2-O-H, and
methoxymethane, CH3-O-CH3,
both have the same molecular
formula, C2H6O.
Solubility
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If there are strong solute-solvent interactions, the
solvent is soluble in the solute.
Most ammonium, nitrate and sulphate salts are
soluble in water since they form hydrogen bonds
with water molecules.
The high solubility of alkanols in water is cause by
the formation of hydrogen bond.
Carbohydrates have many –OH groups which can
form hydrogen bond with water. Therefore
carbohydrates with low relative molecular mass
are soluble in water.
Hydrogen Bonding in an Ice Crystal
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Ice has a lower density
than water as ice has
an open structure. In
ice, each molecule is
tetrahedral bonded to
other molecules by
hydrogen bond.
Hydrogen Bond in Water
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Many other unique properties of
water are due to the hydrogen
bonds. For example, ice floats
because hydrogen bonds hold
water molecules further apart in a
solid than in a liquid, where there
is one less hydrogen bond per
molecule. The unique physical
properties, including a high heat
of vaporization, strong surface
tension, high specific heat, and
nearly universal solvent
properties of water are also due
to hydrogen bonding.
Hydrogen Bonding in DNA
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Hydrogen bonds play an
important role in the ‘basepairing’ duplication of DNA
(A-T,C-G). Matching of the
bases produces an
accurate duplicate of the
original DNA chain.
Boiling Points of Some Hydrides
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The figure shows the normal
boiling point temperatures for
several related substances.
This boiling point diagram tells
us about the intermolecular
forces between a homologous
series of small hydrogen
containing molecules. Although
for the most part the trend is
that the boiling points increase
as going down the group. The
boiling point other hydride of
the first element in each group
is abnormally high. In the cases
of NH3, H2O and HF there are
hydrogen bond attraction,
requiring significantly more
heat energy to break.
Explore an example in depth to show
the significance of existence of
intermolecular forces
The Hardness of Calcium Sulfate
(CaSO4)
Gypsum (Hydrated CaSO4 ,
CaSO4·2H2O )
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In the gypsum crystal
structure, calciums are
coordinated by six oxygens
from sulphate, and by two
oxygens from water (H2O).
Two sheets of sulphates are
bound together by calciums
forming double sheet layers.
At each side of these layers
are water molecules, which
form weak hydrogen bonds
to the next layer in the
structure.
It is sectile and slightly
flexible
Anhydrite (Anhydrous CaSO4)
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Anhydrite has the same
composition as Gypsum, but
contains no water in its
structure. There are only
strong ionic bonds between
ions.
It is very hard and very
difficult to cleave
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Calcined gypsum has an unusual property: when
mixed with water at normal temperatures, it
recombines with the water that was driven off during
calcination, and sets to form a strong gypsum crystal
lattice:
CaSO4·½H2O + 1½H2O → CaSO4·2H2O
This reaction is exothermic.
The anhydrous form, called anhydrous calcium
sulfate (sometimes anhydrite), is produced by
further heating to above approximately 180°C (356°F)
and has the chemical formula CaSO4. Anhydrite
reacts slowly with water to return to the dihydrated
state.
Plaster of Paris (Calcium Sulfate Hemihydrate, CaSO4, ½ H2O)
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Plaster of Paris, or simply plaster, is a type of building
material based on calcium sulfate hemi-hydrate, nominally
(CaSO4)2. ½ H2O. It is created by heating gypsum to about
150°C.
(CaSO4, 2 H2O) + heat = (CaSO4, ½ H2O) + 1.5 H2O
When the dry plaster powder is mixed with water, it re-forms
into gypsum, initially as a paste but eventually drying into a
solid. The structure is made up of sheets of Ca2+ and
SO42- ions held together by hydrogen bonds in the water
molecules. The grip between these sheets is easily broken,
so plaster is fairly soft.
Its major use is in building, statuary, ceramics, dental plates,
fine metal parts for precision instruments, and surgical
splints.
Use of Plaster of Paris
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Plaster is used as a building material similar to mortar or
cement. Like those materials plaster starts as a dry
powder that is mixed with water to form a paste, which
then dries into a hard surface. Unlike those materials
plaster remains quite soft after drying, and can be easily
manipulated with metal tools or even sandpaper. Plaster
was a common building material for wall surfaces in a
process known as lath and plaster, in which a series of
wooden strips were covered with a semi-dry plaster and
then hardened into a flat surface. Today this building
method has been almost completely replaced with
drywall.
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Plaster expands while drying, then contracts slightly just
before hardening completely. This makes plaster
excellent for use in molds, and it is often used as an
artistic material for casting. Plaster is also commonly
spread over an armature (form), usually made of wire,
mesh or other materials. In medicine, it is also widely
used as a support for broken bones; a bandage
impregnated with plaster is moistened and then wrapped
around the damaged limb, setting into a close-fitting yet
easily removed tube.