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Essay/Term paper: Intermolecular bonding essay

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Intermolecular Bonding Essay

Write an essay on intermolecular bonding. Explain how each type of bond arises
and the evidence for the existence of each. Comment on their strengths in
relation to the types of atoms involved; the covalent bond and relative to each
other. Use the concepts of different types and strengths of intermolecular bonds
to explain the following:

There exists four types of intermolecular bonding, they include ionic, covalent,
Van der waals and hydrogen bonding. In order to describe the existence of such
bonding you must also understand the concepts of polarity, polar and non-polar,
and electronegativity.

Ionic bonds are created by the complete transfer of electrons from one atom to
another. In this process of electron transfer, each atom becomes a ion that is
isoelectronic with the nearest noble gas., the substance is held together by
electrostatic forces between the ions. The tendency for these ions to be formed
by elements is corespondent to the octet rule, when atoms react,, they tend to
do so in such a way that they attain an outer shell containing eight electrons.
The factors that effect the formation of ions are ionization energy, electron
affinity, lattice energy.

Figure 1

The transfer of electrons involved in the formation of (a) sodium chloride and
(b) calcium fluoride. Each atom forms an ion with an outer shell containing
eight electrons.

For many elements, compounds cannot be formed by the production of ions, since
the energy released in the formation of the lattice of ions would be
insufficient to overcome the energy required to form the ions would be
insufficient to overcome the energy required to form the ions in the first place.
In order for the atoms to achieve a noble gas configuration they must use
another method of bonding by the process of electron sharing. From figure 2, you
can see that the example of two hydrogen atoms combing. As the atoms get closer
together, each electron experiences an attraction towards the two nuclei and the
electron density shifts so that the most probable place to find the two
electrons is between the two nuclei. Effectively each atom now has a share of
both the electrons. The electron density between the two nuclei exerts an
attractive force on each nucleus keeping them held tightly together in a
covalent bond.

Figure 2

A covalent bond forming between two hydrogen atoms.

It is also possible for two atoms share more than one pair of electrons, sharing
two pairs results in a double bond and sharing three pairs results in a triple
bond. Electronegativity is a measure of how powerful a atom is in a molecule to
attract electrons. Polarization is a term given to name the unequal sharing of
electrons in a covalent bond. Molecules that have unequal sharing of electrons
are called polar molecules and dipole molecules are ones which have the charge
separated, therefore all polar molecules must have a dipole attraction. Non-
polar molecules are ones in which there shapes are symmetrical so the electrons
are evenly distributed. Polar molecules have a permanent dipole in other words
they have a permanent separation of charge. As a result of this, polar molecules
are attracted to one another by forces called permanent dipole-permanent dipole
interactions, in which the negative end of one molecule is attracted towards the
positive end of another. These interactions decrease quite rapidly as the
distance between molecules increases. They are approximately 100 times weaker
than covalent bonds. There are also very strong types of dipole-dipole
interactions called Hydrogen bonds. Evidence for the existence of such
intermolecular forces lies in the properties of hydrides formed by element in
groups 4,5,6 and 7. While all the hydrides formed in group 5 behave in a similar
way, the hydrides of other groups do not. This suggest that the intermolecular
forces in these hydrides are much stronger than expected compared with other
hydrides of the other elements in each group. This type of intermolecular
bonding occurs in two molecules that each contain a polar bond between hydrogen
and another atom.

Figure 3

The variation in boiling points of the hydrides of groups IV, V, VI and VII.

The forces of attraction that exists between two non-polar molecules also arise
due to an uneven charge distribution. If we consider a neutral atom, at any
particular moment the centres of positive and negative charge may not coincide,
due to an instantaneous asymmetry in the electron distribution around the
nucleus. So, there must be an instantaneous dipole in the molecule. Any other
atom next to an atom with an instantaneous dipole will experience an electric
field due to the dipole, and so itself develop an induced dipole. These
instantaneous dipole-induced dipole interactions between neighboring molecules
enable non-polar molecules to come together. This is the basis for another
branch of intermolecular forces known as Van der waals forces. These forces are
weak, short-ranged forces of attraction between molecules. They are the weakest
force of attraction between atoms. Covalent bond strengths are typically between
200 and 500 kJ mol-1. Hydrogen bonds are weak in comparison, they range from 5
to 40 kJ mol-1. Van der waals forces are weaker still having a strength of about
2 kJ mol-1. Hydrogen bonds and Van der waals forces are not strong enough to
influence the chemical behavior of most substances, although they may affect the
physical properties of substances.

a. The heat of solvation arises when an ionic substance is dissolved in a polar
solvent. Intermolecular bonds form between polar solvent and ionic substance
molecules. Bonds that are within the ionic lattice between molecules are broken
as charged molecules are attracted to solvent molecules. Ionic lattice bonds
that are broken release more energy than the energy put into the newly formed
intermolecular bonds this explains why an exothermic reaction occurs.

b. Sodium Chloride is a ionic compound and when mixed in tetrachloromethane, it
does not dissolve. The tetrachloromethane which has a symmetrical tetrahedral
shape is a non-polar substance so no intermolecular attractions between
molecules occur. Since there are no intermolecular attractions, no forces are
created which can attract NaCl molecules away from their ionic lattice. In the
case of NaCl and ethanol the polar molecules forms intermolecular attractions
with charged NaCl molecules, pulling the molecules away from the ionic lattice
and therefore allowing NaCl to dissolve in ethanol.

c. Water is a polar molecule and oil is non-polar. If no intermolecular bonding
occurs, the two substances will be immiscible.

d. All organic acids such as ethanoic acid, CH3 -COOH, are partially polar
molecules. One side of the molecule is non-polar while the other side is polar.
Ethanoic acid has extending hydrogen atoms that form hydrogen bonds with oxygen
atoms from the COOH group of neighboring molecules. So a dimer is formed. When
organic acids are heated, energy is needed to overcome both van der Waals forces
and hydrogen bonds between molecules. This explains why organic acids have a
higher than expected boiling and melting point than other similar compounds.

e. Through the process of condensation polymerization, amino acids form into
polypeptide chains, or proteins. Hydrogen bonds form to stabilize the structure
of these compounds and the more hydrogen bonds present in a polypeptide, the
more stable it is. At 40 degrees Celsius, molecules in protein gain enough
kinetic energy to vibrate rapidly and overcome and break the stabilizing
hydrogen bonds. As the bonds break, the protein loses shape and returns to a
primary structure. This is the process which makes the compound denatured. A
similar process occurs with DNA. DNA is composed of two polynucleotide chains,
attached together by hydrogen bonds. Hydrogen bonds form between the
complimentary base pairs and this occurs throughout the double helical structure.
At 40 degrees Celsius, the structure vibrates so rapidly that the hydrogen
bonds between the base pairs are broken. As these hydrogen bonds break, the DNA
molecule loses its shape and is denatured.

f. The boiling point of water can be explained by the hydrogen bonds present.
Oxygen has a very high electronegativity value and when bonded to hydrogen, a
very polar molecule is formed. The hydrogen bonds occur throughout the liquid so
when water is boiled, enough kinetic energy must be supplied to the atoms to
break all of the hydrogen bonds before water boils. This explains why water has
such a high boiling point.

h. The increased boiling points and melting points of alkanes of increasing size
are due to stronger intermolecular forces. The only significant intermolecular
force in alkanes are the van der Waals interaction. This explains why, as the
size of alkanes increases, their boiling and melting points also increase.

i. Dimethylpropane molecules have a lower boiling point than pentane molecules.
Branching produces less efficient packing and thus weakens the intermolecular
interactions. Dimethylpropane also has a lower melting point because of it's
repeating crystal structure. Pentane has a highly symmetrical structure and
because of the ease with which it packs into the solid crystal structure, it has
a higher boiling point.

j. Iodine is a molecular solid at room temperature. Although individual atoms
are covalently bonded in pairs, weak van der Waals forces act between them and
induced dipoles are formed, these act throughout the structure and are strong
enough to hold the molecules in place. At the same temperature, chlorine
molecules are in a gaseous state. Like iodine, the atoms are bonded covalently
in pairs, but because Cl atoms are smaller in size, van der Waals forces are
even weaker than in iodine and not strong enough to hold chlorine molecules in
place. Therefore Cl molecules remain in a gaseous state at room temperature.


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