IB Chemistry - 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.
IB Chemistry - 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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