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.




--------------------------------------------------------------