INTERMOLECULAR FORCES

INTERMOLECULAR FORCES

• Inter molecular forces are the attraction between the molecules. It is a weak bond.
• Attractions exerted by one molecule on another, such as the force of attraction between water molecules in ice.
• Attractions between atoms of the noble gas elements, helium through radon.
• Attractions between molecules of one substance and molecules of another, as when two liquids are mixed, or a molecular solid such as sugar is dissolved in a liquid.
• Attraction between molecules of one substance and ions of another, as when an ionic compound dissolves in a liquid.

            Intermolecular forces (forces between two molecules) are weak compared to the intramolecular forces (forces keeping a molecule together). For example, the covalent bond present within HCl molecules is much stronger than the forces present between the neighbouring molecules. These forces exist between molecules when they are sufficiently close to each other. The forces consist of following types:

1. Dipole-dipole interactions
2. Hydrogen bonds
3. Dispersion forces
4. Vander walls forces
5. Ion–dipole forces

6. Instantaneous dipole-induced dipole forces or London dispersion forces.

Dipole–dipole interactions

            Dipole–dipole interactions are electrostatic interactions of permanent dipoles in molecules. These interactions tend to align the molecules to increase the attraction (reducing potential energy).

An example of a dipole–dipole interaction can be seen in hydrogen chloride (HCl):

            The positive end of a polar molecule will attract the negative end of the other molecule and cause them to be arranged in a specific arrangement. Polar molecules have a net attraction between them. For example HCl and chloroform (CHCl₃)

            Intermolecular forces that operate between neutral molecules having molecular dipole moments are called dipole-dipole forces. The dipole moments of two neighbouring molecules tend to align with the + end of one dipole near the - end of the other, so that forces of attraction between them are maximized.

            Such forces are obviously much weaker than those operating in ionic or covalent network solids, and give rise to potential wells having depths in the approximate range 5-20 kJ/mole. Many molecular substances with dipolar molecules exist as liquids at ambient temperature, and have relatively low boiling points. In particular, many organic compounds are of this type.

·         For Ex - SiF₄, CHCl₃, CO₂, SO₂ experience dipole-dipole intermolecular forces.

Hydrogen bonding

            Hydrogen bond is the strong dipole-dipole attractions between hydrogen atoms bonded to small, strongly electronegative atoms (O, N, or F) and nonbonding electron pairs on other electronegative atoms.

1) Bond dissociation energy of about 4-38 KJ mol–1 (0.96-9.08 Kcal mol–1).

2) H-bond is weaker than an ordinary covalent bond; much stronger than the dipole-dipole interactions.

3) Hydrogen bonding accounts for the much higher boiling point (78.5 °C) of ethanol than that of dimethyl ether (–24.9 °C).

4) A factor (in addition to polarity and hydrogen bonding) that affects the melting point of many organic compounds is the compactness and rigidity of their individual molecules.

            Adjacent molecules of the compound containing an O-H bond will be attracted to each other by virtue of these opposite charges. This force of attraction is known as the Hydrogen Bond. Usually a hydrogen bond is represented by a dotted line (Fig. 5.39).

            Intermolecular hydrogen bonding is responsible for the high boiling point of water (100°C) compared to the other hydrides that have no hydrogen bonds. Intra molecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids. It also plays an important role in the structure of polymers, both synthetic and natural. Hydrogen bond forces cause potential wells of depth in the range 5-50 kJ/mole.

 Examples  -  CHCl₃, CH₃CH₂OH, HNO₃, PH₃

            It is understandable that substances having nearly the same molecular weights, have the same boiling point. The boiling points of alkanes and ethers of comparable molecular weights are not far apart, but the boiling points of alcohols having almost equal molecular weights are considerably higher.

            This can be explained on the basis of hydrogen bonding. Ethanol forms hydrogen bonds. Extra energy in the form of heat is required to break the hydrogen bonds holding the molecules together before it can be volatilized. Propane and dimethyl ether do not form hydrogen bonds and, therefore, have low boiling points.

Effect on Water-Solubility. A hydrogen-bonded substance is usually soluble in another hydrogen bonded substance.

            For example, alcohols are soluble in water but alkanes are not. This is because a non polar alkane molecule cannot break into the hydrogen bonded sequence in water . It Cannot replace the hydrogen bonds that would have to be broken to let it in.

            An alcohol molecule is capable of hydrogen bonding. It can slip into the hydrogen bonded sequence in water. It can replace the hydrogen bonds that must be broken to let it in.

            Thus alcohols of low molecular weight are water soluble. However, when the alkyl group is four or more carbons in length the alkane nature of the molecule predominates, and water solubility fans off sharply. Alcohols containing more than seven carbons are insoluble in water.

London dispersion forces

            Otherwise known as quantum-induced instantaneous polarization or instantaneous dipole-induced dipole forces, the London dispersion force is caused by correlated movements of the electrons in interacting molecules. The electrons, which belong to different molecules, start "feeling" and avoiding each other at the short intermolecular distances, which is frequently described as formation of "instantaneous dipoles" that attract each other.

Debye (induced dipole) force

            The induced dipole forces appear from the induction (also known as polarization), which is the attractive interaction between a permanent multipole on one molecule with an induced (by the former di/multi-pole) multipole on another. This interaction is called Debye force after Peter J.W. Debye.

            The example of an induction-interaction between permanent dipole and induced dipole is HCl and Ar. In this system, Ar experiences a dipole as its electrons are attracted (to H side) or repelled (from Cl side) by HCl. This kind of interaction can be expected between any polar molecule and non-polar/symmetrical molecule. The induction-interaction force is far weaker than dipole-dipole interaction, however stronger than London force.

Van der Waals Forces

            In addition to chemical bonding between atoms, there is another type of attractive force that exists between atoms, ions, or molecules known as van der Waals forces.

            These forces occur between the molecules of non polar covalent substances such as H₂, Cl₂ and He. These forces are generally to be caused by a temporary dipole, or unequal charge distribution, as electrons constantly move about in an atom, ion, or molecule. At a given instant, more electrons may be in one region than in another region, as illustrated in Figure.

            The temporary dipole induces a similar temporary dipole on a nearby atom, ion, or molecule. Every instant, billions of these temporary dipoles form, break apart, and reform to act as a weak electrostatic force of attraction known as van der Waals forces.

            It is important to note that van der Waals forces exist between all kinds of molecules. Some molecules may have these forces, as well as dipole or other intermolecular forces. Therefore, the strength of the van der Waals forces between substances increases with increasing gram molecular mass.