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Intermolecular Forces

Intermolecular forces are responsible for most of the physical and chemical properties of matter. Although the exact nature of these forces are very complicated, there are no conceptual difficulties.

The basic building block in molecular compounds is the molecule, which is formed by relatively strong covalent bonds. Molecules are then held together in the solid and liquid states by weaker forces that have an effect on the physical properties of the material. Most of the theory as we know it today was developed during very early days of quantum mechanics. With the use of new types of quantum mechanical techniques and the availability of high-speed computing machines, it is now becoming possible to make accurate calculations of intermolecular forces.

The intermolecular forces if defined as

"The forces acting between the atoms due to electrostatic interaction between the changes of the atoms are called inter atomic forces".

During interaction between two atoms the following electrostatic forces will be active.

  1. Attractive force between the nucleus of one atom and electrons of the other atom. These attractive forces tend to decrease the potential energy of the pairs of atoms.
  2. Repulsive forces between the nucleus of one atom with the nucleus of another atom and electrons of one atom with the electrons of the other atom. These repulsive forces tend to increase the potential energy of pair of atoms.

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Influence of intermolecular forces

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Intermolecular forces influence chemistry in many ways.
  1. They are directly related to properties such as melting point, boiling point and the energy required to convert the solid into a liquid or a liquid into vapor.
  2. They are important in determining the solubility of gases, liquids, and solids in various solvents.
  3. They are crucial in determining the structures of biologically important molecules such as DNA and proteins.

Collectively, forces between molecules are called van der walls forces and these include the attractive and repulsive forces between

  1. Molecules with permanent dipoles.(dipole-dipole forces)
  2. Polar molecules and non-polar molecules.(dipole-induced dipole forces)
  3. Non-polar molecules.(induced dipole-induced dipole forces also called as London forces)

Intermolecular forces involve molecules that are in which polarity can be induced. Furthermore, several types of intermolecular forces can be at work in a single type of molecule. Each individual induced dipole/induced dipole force is usually quite small, the sum of these forces over the entire structure of a molecule can actually be quite even in polar molecules.

For example, the polar sulfur-dioxide molecule attracted one another in the following manner. Molecules of SO2 are polar with the partially negative region of one molecule being attracted to the partially positive region of an adjacent molecule.
Intermolecular Forces in SO2

For polar molecules intermolecular forces act between the positive end of one polar molecule and the negative end of an adjacent polar molecule.

Types of intermolecular forces

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Intermolecular forces are electromagnetic forces, and can be further loosely divided into three categories according to their range.
  1. Coulomb forces are the electrostatic interactions between permanent changes and dipoles and have a long range.
  2. Polarization forces, giving rise to the van der waals forces, arise from interactions between dipoles in atoms and molecules induced by nearby charges and permanent dipoles. van der waals are considered long ranged on a molecular scale but short ranged on a colloidal scale.
  3. Very short ranged forces of quantum mechanical nature include chemical bonds and steric or Born repulsions due to Pauli's exclusion principle.

The coulombic forces between molecules and atoms namely intermolecular forces, were described and defined by Van der waals in 1873. He invoked these intermolecular forces to explain why the behavior of real gases deviated from the ideal state. These are weal forces between atoms and molecules that do not involve the transfer or sharing of electrons, and therefore do not result in bonding. They are collectively known as Van der waals forces and can be divided into three categories.

  • Dipole-dipole interactions
  • Dipole-induced dipole interactions
  • Induced dipole- induced dipole interactions or London dispersion forces

Each of these interactions is weaker than interactions between fully charged ions, but they play an important role in determining the physical properties of molecules and their interactions with one another. In most cases a combination of forces acts together to contribute to the overall properties of the system so it is helpful to examine each of them to understand their relative importance and the factors that affect their strength.

In the case of molecules with no polar bonds the only attractive forces possible between the molecules are induced dipole-induced dipole interactions (London dispersion forces). For molecules with polar bonds, all three of the Van der waals forces contribute to the overall attractive forces: Induced dipole- induced dipole, Dipole-induced dipole and Dipole-dipole.


Problem

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Question

Suppose you have a mixture of solid iodine I2, and the liquids water and carbon tetrachloride CCl4. What intermolecular force exist between each possible pair of compounds? Describe what you might see when these compounds are missed.

Strategy

First, decide each substance is polar or non-polar. Second determine the types of intermolecular forces that could exist between the different pairs. Finally, use the 'like dissolves like' guidelines to decide whether iodine will dissolve in water or CCl4 and whether CCl4 will dissolve in water.


Solution

I2 is non-polar. As a molecule composed of large iodine atoms, it has an extensive electron cloud. Thus, the molecule is easily polarized, and iodine could interact with water a polar molecule by dipole/induced dipole forces.
Carbon tetrachloride a tetrahedral molecule, is not polar. As a consequence it can interact with iodine only by dispersion forces. Water and CCl4 could interact by dipole/induced dipole forces, but the interaction is expected to be weak
Iodine does not dissolve to a small extent in water to give a brown solution. When this brown solution is added to a test tube containing CCl4 the liquid layer do not mix (polar water does not dissolve in non-polar CCl4)(notice the more dense the CCl4 layer [d = 1.58 gm/ml] is underneath the less dense water layer). When the test tube is shaken however non polar I2 dissolves preferentially in non-polar CCl4, as evidenced by the disappearance of the color of I2 in the water layer (top) and the appearance of the purple I2 color in the CCl4 layer (bottom).

Intermolecular Forces in Water

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To understand the effect of molecular motion on the structure of water at different temperature we need to differentiate between intramolecular bonds and intermolecular forces, as shown in the diagram.

Intermolecular Forces in H2O

The covalent bonds between the hydrogen and oxygen atoms in a water molecule are called intramolecular bonds. The prefix intra comes from a Latin stem meaning "within or inside". The attractive forces between the neighboring water molecules are called intermolecular forces.
The prefix inter comes from a Latin stem meaning "between".

The intermolecular bonds that hold the atoms in H2O molecules together are much stronger than the intermolecular forces between water molecules. It takes 463kJ to break the H-O bonds in a mole of water molecules, but only about 45kJ to break the intermolecular forces that hold a mole of water molecules to one another. As the temperature of a sample of water increases so does the average kinetic energy of the water molecules. The increase in the motion of water molecules results from increase in the average kinetic energy disrupts the intermolecular forces between water molecules.

Intermolecular Forces of Attraction

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The term intermolecular forces refers to attractions between molecules. Although it is proper to refer to all intermolecular forces as Van der waals forces this concept should be explained for clarity. There are three types of intermolecular force of attraction. These are the forces that hold the molecules in condensed state.

Dipole-dipole attraction

One component of van der waals forces is dipole-dipole attraction. The polar covalent bonding that the unsymmetrical distribution of electronic charges leads to positive and negative charges in the molecules, which are referred to as dipoles. In polar molecular substances, the dipoles line up so that the positive pole of one molecule attracts the negative pole of another. This is much like the lineup of small bar magnets. The force of attraction between polar molecules is called dipole-dipole attraction. These attractive forces are less than the full charges carried by ions in ionic crystals.

Dipole Dipole Attraction


The strength of dipole-dipole attractive forces increases as the polarity of molecules increases. As with all the intermolecular forces, dipole-dipole forces are not as strong as covalent bonds between atoms, yet they play an important role as a force between molecules.

Induced dipole- induced dipole interactions (London forces)

Another component of van der waals forces is called London force. Found in both polar and non polar molecules, it can be attributed to the fact that an atom that usually is non polar sometimes becomes polar because the constant motion of its electron may cause uneven charge distribution at any one instant.

London forces are about one-tenth the force of most dipole interaction and are the weakest of all the electrical forces that act between atoms or molecules. These forces help to explain why non polar substances such as noble gases and the halogens condense into liquids and then freeze into solids when the temperature is lowered sufficiently.

In general they also explain why liquids composed of discrete molecules with no permanent dipole attraction have low boiling points relative to their molecular masses. Because of the low intermolecular forces the melting points are low and evaporation takes place so easily that it may occur at room temperature. The following figure shows that the London forces are attractive forces.

London Forces

Hydrogen bonds

A proton or hydrogen nucleus has a high concentration of positive charge. When a hydrogen atom id bonded to a highly electronegative atom, its positive charge will have an attraction for neighboring electron pairs. This special kind of dipole-dipole attraction is called a hydrogen bond. The more strongly polar the molecule is the more effective the hydrogen bonding is in binding the molecules into larger unit. As a result the boiling points of such molecules are higher than those of similar non polar molecules.

Hydrogen Bonding in Water

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