The Hydrogen Bond: And Other Intermolecular Forces

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This occurs in molecules such as tetrachloromethane and carbon dioxide. The dipole—dipole interaction between two individual atoms is usually zero, since atoms rarely carry a permanent dipole.

Structural Biochemistry/Strong Intermolecular Forces

These forces are discussed further in the section about the Keesom interaction, below. Ion—dipole and ion—induced dipole forces are similar to dipole—dipole and dipole—induced dipole interactions but involve ions, instead of only polar and non-polar molecules. Ion—dipole and ion—induced dipole forces are stronger than dipole—dipole interactions because the charge of any ion is much greater than the charge of a dipole moment. Ion—dipole bonding is stronger than hydrogen bonding. An ion—dipole force consists of an ion and a polar molecule interacting.

They align so that the positive and negative groups are next to one another, allowing maximum attraction. An important example of this interaction is hydration of ions in water which give rise to hydration enthalpy. The polar water molecules surround themselves around ions is water and the energy released during the process is known as hydration enthalpy.

Worked example 1: Intermolecular forces

An ion—induced dipole force consists of an ion and a non-polar molecule interacting. Like a dipole—induced dipole force, the charge of the ion causes distortion of the electron cloud on the non-polar molecule. The van der Waals forces arise from interaction between uncharged atoms or molecules, leading not only to such phenomena as the cohesion of condensed phases and physical adsorption of gases, but also to a universal force of attraction between macroscopic bodies. The first contribution to van der Waals forces is due to electrostatic interactions between charges in molecular ions , dipoles for polar molecules , quadrupoles all molecules with symmetry lower than cubic , and permanent multipoles.

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It is termed the Keesom interaction , named after Willem Hendrik Keesom. They consist of attractive interactions between dipoles that are ensemble averaged over different rotational orientations of the dipoles. It is assumed that the molecules are constantly rotating and never get locked into place.

This is a good assumption, but at some point molecules do get locked into place.

Hydrogen Bonding

The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance. The Keesom interaction can only occur among molecules that possess permanent dipole moments, i. Also Keesom interactions are very weak van der Waals interactions and do not occur in aqueous solutions that contain electrolytes. The angle averaged interaction is given by the following equation:.

The second contribution is the induction also termed polarization or Debye force, arising from interactions between rotating permanent dipoles and from the polarizability of atoms and molecules induced dipoles.

Hydrogen Bonding

These induced dipoles occur when one molecule with a permanent dipole repels another molecule's electrons. A molecule with permanent dipole can induce a dipole in a similar neighboring molecule and cause mutual attraction. Debye forces cannot occur between atoms. The forces between induced and permanent dipoles are not as temperature dependent as Keesom interactions because the induced dipole is free to shift and rotate around the polar molecule.

Associated Data

The Debye induction effects and Keesom orientation effects are termed polar interactions. One example of an induction interaction between permanent dipole and induced dipole is the interaction between HCl and Ar. The induction-interaction force is far weaker than dipole—dipole interaction, but stronger than the London dispersion force.

The third and dominant contribution is the dispersion or London force fluctuating dipole—induced dipole , which arises due to the non-zero instantaneous dipole moments of all atoms and molecules.

Dipole–induced-dipole interaction

Such polarization can be induced either by a polar molecule or by the repulsion of negatively charged electron clouds in non-polar molecules. Thus, London interactions are caused by random fluctuations of electron density in an electron cloud. An atom with a large number of electrons will have a greater associated London force than an atom with fewer electrons.

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The dispersion London force is the most important component because all materials are polarizable, whereas Keesom and Debye forces require permanent dipoles. The London interaction is universal and is present in atom-atom interactions as well.

Chapter 9 Solutions

For various reasons, London interactions dispersion have been considered relevant for interactions between macroscopic bodies in condensed systems. Hamaker developed the theory of van der Waals between macroscopic bodies in and showed that the additivity of these interactions renders them considerably more long-range. This comparison is approximate.

The actual relative strengths will vary depending on the molecules involved.

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Ionic bonding and covalent bonding will always be stronger than intermolecular forces in any given substance. Intermolecular forces are repulsive at short distances and attractive at long distances see the Lennard-Jones potential. In a gas, the repulsive force chiefly has the effect of keeping two molecules from occupying the same volume. This gives a real gas a tendency to occupy a larger volume than an ideal gas at the same temperature and pressure.

The attractive force draws molecules closer together and gives a real gas a tendency to occupy a smaller volume than an ideal gas. Which interaction is more important depends on temperature and pressure see compressibility factor. In a gas, the distances between molecules are generally large, so intermolecular forces have only a small effect. The attractive force is not overcome by the repulsive force, but by the thermal energy of the molecules.

Temperature is the measure of thermal energy, so increasing temperature reduces the influence of the attractive force.

In contrast, the influence of the repulsive force is essentially unaffected by temperature. When a gas is compressed to increase its density, the influence of the attractive force increases. If the gas is made sufficiently dense, the attractions can become large enough to overcome the tendency of thermal motion to cause the molecules to disperse. Then the gas can condense to form a solid or liquid, i. Lower temperature favors the formation of a condensed phase. In a condensed phase, there is very nearly a balance between the attractive and repulsive forces. Intermolecular forces observed between atoms and molecules can be described phenomenologically as occurring between permanent and instantaneous dipoles, as outlined above.

Alternatively, one may seek a fundamental, unifying theory that is able to explain the various types of interactions such as hydrogen bonding, van der Waals forces and dipole—dipole interactions. This ends up forming a partial bond, which we describe as the hydrogen bond.

The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces except for the ionic bond. Could the CH 2 O molecule exhibit hydrogen bonding? The answer is no, since the hydrogen must be bound to either N, O, or F. Just having one of those species in the molecule is not enough. Now, as these things increase in strength it becomes harder to remove the molecules from each other. Therefore, one would expect the melting and boiling points to be higher for those substances which have strong intermolecular forces.

We know that it takes energy to go from a solid to a liquid to a gas. This energy is directly related to the strength of attraction between molecules in the condensed phases. Since energy is directly proportional to the temperature, the above trends ought to hold true. Each of these processes are endothermic, and scale with the magnitude of the intermolecular forces.

Thus, as these intermolecular forces increase, so do the energies requires to melt, vaporize, or sublime go from solid to a gas a species. Every substance also has an associated vapor pressure with it. The vapor pressure is defined to be the amount of gas of a compound that is in equilibrium with the liquid or solid.