The Intermolecular Forces of Attraction

Intermolecular forces are sometimes referred to as van der Waals forces, after the Dutch scientist Johannes van der Waals (1837-1923). The three most important intermolecular forces that can exist between neutral molecules are  

As you might expect, 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. 

The following figure shows that London dispersion forces are attractive forces between instantaneous dipoles that result from the flexing of the negative electron cloud about the positive nuclear framework of an atom or molecule. The short-lived dipoles induce formation of more dipoles in adjacent molecules, which attracts them to one another. After an instant, the whole attractive structure of instantaneous dipoles will form again in a different direction. 

These are the London dispersion forces of attraction, or London forces, proposed in 1928 by Fritz London. Every atom, ion, or molecule can engage in London forces, as long as it has at least one electron. London forces are the only intermolecular forces possible in nonpolar substances. London forces are significant only when molecules are veiy close together, essentially touching. They are the forces that are responsible for carbon tetrachloride being a liquid at room temperature. The magnitude of London forces increases as the molecular masses of molecules increase. 

For polar molecules with large molecular masses, like HCC13, IQ, or SC12, the London forces can be a more significant attractive force between molecules than the dipole-dipole forces, although with smaller polar molecules, the dipole-dipole forces are usually stronger than the London forces. 

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This expression applies to the transport of any conserved quantity Q, e.g., mass, energy, momentum, or charge. The rate of transport of Q per unit area normal to the direction of transport is called the flux of Q. This transport equation can be applied on a microscopic or molecular scale to a stationary medium or a fluid in laminar flow, in which the mechanism for the transport of Q is the intermolecular forces of attraction between molecules or groups of molecules. It also applies to fluids in turbulent flow, on a turbulent convective scale, in which the mechanism for transport is the result of the motion of turbulent eddies in the fluid that move in three directions and carry Q with them. 

Viscosity is the resistance to flow of a liquid. The stronger the intermolecular forces of attraction, the more viscous the liquid will be. 

The molar heat of vaporization, AHvap, at the boiling point is a measure of the heat required to change 1 mole of a liquid into a gas. The stronger the intermolecular forces of attraction between the molecules, the greater will be the value of AHvap. Since vapor pressure at a given temperature decreases with increasing intermolecular force, we expect the AHvap values to be the reverse of the vapor pressure trend. 

Remember that one of the principal properties used to define an ideal solution is that the intermolecular forces of attraction and repulsion are the same between unlike as between like molecules. This property does not exist in real solutions. Molecular behavior in a real solution depends on the types and sizes of the molecules which are interacting. 

Rather than the like dissolves like rule, it is the intermolecular interaction, between solvent and solute molecules, which determines the mutual solubility. A compound A dissolves in a solvent B only when the intermolecular forces of attraction Kaa and Kbb for the pure compounds can be overcome by the forces KAb in solution [13],... 

But at high pressures or at low temperatures, the volume is small and molecules lie closer to one another. The intermolecular forces of attraction, therefore, are appreciable and cannot be ignored. 

Effect of Temperature on Surface Tension According to the kinetic theory, molecular kinetic energy is proportional to absolute temperature. The rise in temperature of a liquid, therefore, is accompanied by increase in energy of its molecules. Since intermolecular forces decrease with increase in the energy of molecules, the intermolecular forces of attraction decrease with rise of temperature. 

Hydrogen bonding is the intermolecular force of attraction between a hydrogen atom in one molecule and a small, highly electronegative atom with an... 

Gases have no definite shape and no definite volume. They will expand to fill the container that they are made to occupy. Ten molecules of a gas can occupy as much space as ten million molecules of a gas, as they move freely around a container at relatively high speeds. The intermolecular forces of attraction between molecules in the gas phase are so weak that the particles are allowed to travel quite far away from each other. Gases differ significantly from the other phases of matter in that, the particles of a gas are often separated by very great distances. You already know that most of an atom is made up of empty space. So, too, most of a gas sample is made up of empty space, to the point that the size of a gas sample typically has little to do with the size of the actual particles. 

This reflects the strength of the intermolecular forces of attraction between the water molecules, mainly hydrogen bonding. 

The intermolecular forces of attraction in hexatriacontane cited above and in polyethylene are essentially van der Waals forces. However, in polymers of large molecular size, there are so many intermolecular contacts that the sum of van der Waals forces holding each molecule to its neighbors is appreciable. It thus requires a much- larger stress to break a polymeric material as this involves separating the constituent molecules. Deformation of a polymeric structure for the same reason requires more force as the macromolecules become larger. 

Polar covalent bonds can cause a molecule to act like an electric dipole, possessing a permanent dipole, partially negative on one end, partially positive on the other. When molecules with permanent dipoles get close together, the natural attraction of 8+ for 8- and 8- for 8+ draw them together, and this is the basis of the intermolecular forces of attraction. 

If you answered incorrectly, review The Intermolecular Forces of Attraction, page 334. 

To determine whether a solute will dissolve in a given solvent, it is necessary to consider the balance of the intermolecular forces of attraction between solvent-solvent, solute-solute, and solvent-solute pairs of molecules in solution. This balance of forces is given as the interchange energy E of mixing i-j pairs of molecules... 

In the absence of chemical reaction between adsorbed species, it is instructive to analyze adsorption/desorption equilibria via steps 3 and 6. The overall objective here is to develop expressions between the partial pressnie / a of gas A above a solid surface and the fraction of active sites a on the catalyst that are occnpied by this gas when it adsorbs. The phenomenon of chemisorption and the relation between pa and a apply to a unimolecular layer of adsorbed molecnles on the catalytic surface. This is typically referred to as a monolayer, where the intermolecular forces of attraction between adsorbed molecules and active snrface sites are characteristic of chemical bonds. When complete monolayer coverage of the surface exists, subsequent adsorption on this saturated surface corresponds to physisorption, which is analogous to condensation of a gas on a cold snbstrate. The enthalpy change for chemisorption is exothermic with valnes between 10 and 100 kcal/mol. The Langmuir adsorption isotherm, first proposed in 1918 (see Langmuir, 1918), is based on the following reversible elementary step that simulates single site adsorption on a catalytic surface when there is only one adsorbate (i.e., gas A) present ... 

Introduction of a lateral substituent into the mesogen will also lower and Tj, as the bulky side groups will tend to force the chains apart, thereby reducing the intermolecular forces of attraction. Ring substimtion is the easiest way to achieve this, and Lenz has shown that the structure VI,... 

The measurement of the strength of adhesion between adherend and adhesive requires a measurement of the intermolecular forces of attraction this is not currently possible. This aspect of quality control is therefore essentially reduced to assessing the adherend surface characteristics prior to bonding, although some post-bonding simple mechanical tests are appropriate. 

The melting temperamre is a temperature at which the thermal energy in a sohd material is just sufficient to overcome the intermolecular forces of attraction in the crystalline lattice so that the lattice breaks down and the material becomes a liquid, i.e., it melts. 

A more accurate and rigorous treatment must consider the intermolecular forces of attraction and repulsion between molecules and also the different sizes of molecules A and B. Chapman and Enskog (H3) solved the Boltzmann equation, which does not utilize the mean free path X but uses a distribution function. To solve the equation, a relation between the attractive and repulsive forces between a given pair of molecules must be used. For a pair of nonpolar molecules a reasonable approximation to the forces is the Lennard-Jones function. 

Ideal Solutions Similar to the concept of ideal gases, in the theory of solutions, the concept of ideal solutions can be visualized. In an ideal gas, the intermolecular forces of attraction are assumed to be completely absent. Since liquids exist only because of molecular interactions, in an ideal solution the molecules can be expected to be of similar size and intermolecular attraction. Thus, in an ideal solution A in B, the forces of attraction between A and A, A and B, B and B should be the same. 

Increasing the pressure on a substance forces the molecules closer together, which in turn increases the strength of the intermolecular forces of attraction. Propane (CsHr) is a gas at room temperature and 1 atm pressure, whereas liquefied propane (LP) gas is a liquid at room temperature berause it is stored under much higher pressure. 

This reflects the lower strength of the intermolecular forces of attraction between the water molecules, compared to the covalent bonds within the water molecules. The attractive forces between water molecules are mainly due to hydrogen bonding. 

As it turns out, geckos do not use any chemical adhesives, nor do they use suction. Instead, their abilities arise from the intermolecular forces of attraction between the molecules in their feet and the molecules in the surface on which they are walking. When you place your hand on a surface, there are certainly intermolecular forces of attraction between the molecules of your hand and the surface, but the microscopic topography of your hand is quite bumpy. As a result, your hand only makes contact with the surface at perhaps a few thousand points. In contrast, the foot of a gecko has approximately half a million microscopic flexible hairs, called setae, each of which has even smaller hairs. 

Vapor Pressure. Individual molecules in the liquid have different amounts of kinetic energy, distributed roughly along a normal curve. Some molecules will have energy exceeding the intermolecular forces of attraction. These molecules will vaporize. The number of molecules, per unit time and unit area, that vaporize dictates the vapor pressure of the substance. Since kinetic energy is directly proportional to temperature, vapor pressure is dependent solely on temperature. 

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