Nonpolar molecules dispersion forces

II. Complex nonpolar molecules Dispersion forces CCI4, iC5Hio... 

Nonpolar Atoms or nonpolar molecules Dispersion forces Soft extremely low to moderate melting points (depending on molar mass) sublime in some cases soluble in some nonpolar solvents He, Ar, H2, CO2, CCI4, CH4,12... 

In nonpolar molecules, dispersion is the only intermolecular force. 

London dispersion forces are the weakest of the intermolecular forces and occur between all molecules. These are the only types of intermolecular forces that are possible between nonpolar molecules and are caused by momentary dipoles. Experimental evidence suggests that electrons are not symmetrically distributed about the nucleus at all times. On average, the electrons may be spread out evenly around the nucleus, but there are brief instants when the electron density may be greater on one side of the atom than another. During these periods of time, the atoms develop a temporary or instantaneous polarity. The temporary polarity (which is the cause of the momentary dipole) allows for attraction between particles that are normally nonpolar. London dispersion forces tend to increase as the size and mass of the molecule increase. 

Dispersion forces are weak attractive forces that are important only over extremely short distances because they vary as Xld". They are present between all types of molecules in condensed phases but are weak for small molecules. Dispersion forces are the only kind of intermolecular forces present among symmetrical nonpolar substances such as SO3, CO2, O2, N2, Bt2, H2, and monatomic species such as the noble gases. Without dispersion forces, such substances could not condense to form liquids or solidify to form solids. Condensation of some substances occurs only at very low temperatures and/or high pressures. 

As van der Waals postulated, the attractive forces between neutral molecules also originate from electrical interactions (Hiemenz 1986). Although there are several types of van der Waals attractive forces that originate from electrical interactions, the most important for colloids is that operating between nonpolar molecules. These forces are due to the polarization of one molecule by quantum fluctuations in the charge distribution in the second molecule, and vice versa. They are known as the London dispersion forces, their origin having first been explained by F. London in 1930. 

There are different types of intermolecular forces. Between nonpolar molecules, the force is weak and is called a dispersion force, or induced dipole. The force between oppositely charged ends of two polar molecules is called a dipole-dipole force. The more polar the molecule, the stronger the dipole-dipole force. The third force, a hydrogen bond, is especially strong. It forms between the hydrogen end of one dipole and a fluorine, oxygen, or nitrogen atom on another dipole. 

London dispersion forces are relatively weak forces that arise among noble gas atoms and in nonpolar molecules. London forces arise horn instantaneous dipoles that develop when one atom (or molecule) momentarily distorts the electron cloud of another atom (or molecule). London forces are typicaUy weaker than either permanent dipole-dipole forces or covalent bonds. 

London dispersion forces between nonpolar molecules Repulsive forces between nonpolar molecules Coulombic ion/ion interactions... 

The physical properties of alkenes are similar to those of alkanes. The only attractive forces between alkene molecules, which are nonpolar, are dispersion forces (Section 2.7B). Two, three, and four carbon alkenes are gaseous at room temperature the larger ones are colorless liquids less dense than water. Alkenes are insoluble in water but soluble in one another, in other nonpolar organic liquids, and in ethanol.Table 5.1 lists physical properties of some alkenes. 

Prevalence. While they are the only force existing between nonpolar particles, dispersion forces contribute to the energy of attraction in all substances because they exist between all particles. In fact, except for the forces between small, highly polar molecules or between molecules forming H bonds, the dispersion force is the dominant intermolecular force. Calculations show, for example, that 85% of the attraction between HCl molecules is due to dispersion forces and only 15% to dipole-dipole forces. Even for water, 75% of the intermolecular attraction comes from H bonds and 25% from dispersion forces. 

Van der Waals Forces np [Johannes D. van der Waals 1923 Dutch physicist] (1926) (secondary valence force, intermolecular force) An attractive force, much weaker than primary covalence bonds, between molecules of a substance in which all the primary valences are saturated. They are believed to arise mainly from the dispersion effect, in which temporary dipoles induce other dipoles in phase with themselves. The primary van der Waals forces are dipole-dipole (polar molecules) and London forces (nonpolar molecules). These forces are attributed to the attractions between molecules and from noncovalent bonds (Goldberg, D. E., Fundamentals of Chemistry, McGraw-Hill Science/Engineering/ Math, New York, 2003). 

Besides the most basic and predominant nonpolar interactions (dispersion forces), there are polarization or polar interactions between molecules of counter bodies, such as dipole-dipole interactions (Keesom 1922) and dipole-induced dipole interactions (Debye 1921). The essential difference between dispersion and polarization forces is that, while the former involve simultaneous excitation of both molecules, those for the latter involve only a passive partner. The Keesom orientation interaction energy between two molecules with permanent dipoles is temperature dependent and proportional to the dipole moments as follows ... 

Polar Polar molecules Dispersion forces and dipole-dipole attractions Low to moderate melting points soluble in some polar and some nonpolar solvents (CH3)20, CHCI3, HCl... 

The (dispersion plus repulsion) terms are known as the London or van der Waals forces. Spherical, nonpolar molecules are well described... 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

The surface tension 7 is a measure of the work required to create unit area of surface from molecules in the bulk it is expressed in ergs per square centimeter or dynes per centimeter. The surface tension is a bulk property, not a molecular property. There appears to be some trend of y with other measures of polarity, but a lower limit of y is reached with very nonpolar liquids this limit (evidently about 15 dyn/cm) reflects the ever-present dispersion force between the molecules of liquid. 

Figure 2.8 Attractive dispersion forces in nonpolar molecules are caused by temporary dipoles, as shown in these models of pentane, C5H12-...

Polar molecules, like nonpolar molecules, are attracted to one another by dispersion forces. In addition, they experience dipole forces as illustrated in Figure 9.9, which shows the orientation of polar molecules, such as Id, in a crystal. Adjacent molecules line up so that the negative pole of one molecule (small Q atom) is as dose as possible to the positive pole (large I atom) of its neighbor. Under these conditions, there is an electrical attractive force, referred to as a dipole force, between adjacent polar molecules. 

Strategy Determine whether the molecule is polar or nonpolar and identify the intermolecular forces present. Remember, dispersion forces are always present and increase with molar mass. 

Strategy Determine whether the molecules are polar or nonpolar only polar molecules show dipole forces. Check the Lewis structures for H atoms bonded to F, N, or O. All molecules have dispersion forces. 

SOLUTION The N2 and C02 molecules are nonpolar (Chapter 7), so only dispersion forces are present. Both CHC13 and NH3 are polar molecules. Chloroform contains dipole forces as well as dispersion forces. Ammonia contains hydrogen bonds as well as dispersion forces. 

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