Polar molecules intermolecular forces between

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

In dimethyl ether, the oxygen atom is sp3 hybridized. In creating two single bonds, each bond is formed by the overlap of one of its sp3 hybrid orbitals with the sp3 hybrid orbital on the adjacent carbon atom. Each of the remaining two hybrid orbitals on the oxygen atom contain a lone pair of electrons. The resulting molecule is polar. The intermolecular forces found operating between molecules of dimethyl ether are therefore dipole-dipole interactions and London forces. 

C—Deviations from ideal behavior depend on the size and the intermolecular forces between the molecules. The greatest deviation would be for a large polar molecule. Sulfur tetrafluoride is the largest molecule, and it is the only polar molecule listed. 

Alkanes have similar chemical properties, hut their physical properties vary with molecular weight and the shape of the molecule. The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the weak dipole-dipole forces (see 2.5.1), which are easily overcome. As a result, compared with other functional groups, alkanes have low melting and boihng points, and low solubility in polar solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and dichloromethane. Most cycloalkanes also have low polarity. 

If the intermolecular forces between solute particles and solvent molecules are weaker than the forces between solvent molecules alone, then the solvent molecules are less tightly held in the solution and the vapor pressure is higher than Raoult s law predicts. Conversely, if the intermolecular forces between solute and solvent molecules are stronger than the forces between solvent molecules alone, then the solvent molecules are more tightly held in the solution and the vapor pressure is lower than predicted. Solutions of ionic substances, in particular, often have a vapor pressure significantly lower than predicted, because the ion-dipole forces between dissolved ions and polar water molecules are so strong. 

Because unbranched alkanes are neutral, nonpolar molecules, it is difficult to explain the existing intermolecular force between such alkanes that increases as the alkane molecules become larger. We will see that this attractive force is weak and tenuous. These molecules do not become overly friendly with each other. In theory, as atoms within one alkane molecule approach the atoms of another alkane molecule, the electrons around these atoms, for an instant, arrange themselves asymmetrically around the atoms so that instant dipoles are formed—the positive side of one atom attracts the negative side of another atom. This weak intermolecular attractive force is called a London Force. When there is a weak intermolecular attractive force between polar molecules, the force is called a dipole-dipole force. Together, London forces and dipole-dipole forces are called Van der Waals forces. 

Dipole-dipole interactions The intermolecular forces between polar molecules are known as dipole-dipole interactions. The partial positive charge of one molecule is attracted to the partial negative charge of its neighbor. 

In the Thought Lab in section 8.1, you observed that solid iodine is insoluble in water. Only a weak attraction exists between the non-polar iodine molecules and the polar water molecules. On the other hand, the intermolecular forces between the water molecules are very strong. As a result, the water molecules remain attracted to each other rather than attracting the iodine molecules. 

The intermolecular forces between polar molecules are known as dipole-dipole interactions. The partial positive charge of one molecule is attracted to the partial negative charge of its neighbor. Polar molecules (such as CH3CI) form molecular solids with dipole-dipole bonds between units. Polar molecules with H on one molecule attracted to 0, N, or F on an adjacent molecule (like H2O) form relatively strong dipole-dipole bonds known as hydrogen bonds between molecules. 

The nature of the solvent must also be considered. How well does the solvent accommodate the solute If a substance is to be soluble, it needs to interact with the solvent in a manner similar to the way the solvent molecules interact with themselves. The intermolecular forces between molecules of the solvent that are disrupted (this requires energy) should be replaced, to some degree, by the new attractive forces between the solute and the solvent (this produces energy). Clearly, the characteristics of both the solute and the solvent contribute to the solubility of a substance, and one of the most important is the polarity or lack of polarity of the solute and solvent. 

This arrangement of atoms produces a very polar bond, often resulting in a polar molecule with strong intermolecular attractive forces. Although the hydrogen bond is weaker than bonds formed within molecules (covalent and polar covalent intramolecular forces), it is the strongest attractive force between molecules (intermolecular force). 

Most organic species form molecular crystals in which discrete molecules are arranged in fixed positions relative to the lattice points. This of course means that the individual atoms making up the molecules are each arranged at fixed positions relative to each other, the lattice point, and the other molecules. The forces between molecules in molecular crystals are generally weak when compared with the forces within a molecule. The structure of molecular crystals is affected by both the intermolecular forces and the intramolecular forces since the shapes of the individual molecules will affect the way the molecules pack together. In addition, the properties of the individual molecule, such as the polarity, will affect the intermolecular forces. The forces between the molecules in molecular crystals include electrostatic interactions between dipoles, dispersion forces, and hydrogen bond... 

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. 

The forces that bind the adsorber to adsorbent may be physical (i.e. intermolecu-lar forces) or chemical (when chemical bonds are formed). The adsorption of gases by charcoal is physical, whereas the adsorption of gases on some catalysts is chemical. Toxic gases adsorb to charcoal better than the oxygen and nitrogen of the air because they are often relatively large molecules and are frequently polar. This increases the intermolecular forces between adsorbate and adsorbent. 

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