Force dipole solution

Polar molecules attract other polar molecules through dipole-dipole intermolecular forces. Polar solutes tend to have higher solubilities in polar solvents than in nonpolar solvents. Which of the following pairs of compounds would be expected to have the higher solubility in hexafluorobenzene, Cf,I... 

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. 

For a solute to dissolve in a solvent, the cohesive forces that hold the solute molecules together (e.g., London forces, dipole-dipole interactions) should be the same as those that hold the solvent molecules together. 

The solubilities of the solutes in an aqueous system determined from Equation (3.20) are usually larger than experimental values, as shown in Figure 3.2. This normally occurs for solutes (especially crystalline solids) in polar solvents. There are many interactions between the solute and the polar solvent self-association of the solute or the solvent, solvation of the solute by the polar solvent, complexation in solution, etc. A modification of Equation (3.20), known as the extended Hildebrand solubility approach, has been developed. In this approach, it is assumed that the activity coefficient is partitioned into two forces van der Waals forces and residual forces (dipole-dipole and hydrogen-bonding forces). 

The interactions between molecules which produce the cohesive energy characteristic of the liquid phase are described in the section entitled Secondary Forces Between Solvent and Solute Molecules. These involve the dispersion forces, dipole-dipole and dipole-induced dipole interactions, and specific interactions, especially hydrogen bonding. If it is assumed that the intermolecular forces are the same in the vapor and liquid states, then -E is the energy of a liquid relative to its ideal vapor at the same temperature. It can be described as the energy required to vaporize 1 mole of liquid to the saturated vapor phase (Af U) plus the energy required for the isothermal expansion of the saturated vapor to infinite volume. Detailed discussion of the theory and derivations is given in the publications by Hildebrand and associates cited above. 

I. Fundamental Solutions for a Force Dipole and Other Higher-Order Singularities... 

In view of the properties of the stokeslet and potential dipole solutions, the force acting on the spheroid is simply represented by the accumulative strength of the stokeslet distribution along the centerline of the spheroid. Thus, in dimensionless terms,... 

Ions and charged surfaces can break down the ice slurry structure of water. The electric charges are stronger than dipole forces and tend to pull water molecules away from their groups by attracting the positive or negative ends of the water dipoles. Solutes and water molecules are constantly in motion, but they remain in the vicinity of each other for some period of time. If water molecules remain near an ion longer than the time required for the water molecules to dissociate from the water structure, the ion will have a sphere of water molecules (a solvation sphere or sheath) around itself. The number of water molecules in the closest solvation sphere is called the primary hydration number. 

When an ion and a nearby polar moleeule (dipole) attraet eaeh other, an ion-dipole force results. The most important example takes plaee when an ionic compound dissolves in water. The ions become separated because the attractions between the ions and the oppositely charged poles of the H2O molecules overcome the attractions between the ions themselves. Ion-dipole forces in solutions and their associated energy are discussed fully in Chapter 13. 

The very large dipole moment of polymers results in strong intermolecular forces in solution. Atactic and isotatic polymers have different dipole moments. The dipole moment of the atactic poly(vinyl isobutyl ether) is 10% lower than that of the isotactic form, showing that the isotactic polymer adopts a more ordered structure with group dipoles tending to align parallel to each other. 

Like dissolves like (i) A solute dissolves when the attraction of solvent molecules to solute molecules (or ions) overcomes the intermolecular forces (or ionic bonds) holding solute molecules (or ions) together, (ii) For a solute to dissolve in a solvent, the cohesive forces that hold the solute molecules together (e.g. London forces, dipole-dipole interactions) should be the same as those that hold the solvent molecules together, (iii) Non-polar solvents dissolve non-polar solutes, while polar solvents dissolve polar solutes. 

Three types of intermolecular attractions exist between electrically neutral molecules dispersion forces, dipole-dipole attractions, and hydrogen bonding. The first two are collectively called van der Wacds forces after Johannes van der Waals, who developed the equation for predicting the deviation of gases from ideal behavior, ooo (Section 10.9) Another kind of attractive force, the ion-dipole force, is important in solutions. 

In which mixture do you expect to find ion-dipole forces between solute and... 

Two broad categories of sorption phenomena, adsorption and absorption, can be differentiated by the degree to which the sorbet molecule interacts with the sorbent phase and its freedom to migrate within the sorbent. In adsorption, solute accumulation is in general restrict to the surface or interface between the solution and adsorbent. In contrast, absorption is a process in which solute, transferred from one phase to the other, interpenetrates the sorbent phase by at least several nanometers. Sorption results from a variety of different types of attractive forces between solute, solvent and sorbent molecules. Chemical (covalent or hydrogen bonds), electrostatic (ion-ion, ion-dipole) and physical (Coulombic, Kiesom en-... 

The simplicity of idealized electrostatic solvation models has led to the use of dielectric constant (e) and of the permanent dipole moment (p) as parameters of the so-called solvent polarity. However, the dielectric constant describes only the change in the electric field intensity that occurs between the plates of a condenser, when the latter is removed from vacuum and placed into a solvent. This induces a dipole moment in nonpolar solvent molecules and dipolar molecules are aligned. Hence, the dielectric constant describes only the ability of a solvent to separate electrical charges and orient its dipolar molecules. The intermolecular forces between solute and solvent molecules are, however, much more complicated in addition to the non-specific coulombic, directional, inductive and dispersion interactions, can also be present specific hydrogen bond, electron-pair donor (EPD)/electron-pair acceptor (EPA), and solvophobic interactions in solutions. 

We have just seen that, in the absence of intermolecular forces, two substances spontaneously mix to form a homogeneous solution. We know from Chapter 11, however, that solids and liquids exhibit a number of different types of intermolecular forces including dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole forces (Figure 12.4) t. These forces may promote the formation of a solution or prevent it, depending on the nature of the forces in the particular combination of solute and solvent. 

If the forces between solute and solvent are somewhat weaker than between molecules of the same kind, complete mixing may still occur, the solution formed is nonideal. The solution has a higher enthalpy than the pure components, and the solution process is endothermic. This type of behavior is observed in mixtures of carbon disulfide (CS2), a nonpolar liquid, and acetone, a polar liquid. In these mixtures, the acetone molecules are attracted to other acetone molecules by dipole-dipole interactions and hence show a preference for other acetone molecules as neighbors. A possible explanation of how a solution process can be endothermic and still occur is found on page 649. 

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