Dipole, forces

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

We can identify the intermolecular forces operative in a substance by considering its composition and structure. Dispersion forces are found in all substances. The strength of these attractive forces increases with increasing molecular weight and depends on molecular shapes. With polar molecules dipole-dipole forces are also operative, but these forces often make a smaller contribution to the total intermolecular attraction than do dispersion forces. For example, in liquid HCl, dispersion forces are estimated to accoimt for more than 80% of the total attraction between molecules, while dipole-dipole attractions account for the rest. Hydrogen bonds, when present, make an important contribution to the total intermolecular interaction. 

Positive ends of polar molecules are oriented toward negatively charged 

When comparing the relative strengths of intermolecular attractions, consider these generalizations  

There are three primary categories of secondary forces, also called van der Waals forces dispersion forces, induction forces, and dipole forces—from weakest to strongest. These forces are highly sensitive to the distance between the molecules, with an approximate range of action between 3 and 5 A (1 A = 1 x 10 m). This 

Intermolecular Forces Cohesive Energy Density Typical Polymer Characteristics 

Small Low Relatively flexible rubbery behavior, high permeabiiity. Ex PE, PP 

Large Higher High resistance to stress, high strength, good mechanical properties, low permeability. Ex nylon, EVOH 

Dispersion forces, also called London forces or London dispersion forces, are the most prevalent type of intermolecular forces. They are found in all substances, resulting from the natural fluctuation of the electron cloud in an atom that causes time-varying partial positive and negative charges. These charges sum to zero over time, but at any instant lead to electrostatic attractions between neighboring atoms with opposite charges. Dispersion forces are typically about 0.4-0.8 kj/mol. 

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. 

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The first tenn is tire dipole force, sometimes called tire trapping force, Fj, because it is a conservative force and can be integrated to define a trapping potential for tire atom ... 

A far-off resonance trap (FORT), in contrast, uses tire dipole force ratlier tlian tire spontaneous force to confine atoms and can therefore operate far from resonance witli negligible population of excited states. A hybrid MOT/dipole-force trap was used by a NIST-Maryland collaboration [26] to study cold collisions, and a FORT was... 

Dalibard J and Cohen-Tannoudji C 1985 Dressed-atom approach to atomic motion in laser light the dipole force revisited J.Opt.Soc.Am. B 21707-20... 

Induced dipole/induced dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes In addition to these forces polar molecules engage m dipole-dipole and dipole/mduced dipole attractions The dipole-dipole attractive force is easiest to visualize and is illustrated m Figure 4 3 Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other As its name implies the dipole/induced dipole force combines features of both the induced dipole/mduced dipole and dipole-dipole attractive forces A polar region of one mole cule alters the electron distribution m a nonpolar region of another m a direction that produces an attractive force between them... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

Van der Waals forces (Section 2 17) Intermolecular forces that do not involve ions (dipole-dipole dipole/mduced dipole and induced dipole/induced dipole forces)... 

While neat-resonant light exerts both scattering forces and dipole forces on single atoms, similar forces ate also exerted on larger dielectric objects. 

Ion-Dipole Forces. Ion-dipole forces bring about solubihty resulting from the interaction of the dye ion with polar water molecules. The ions, in both dye and fiber, are therefore surrounded by bound water molecules that behave differently from the rest of the water molecules. If when the dye and fiber come together some of these bound water molecules are released, there is an increase in the entropy of the system. This lowers the free energy and chemical potential and thus acts as a driving force to dye absorption. 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg s of commercial polymers (approx. 104°C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups. 

Dipol-kraft,/. dipole force, -messung,/. dipole measurement, -strahlung, /. dipole radiation. 

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

Figure 2.7 Dipole-dipole forces la) cause polar molecules (a) to 8+ 8- a+ 8- a+ lb)...

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