In essence the three elements listed above will grab the electrons for itself, and leave the hydrogen atom with virtually no electron density since it had only the one. Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back.
This ends up forming a partial bond, which we describe as the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces except for the ionic bond. Could the CH 2 O molecule exhibit hydrogen bonding?
The answer is no, since the hydrogen must be bound to either N, O, or F. Just having one of those species in the molecule is not enough. Now, as these things increase in strength it becomes harder to remove the molecules from each other. Therefore, one would expect the melting and boiling points to be higher for those substances which have strong intermolecular forces. We know that it takes energy to go from a solid to a liquid to a gas. This energy is directly related to the strength of attraction between molecules in the condensed phases.
Since energy is directly proportional to the temperature, the above trends ought to hold true. Each of these processes are endothermic, and scale with the magnitude of the intermolecular forces. Thus, as these intermolecular forces increase, so do the energies requires to melt, vaporize, or sublime go from solid to a gas a species. Every substance also has an associated vapor pressure with it. Number of electrons Greater the number of electrons, the larger the distance between the valence electrons and the nucleus. Attraction of valence electrons to nucleus will be reduced and hence the electron cloud will be polarized more easily increasing the boiling point.
Size volume of the electron cloud In a large electron cloud, the attraction of electrons to the nucleus will not be as great as in smaller cloud, hence the electrons in larger clouds can be polarized easily increasing boiling point. In small atoms such as He, the two 1 s electrons are held close to the nucleus in a very small volume, and electron—electron repulsions are strong enough to prevent significant asymmetry in their distribution.
In larger atoms such as Xe, however, the outer electrons are much less strongly attracted to the nucleus because of filled intervening shells. As a result, it is relatively easy to temporarily deform the electron distribution to generate an instantaneous or induced dipole. The ease of deformation of the electron distribution in an atom or molecule is called its polarizability The ease of deformation of the electron distribution in an atom or molecule.
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- London Dispersion Forces.
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Because the electron distribution is more easily perturbed in large, heavy species than in small, light species, we say that heavier substances tend to be much more polarizable than lighter ones. For similar substances, London dispersion forces get stronger with increasing molecular size. The polarizability of a substance also determines how it interacts with ions and species that possess permanent dipoles, as we shall see when we discuss solutions in Chapter 13 "Solutions". Thus London dispersion forces are responsible for the general trend toward higher boiling points with increased molecular mass and greater surface area in a homologous series of compounds, such as the alkanes part a in Figure The strengths of London dispersion forces also depend significantly on molecular shape because shape determines how much of one molecule can interact with its neighboring molecules at any given time.
For example, part b in Figure Neopentane is almost spherical, with a small surface area for intermolecular interactions, whereas n -pentane has an extended conformation that enables it to come into close contact with other n -pentane molecules. As a result, the boiling point of neopentane 9. As a result, neopentane is a gas at room temperature, whereas n -pentane is a volatile liquid.
All molecules, whether polar or nonpolar, are attracted to one another by London dispersion forces in addition to any other attractive forces that may be present. In general, however, dipole—dipole interactions in small polar molecules are significantly stronger than London dispersion forces, so the former predominate. Arrange n -butane, propane, 2-methylpropane [isobutene, CH 3 2 CHCH 3 ], and n -pentane in order of increasing boiling points. Determine the intermolecular forces in the compounds and then arrange the compounds according to the strength of those forces. The substance with the weakest forces will have the lowest boiling point.
The four compounds are alkanes and nonpolar, so London dispersion forces are the only important intermolecular forces. These forces are generally stronger with increasing molecular mass, so propane should have the lowest boiling point and n -pentane should have the highest, with the two butane isomers falling in between. Of the two butane isomers, 2-methylpropane is more compact, and n -butane has the more extended shape. Consequently, we expect intermolecular interactions for n -butane to be stronger due to its larger surface area, resulting in a higher boiling point.
Molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F and to a much lesser extent Cl and S tend to exhibit unusually strong intermolecular interactions. These result in much higher boiling points than are observed for substances in which London dispersion forces dominate, as illustrated for the covalent hydrides of elements of groups 14—17 in Figure Methane and its heavier congeners in group 14 form a series whose boiling points increase smoothly with increasing molar mass.
This is the expected trend in nonpolar molecules, for which London dispersion forces are the exclusive intermolecular forces. These plots of the boiling points of the covalent hydrides of the elements of groups 14—17 show that the boiling points of the lightest members of each series for which hydrogen bonding is possible HF, NH 3 , and H 2 O are anomalously high for compounds with such low molecular masses. Why do strong intermolecular forces produce such anomalously high boiling points and other unusual properties, such as high enthalpies of vaporization and high melting points?
The answer lies in the highly polar nature of the bonds between hydrogen and very electronegative elements such as O, N, and F. The large difference in electronegativity results in a large partial positive charge on hydrogen and a correspondingly large partial negative charge on the O, N, or F atom.
Because a hydrogen atom is so small, these dipoles can also approach one another more closely than most other dipoles. The combination of large bond dipoles and short dipole—dipole distances results in very strong dipole—dipole interactions called hydrogen bonds An unusually strong dipole-dipole interaction intermolecular force that results when hydrogen is bonded to very electronegative elements, such as O, N, and F.
A hydrogen bond is usually indicated by a dotted line between the hydrogen atom attached to O, N, or F the hydrogen bond donor and the atom that has the lone pair of electrons the hydrogen bond acceptor.
Because each water molecule contains two hydrogen atoms and two lone pairs, a tetrahedral arrangement maximizes the number of hydrogen bonds that can be formed. In the structure of ice, each oxygen atom is surrounded by a distorted tetrahedron of hydrogen atoms that form bridges to the oxygen atoms of adjacent water molecules. The bridging hydrogen atoms are not equidistant from the two oxygen atoms they connect, however. Instead, each hydrogen atom is pm from one oxygen and pm from the other. The resulting open, cagelike structure of ice means that the solid is actually slightly less dense than the liquid, which explains why ice floats on water rather than sinks.
Each water molecule accepts two hydrogen bonds from two other water molecules and donates two hydrogen atoms to form hydrogen bonds with two more water molecules, producing an open, cagelike structure. The structure of liquid water is very similar, but in the liquid, the hydrogen bonds are continually broken and formed because of rapid molecular motion. Hydrogen bond formation requires both a hydrogen bond donor and a hydrogen bond acceptor. Because ice is less dense than liquid water, rivers, lakes, and oceans freeze from the top down.
In fact, the ice forms a protective surface layer that insulates the rest of the water, allowing fish and other organisms to survive in the lower levels of a frozen lake or sea.
If ice were denser than the liquid, the ice formed at the surface in cold weather would sink as fast as it formed. Bodies of water would freeze from the bottom up, which would be lethal for most aquatic creatures.
Compounds such as HF can form only two hydrogen bonds at a time as can, on average, pure liquid NH 3. Consequently, even though their molecular masses are similar to that of water, their boiling points are significantly lower than the boiling point of water, which forms four hydrogen bonds at a time. Draw the hydrogen-bonded structures. Asked for: formation of hydrogen bonds and structure. A Identify the compounds with a hydrogen atom attached to O, N, or F.
These are likely to be able to act as hydrogen bond donors. B Of the compounds that can act as hydrogen bond donors, identify those that also contain lone pairs of electrons, which allow them to be hydrogen bond acceptors.
If a substance is both a hydrogen donor and a hydrogen bond acceptor, draw a structure showing the hydrogen bonding. A Of the species listed, xenon Xe , ethane C 2 H 6 , and trimethylamine [ CH 3 3 N] do not contain a hydrogen atom attached to O, N, or F; hence they cannot act as hydrogen bond donors. B The one compound that can act as a hydrogen bond donor, methanol CH 3 OH , contains both a hydrogen atom attached to O making it a hydrogen bond donor and two lone pairs of electrons on O making it a hydrogen bond acceptor ; methanol can thus form hydrogen bonds by acting as either a hydrogen bond donor or a hydrogen bond acceptor.
The hydrogen-bonded structure of methanol is as follows:. Identify the intermolecular forces in each compound and then arrange the compounds according to the strength of those forces. Electrostatic interactions are strongest for an ionic compound, so we expect NaCl to have the highest boiling point. To predict the relative boiling points of the other compounds, we must consider their polarity for dipole—dipole interactions , their ability to form hydrogen bonds, and their molar mass for London dispersion forces. Helium is nonpolar and by far the lightest, so it should have the lowest boiling point.
Consequently, N 2 O should have a higher boiling point. Because the boiling points of nonpolar substances increase rapidly with molecular mass, C 60 should boil at a higher temperature than the other nonionic substances. Molecules in liquids are held to other molecules by intermolecular interactions, which are weaker than the intramolecular interactions that hold the atoms together within molecules and polyatomic ions. Transitions between the solid and liquid or the liquid and gas phases are due to changes in intermolecular interactions but do not affect intramolecular interactions.
The three major types of intermolecular interactions are dipole—dipole interactions, London dispersion forces these two are often referred to collectively as van der Waals forces , and hydrogen bonds. London dispersion forces are due to the formation of instantaneous dipole moments in polar or nonpolar molecules as a result of short-lived fluctuations of electron charge distribution, which in turn cause the temporary formation of an induced dipole in adjacent molecules.
Larger atoms tend to be more polarizable than smaller ones because their outer electrons are less tightly bound and are therefore more easily perturbed. Hydrogen bonds are especially strong dipole—dipole interactions between molecules that have hydrogen bonded to a highly electronegative atom, such as O, N, or F.
The resulting partially positively charged H atom on one molecule the hydrogen bond donor can interact strongly with a lone pair of electrons of a partially negatively charged O, N, or F atom on adjacent molecules the hydrogen bond acceptor. What is the main difference between intramolecular interactions and intermolecular interactions? Which is typically stronger? How are changes of state affected by these different kinds of interactions?
Related Chapter 004, Intermolecular Interactions
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