Chemistry
Intermolecular Forces
Intermolecular forces are the attractive or repulsive forces between molecules. These forces play a crucial role in determining the physical properties of substances, such as boiling and melting points, viscosity, and solubility. The main types of intermolecular forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
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11 Key excerpts on "Intermolecular Forces"
eBook - PDF- Geoffrey M. Bowers, Ruth A. Bowers(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
For example, the very strong intermolec-ular forces between H 2 O molecules in water are the reason behind the anomalously high boiling point for this small molecule. In this chapter, we will introduce the types of Intermolecular Forces and how they manifest in the chemistry of your car. 4.1 Types of Intermolecular Forces Chemistry Concepts : Intermolecular Forces, organic chemistry Expected Learning Outcomes : • Name the common types of Intermolecular Forces • Understand the origin of the various forces • List some common material properties that depend upon IM forces Molecules are typically considered to be atoms held together in fixed atomic arrangements via chemical bonds, but they can also be con-sidered collections of protons, neutrons, and electrons with relatively well-defined spatial relationships. Sometimes these arrangements of pro-tons, neutrons, and electrons are not symmetrically distributed in space, 90 Understanding chemistry through cars leading to molecular dipole moments that cause individual molecules to function on some basic level as small magnets. These asymmetric distributions of particles/charge also give rise to several types of elec-tronic interactions, such as electrostatic attractions between regions of partial positive and partial negative charge in an H 2 O molecule. Most chemistry students have a fairly well-developed idea about molecular structure, molecular dipole moments, and properties of molecules in isolation by the end of general chemistry, but what about understand-ing what happens when two molecules approach one another in space? For example, how and why does a droplet of water stay together on the surface of your car rather than dispersing completely? The answer is that molecules can and do interact via electronic and magnetic attractions/ repulsions that are too weak to form chemical bonds. It is these relatively weak attractive and repulsive forces that make up the Intermolecular Forces we discuss in this chapter.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
For example, to overcome the IMFs in one mole of liquid HCl and convert it into gaseous HCl requires only about 17 kilojoules. However, to break the covalent bonds between the hydrogen and chlorine atoms in one mole of HCl requires about 25 times more energy—430 kilojoules. Figure 10.5 Intramolecular forces keep a molecule intact. Intermolecular Forces hold multiple molecules together and determine many of a substance’s properties. All of the attractive forces between neutral atoms and molecules are known as van der Waals forces, although they are usually referred to more informally as intermolecular attraction. We will consider the various types of IMFs in the next three sections of this module. Dispersion Forces One of the three van der Waals forces is present in all condensed phases, regardless of the nature of the atoms or molecules composing the substance. This attractive force is called the London dispersion force in honor of German- born American physicist Fritz London who, in 1928, first explained it. This force is often referred to as simply the dispersion force. Because the electrons of an atom or molecule are in constant motion (or, alternatively, the electron’s location is subject to quantum-mechanical variability), at any moment in time, an atom or molecule can develop a temporary, instantaneous dipole if its electrons are distributed asymmetrically. The presence of this dipole can, in turn, distort the electrons of a neighboring atom or molecule, producing an induced dipole. These two rapidly Link to Learning 520 Chapter 10 | Liquids and Solids This OpenStax book is available for free at http://cnx.org/content/col11760/1.9 fluctuating, temporary dipoles thus result in a relatively weak electrostatic attraction between the species—a so-called dispersion force like that illustrated in Figure 10.6. Figure 10.6 Dispersion forces result from the formation of temporary dipoles, as illustrated here for two nonpolar diatomic molecules.
eBook - PDF- William R. Robinson, Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
For example, to overcome the IMFs in one mole of liquid HCl and convert it into gaseous HCl requires only about 17 kilojoules. However, to break the covalent bonds between the hydrogen and chlorine atoms in one mole of HCl requires about 25 times more energy—430 kilojoules. Figure 10.5 Intramolecular forces keep a molecule intact. Intermolecular Forces hold multiple molecules together and determine many of a substance’s properties. All of the attractive forces between neutral atoms and molecules are known as van der Waals forces, although they are usually referred to more informally as intermolecular attraction. We will consider the various types of IMFs in the next three sections of this module. Dispersion Forces One of the three van der Waals forces is present in all condensed phases, regardless of the nature of the atoms or molecules composing the substance. This attractive force is called the London dispersion force in honor of German- born American physicist Fritz London who, in 1928, first explained it. This force is often referred to as simply the dispersion force. Because the electrons of an atom or molecule are in constant motion (or, alternatively, the electron’s location is subject to quantum-mechanical variability), at any moment in time, an atom or molecule can develop a temporary, instantaneous dipole if its electrons are distributed asymmetrically. The presence of this dipole can, in turn, distort the electrons of a neighboring atom or molecule, producing an induced dipole. These two rapidly Link to Learning 522 Chapter 10 | Liquids and Solids This OpenStax book is available for free at http://cnx.org/content/col12012/1.7 fluctuating, temporary dipoles thus result in a relatively weak electrostatic attraction between the species—a so-called dispersion force like that illustrated in Figure 10.6. Figure 10.6 Dispersion forces result from the formation of temporary dipoles, as illustrated here for two nonpolar diatomic molecules.
eBook - PDF- John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
In general, the strongest force that exists between two molecules is the one we are interested in since it will dictate the magnitudes of properties such as the melting and boiling points. Key Concept All Intermolecular Forces are electrostatic and under most conditions are attractive. Key Concept Intermolecular Forces exist among all ions, dipoles, and uncharged non-polar species. TABLE 8.2 Intermolecular Forces Name of Force Approximate Potential Energy Range ( k J/mol ) Attraction Examples Ion–dipole 40–600 Charged ion is attracted to the oppositely charged end of the dipole Cl − −H 2 O Na + −H 2 O Hydrogen bond 5–50 Positively polarized H atom is attracted to a lone pair on an electronegative atom (CH 3 ) 2 CO−H 2 O Dipole–dipole 5–25 Mutual attraction of two dipoles HCl−HCl Ion-induced dipole 3–15 The ion induces a dipole in the other species and is then attracted to it Ca 2+ −O 2 Dipole-induced dipole 2–10 A dipole induces a dipole in the other species and is then attracted to it HCl−Cl 2 Dispersion (London) 0.1–5 Momentary shifts in electron clouds produce momentary dipoles, which attract each other He−He Ion–Dipole Forces The strongest intermolecular force is that between an ion and a permanent dipole. For instance, when an ionic substance dissolves in water, strong forces of attraction between the 8. 2 Types of Intermolecular Forces 363 ions in the solid material and the (numerous) polar water molecules overcome the ion–ion forces in the solid. This is discussed in more detail in Section 9.2. Dispersion Forces The weakest forces between two molecules are dispersion forces. Dispersion forces exist because the electron cloud of any molecule distorts easily. For example, consider what happens when two halogen molecules approach each other. Each mol- ecule contains positive nuclei surrounded by a cloud of negative electrons. As two molecules approach each other, the nuclei of one molecule attract the electron cloud of the other.
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
• explain how the molecular attractive forces affect macroscopic properties. • apply the concept of dynamic equilibrium to explain changes of state. • explain what is meant by vapor pressure and how it reveals the relative strengths of intermolecular attractions. • utilize the concepts of boiling points and normal boiling points. • calculate energy changes involved in changes of state. • interpret and effectively use phase diagrams. • use Le Châtelier’s principle to explain changes of state. • explain how heats of vaporization are determined experimentally. • describe how crystal structures are based on repeating molecular geometries. • illustrate the basics of X-ray crystallography. • describe that Intermolecular Forces determine the properties of crystal types. 11.1 Intermolecular Forces 535 11.1 Intermolecular Forces There are differences among gases, liquids, and solids that are immediately obvious and famil- iar to everyone. For example, any gas will expand to fill whatever volume is available, even if it has to mix with other gases to do so. Liquids and solids, however, retain a constant volume when transferred from one container to another. A solid, such as an ice cube, also keeps its shape, but a liquid such as soda conforms to the shape of whatever bottle or glass we put it in. Properties such as the ones we’ve described can be understood in terms of the way the particles are distributed in the three states of matter, which is summarized in Figure 11.1. Distance and Intermolecular Forces If you’ve ever played with magnets, you know that their mutual attraction weakens rapidly as the distance between them increases. Intermolecular attractions are similarly affected by the distance between molecules. In gases, the molecules are far apart and intermolecular attrac- tions negligible; as a result, chemical composition has little effect on the properties of a gas.
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
With this knowledge, you should now be able to make some estimate of the nature and relative strengths of intermolecular attractions if you know the molecular structure of a substance. This will enable you to understand and sometimes predict how the physical properties of different substances compare. For example, we’ve already mentioned that boiling point is a property that depends on the strengths of intermolecular attractions. By being able to compare Intermolecular Forces in different substances, we can sometimes predict how their boiling points compare. This is illustrated in Example 11.1. Figure 11.10 | Ion–dipole attractions between water molecules and ions. (a) The negative ends of water dipoles surround a cation and are attracted to the ion. (b) The positive ends of water molecules surround an anion, which gives a net attraction. (a) (b) δ- δ- δ- δ+ δ+ δ+ δ+ δ+ δ+ δ- δ+ δ+ - δ+ δ- δ- δ- δ- δ+ δ+ δ+ + δ+ δ+ δ+ δ+ Figure 11.11 | Ion–dipole attractions hold water molecules in a hydrate. Water molecules are arranged at the vertices of an octahedron around an aluminum ion in AlCl 3 # 6H 2 O. Al 3+ 524 Chapter 11 | Intermolecular Attractions and the Properties of Liquids and Solids Summary of Intermolecular Attractions Intermolecular Attraction Types of Substances that Exhibit Attraction Strength Relative to a Covalent Bond Dipole–dipole attractions Occur between molecules that have permanent dipoles (i.e., polar molecules) 1%–5% Hydrogen bonding Occurs when molecules contain NOH, OOH, and F OH bonds 5%–10% London dispersion forces All atoms, molecules, and ions experience these kinds of attractions. They are present in all substances Depends on sizes and shapes of molecules.
eBook - PDF- George C. King(Author)
- 2023(Publication Date)
- Wiley(Publisher)
2 The forces that bind atoms together At the beginning of Chapter 1, we made the statement that atoms attract each other when they are a little distance apart, but repel when they are squeezed together. Evidence that atoms are attracted to each other comes from the fact that atoms combine together to form solids and liquids. Evidence that atoms repel each other comes from the fact that solids and liquids resist being compressed. These physical properties arise from the forces that act between the atoms. These forces and the resulting bonding of the atoms are the subjects of the present chapter. We will describe the general characteristics of the forces that bind atoms together and the resulting potential energy of the atoms. And we will describe the principal kinds of inter- atomic bonding: van der Waals, ionic, covalent, and metallic bonding. We will see that all bonding is a con- sequence of the electrostatic interaction between nuclei and electrons. And we will see that the interatomic interactions that we will describe on the microscopic scale relate directly to the properties of matter that are observed in the laboratory. 2.1 General characteristics of interatomic forces Before going into detail about particular types of interatomic force, we describe some general features of such forces. We are interested in whether the force acting between atoms is attractive or repulsive. We are also interested in the way the strength of the force varies with interatomic separation. To discuss these two aspects, we imagine an atom fixed in place at the origin (r = 0) of a coordinate system and a second atom a distance r away, where r is the distance between the centres of the two atoms. This arrangement is illus- trated in Figure 2.1. We make the assumption that the force depends only on distance r. If the force is repul- sive, the force acts to increase the separation of the two atoms, i.e.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Library Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 5 Intermolecular Force Intermolecular Forces are relatively weak forces between molecules or between different chemical groups of the same large molecule which act at the distances of Van der Waals radii or larger. This is in contrast to chemical bonds which are stronger, act at the shorter distances, and are formed between different atoms of the same molecule. London dispersion forces Interaction energy of argon dimer. The long-range part is due to London dispersion forces London dispersion forces (LDF, also known as dispersion forces , London forces , induced dipole–induced dipole forces ) is a type of force acting between atoms and ________________________ WORLD TECHNOLOGIES ________________________ molecules. They are part of the van der Waals forces. The LDF is named after the German-American physicist Fritz London. The LDF is a weak intermolecular force arising from quantum induced instantaneous polarization multipoles in molecules. They can therefore act between molecules without permanent multipole moments. London forces are exhibited by nonpolar molecules because of the correlated movements of the electrons in interacting molecules. Because the electrons from different molecules start feeling and avoiding each other, Electron density in a molecule becomes redis-tributed in proximity to another molecule. This is frequently described as formation of instantaneous dipoles that attract each other. London forces are present between all chemical groups and usually represent main part of the total interaction force in condensed matter, even though they are generally weaker than ionic bonds and hydrogen bonds. This is the only attractive intermolecular force present between neutral atoms (e.g., a noble gas). Without London forces, there would be no attractive force between noble gas atoms, and they wouldn't exist in liquid form.
eBook - PDF- Milo D. Koretsky(Author)
- 2012(Publication Date)
- Wiley(Publisher)
220 ► Chapter 4. Equations of State and Intermolecular Forces Dispersion (London) Forces Nonpolar molecules, such as N 2 and O 2 , show forces of attraction; otherwise they would not condense or freeze at low temperatures. Yet they do not have a dipole moment, and the pure species are not subject to dipole–dipole interactions and induction. The attrac- tive interactions between nonpolar molecules result from a third type of interaction; dispersion (or London) forces. Dispersion is inherently a quantum-mechanical phenomenon; we would need to understand quantum electrodynamics to develop a rigorous model of dispersion. How- ever, it can be viewed “classically” as follows: Nonpolar molecules are really only non- polar when the electron cloud is averaged over time. In a given “snapshot” of time, the molecule has a temporary dipole moment. Dispersion forces result from the instantane- ous nonsymmetry of the electron cloud surrounding a nucleus. The instantaneous dipole moment induces a dipole in a neighboring molecule, leading to an attractive force. Using quantum mechanics and perturbation theory, London developed the follow- ing expression for the energy of attraction of symmetric molecules i and j: G ij < 2 3 2 a i a j r 6 ¢ I i I j I i 1 I j ≤ CGS units (4.8) where I is the first ionization potential, that is, the energy required for the following reaction: M S M 1 1 e. Ionization potentials of common species are presented in Table 4.1. These “temporary dipole” interactions also have a 1/r 6 dependence on position. They also depend on the polarizability of each species involved, since the extent of the instantaneous dipole is related to the looseness of the nucleus’s control of the valence electrons; similarly, the induc- tion in the neighboring molecule depends on its polarizability. While Equation (4.8) was developed for nonpolar species, polar molecules are subject to dispersion interactions as well.
eBook - PDF- Iam Torrens(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
CHAPTER I THE NATURE OF INTERATOMIC FORCES 1.1 ATOMIC INTERACTIONS IN PHYSICAL PHENOMENA Almost all physical phenomena, discounting the world inside the atomic nucleus, may be attributed directly or indirectly to the forces between atoms. Basic concepts such as temperature and pressure, the strength of a solid or the viscosity of a liquid, as well as our own physical form and that of this book, are intimately related to the forces between atoms. It has long been known that an atom, made up of nucleus and orbital electrons, is largely empty space, like the solar system. Therefore, the idea of an atom as a basic entity is rather nebulous. But such are the forces binding the electrons to the nucleus that except for quite high energy collisions the atom may be thought of as an entity about as penetrable, from the point of view of another atom, as a solid hard rubber ball. This is, of course, an over-simplification. In order to estimate the force acting between atoms in an ensemble or in a collision process, it is often necessary to take into account the elementary components of the atom. Thus, in the rubber ball analogy, we 3 4 I The Nature of Interatomic Forces would have to explain why the hardness varies with the energy of a collision and why under certain circumstances two rubber balls attract each other when they are not touching. With the normal concept of an atom made up of a central nucleus and orbital electrons, let us consider the forces intervening when it interacts with another atom. We discount any subnuclear phenomena since the forces holding together the nuclear components—neutrons, protons, and mesons— are of a completely different nature and orders of magnitude stronger than any interatomic forces. Thus the nucleus is effectively a solid body of diameter ~ 1 0 1 2 cm with a positive charge depending on the number of protons present.
eBook - PDF- Joseph O. Hirschfelder(Author)
- 2009(Publication Date)
- Wiley-Interscience(Publisher)
The permanent moments produce a field that distorts the electronic structures of neighboring molecules leading to an additional interaction, the induction energy. Since the distortions of molecules in their ground electronic states always lower the total energy, the induction energy is associated 107 108 A. D. BUCKINGHAM with an attractive intermolecular force. Both the electrostatic and induction energies are determined by the properties of the free mole- cules ; also the dispersion energy of Fritz London may be approximately related to the polarizabilities describing the distortion of the free molecules by external fields.' Therefore, detailed knowledge of molec- ular charge distributions and polarizabilities is essential for an under- standing of Intermolecular Forces. In the study of molecular interactions, we are normally concerned with the question: What is the difference between the energy of a group of molecules (preferably a pair, although larger clusters are often of interest) and the energy of separate molecules for fixed molecular positions and orientations? This interaction energy is usually small com- pared to molecular-electronic and vibrational energies, so there is no difficulty in assigning the molecules to particular internal states; the energy is averaged over the nuclear vibrational motion. However, the interaction energy may be much larger than the difference between rotational energy levels; the rotational and translational motions of the interacting molecules can, therefore, be very different from those of the free molecules. The basic problem is the evaluation of the energy as a function of relative molecular position and orientation. When this has been solved, the effects of the interaction can be determined by con- sidering the translational and rotational motion; in some cases this is a formidable task, but often the occupied states have energy separations that are small compared to kT, and a classical treatment suffices.
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