Strength of Intermolecular Forces

11 Key excerpts on "Strength of Intermolecular Forces"

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  eBook - PDF

  Chemistry

  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.

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  Chemistry

  • 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.

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  Understanding Chemistry through Cars

  • 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.

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  Chemistry

  The Molecular Nature of Matter

  • Neil D. Jespersen, Alison Hyslop(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  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+ 542 CHAPTER 11 Intermolecular Attractions and the Properties of Liquids and Solids TABLE 11.3 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 N—H, O—H, and F—H 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.

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  Survival Guide to General Chemistry

  • Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  2 is a gas.

  Example: Compare the strength of total dispersion force between C3 H8 and C22 H46 .

  Each of these molecules are very weakly polar and comparison is between hydrocarbons of similar chemical structure. Dispersion force is the dominant intermolecular force operating. The total strength of intermolecular force is predicted to be proportional to the molar mass of each based on the number of similar atoms in the molecule: MM of C3 H8 = 44 g/mole; MM of C22 H46 = 310 g/mole. The order of expected intermolecular force strength is C22 H46 > C3 H8 ; the phases of matter at room temperature confirm this order: C22 H46 is a solid; C3 H8 is a gas.

  II	A GUIDELINE FOR COMPARING TOTAL Strength of Intermolecular Forces IN INDIVIDUAL COMPOUNDS

  Boiling point (b.p.) and melting point (m.p.) temperatures for compounds indicate the energy required to break the attractive forces between molecules. Thus, these measures are proportional to the total Strength of Intermolecular Forces: a higher melting or boiling temperature corresponds to a greater total intermolecular force. A very general representative mid-point range for relative strengths of intermolecular force is shown below.

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  The Specificity of Serological Reactions

  • Karl Landsteiner(Author)
  • 2013(Publication Date)
  • Dover Publications

    (Publisher)

  VIII MOLECULAR STRUCTURE AND INTERMOLECULAR FORCES

  BY LINUS PAULING

  As our knowledge of the structure of molecules has greatly increased in recent years it has become clear that the physiological activity of substances is correlated not alone with their ability to take part in reactions in which strong chemical bonds are broken and formed, but also with the relatively weak forces which their molecules exert on other molecules. It is, indeed, probable that the high specificity which often characterizes physiological activity is in most cases specificity of intermolecular interaction rather than primarily of chemical reaction with the rupture and formation of strong bonds.

  Atoms interact with other atoms in many ways. In the present discussion we divide interatomic forces into two classes, strong forces and weak forces. The strong forces are those which are responsible for the existence of stable molecules; they are the forces which lead to the formation of strong chemical bonds, with bond energies between 10 and 100 kilocalories per mole. The weak interatomic forces, including van der Waals forces and “hydrogen-bond” forces, have energies of a few kilocalories per mole of interacting atom pairs; these forces are effective in holding molecules together without disrupting their individual structures, and also in operating between different parts of a large “loose-jointed” molecule in such a way as to hold it to a particular configuration.

  The selection of the energy value of about 10 kilocalories per mole as the transition value between the two classes of interatomic interactions has its justification in statistical considerations based on thermal equilibrium at room temperature. Under ordinary circumstances at room temperature (that is, with the reacting substances present in reasonable concentrations) interaction of the “strong” class between two atoms will hold them together as a complex which is not significantly dissociated by the disrupting action of thermal agitation, whereas interaction of the “weak” class will not do this. Only by cooperation of the interactions between many atoms of one molecule and many atoms of another molecule can weak interatomic forces give rise to a stable intermodular bond.

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  Chemistry: Atoms First

  • 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.

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  Chemical Physics & Physical Chemistry

  • (Author)
  • 2014(Publication Date)
  • Library Press

    (Publisher)

  • Strength of nylon and cellulose fibres. Relative strength of forces Bond type Dissociation energy (kcal), Covalent 400 ________________________ WORLD TECHNOLOGIES ________________________ Hydrogen bonds 12-16 Dipole-dipole 0.5 - 2 London (van der Waals) Forces <1 Note: this comparison is only approximate- the actual relative strengths will vary depending on the molecules involved. Quantum mechanical explanation of intermolecular interactions In the natural sciences, an intermolecular force is an attraction between two molecules or atoms. They occur from either momentary interactions between molecules (the London dispersion force) or permanent electrostatic attractions between dipoles. They can be explained using a simple logical approach, or using a quantum mechanical approach. Perturbation theory Hydrogen bonding, dipole–dipole interactions, and London (Van der Waals) forces are most naturally accounted for by Rayleigh–Schrödinger perturbation theory (RS-PT). In this theory—applied to two monomers A and B —one uses as unperturbed Hamiltonian the sum of two monomer Hamiltonians, In the present case the unperturbed states are products with and Supermolecular approach The early theoretical work on intermolecular forces was invariably based on RS-PT and its antisymmetrized variants. However, since the beginning of the 1990s it has become possible to apply standard quantum chemical methods to pairs of molecules. This approach is referred to as the supermolecule method . In order to obtain reliable results one must include electronic correlation in the supermolecule method (without it dispersion is not accounted for at all), and take care of the basis set superposition error . This is the effect that the atomic orbital basis of one molecule improves the basis of the other. Since this improvement is distance dependent, it easily gives rise to artifacts.

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  Fundamentals of Ceramics

  • Michel Barsoum(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  4

  Effect of Chemical Forces on Physical Properties

  Now how curiously our ideas expand by watching these conditions of the attraction of cohesion! — how many new phenomena it gives us beyond those of the attraction of gravitation! See how it gives us great strength.

  Michael Faraday, On the Various Forces of Nature

  4.1 Introduction

  The forces of attraction between the various ions or atoms in solids determine many of their properties. Intuitively, it is not difficult to appreciate that a strongly bonded material would have a high melting point and be stiff. In addition, it can be shown, as is done below, that its theoretical strength and surface energy will also increase, with a concomitant decrease in thermal expansion. In this chapter, semiquantitative relationships between these properties and the depth and shape of the energy well, described in Chap. 2, are developed.

  In Sec. 4.2, the importance of bond strengths on the melting points of ceramics is elucidated. In Sec. 4.3, how strong bonds result in solids with low coefficients of thermal expansion is discussed. In Sec. 4.4, the relationships between bond strengths, stiffness and theoretical strengths is developed. Sec. 4.5 relates bond strengths to surface energies.

  4.2 Melting Points

  Fusion, evaporation and sublimation result when sufficient thermal energy is supplied to a crystal to overcome the potential energy holding its atoms together. Experience has shown that, at constant pressure, a pure substance will melt at a fixed temperature, with the absorption of heat. The amount of heat absorbed is known as the heat of fusion, Δ H

  f

  , and it is the heat required for the transformation

  Solid → Liquid

  Δ Hf is a measure of the enthalpy difference between the solid and liquid states at the melting point. The entropy difference, Δ Sf , between the liquid and solid is defined by

  Δ S f = Δ H f T m

  (4.1)

  where Tm is the melting point in degrees Kelvin. The entropy difference Δ Sf is a direct measure of the degree of disorder that arises in the system during the melting process and is by necessity positive, since the liquid state is always more disordered than the solid. The melting points and Δ Sf values for a number of ceramics are listed in Table 4.1 . Inspection of Table 4.1 reveals that there is quite a bit of variability in the melting points or ceramics.35

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  Chemical Principles of Nanoengineering

  • Andrea R. Tao(Author)
  • 2023(Publication Date)
  • Wiley-VCH

    (Publisher)

  Intermolecular forces are responsible for these “weak” or secondary bonds that occur between molecules, particles, and surfaces. The bonds that result from intermolecular forces lack specificity, stoichiometry, and directionality. These forces can also result in interactions that occur over long distances – much longer than interatomic bond lengths.

  As we will see throughout Chapter 1 , intermolecular forces play an important role in dictating materials and molecular behavior at the nanoscale. We will cover five different types of intermolecular forces: electrostatic, hydrogen bonding, van der Waals (vdW ), hydrophobic, and steric forces. For each of these, we will derive and discuss their universal force laws. We will also discuss the differences between these forces for molecules versus nanoscale objects. Finally, we will develop an understanding of how potential energy diagrams can be used to predict the overall intermolecular interactions between two objects as a function of separation distance. This knowledge will be applied toward understanding the behavior of nanosystems ranging from atoms and molecules (e.g. DNA and polymers) to particles and other nanomaterials (e.g. liposomes, metal nanoparticles, C60 ).

  1.1 The Pairwise Potential

  Intermolecular forces can lead to attraction or repulsion between atoms, molecules, particles, and surfaces, and contribute significantly to how nanoscale materials and systems behave. These forces are classified as conservative forces, meaning that they satisfy the relationship:

  (1.1)

  where F is the force, V(r) is the potential energy of the object, and r is distance. Because of this relationship, potential energy can be used as a descriptor of whether the force between two objects is attractive or repulsive.

  We often consider pairwise potentials that describe V(r) as a function of separation distance to determine attraction or repulsion. For example, two possible pairwise potentials between two spherical particles of radius R

  s

  are depicted in Figure 1.2

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  Intermolecular and Surface Forces

  • Jacob N. Israelachvili(Author)
  • 2010(Publication Date)
  • Academic Press

    (Publisher)

  Physical bonds usually lack the specificity, stoichiometry, and strong directionality of covalent bonds. They are therefore the ideal candidates for holding molecules together in liquids, since the molecules can move about and rotate while still remaining “bonded” to one another. Strictly, physical “bonds” should not be considered as bonds at all, for during covalent binding the electron charge distributions of the uniting atoms change completely and merge, whereas during physical binding they are merely perturbed, the atoms remaining as distinct entities. Nevertheless, physical binding forces can be as strong as covalent bonds, and even the weakest is strong enough to hold together all but the smallest atoms and molecules in solids and liquids at STP as well as in colloidal and biological assemblies. These properties, coupled with the long-range nature of physical forces, make them the regulating forces in all phenomena that do not involve chemical reactions.

  3.3 Coulomb Forces or Charge-Charge Interactions, Gauss’s Law

  The inverse-square Coulomb force between two charged atoms, or ions, is by far the strongest of the physical forces that we are considering here; it is even stronger than most chemical binding forces.

  The electric field E at a distance r away from a charge Q 1 is defined by3

  (3.1) where ɛ is the dielectric permittivity or constant of the medium.4 This field, when acting on a second charge Q 2 at r , gives rise to a force known as the Coulomb force or Coulomb Law:

  (3.2) The free energy for the Coulomb interaction between two charges Q 1 and Q 2 is therefore given by

  (3.3) where the “reference state” of zero energy is taken to be at r = ∞. The expression on the right of Eq. (3.3) is commonly used for ionic interactions in aqueous solutions where the magnitude and sign of each ionic charge is given in terms of the elementary electron charge (e = 1.602 × 10−19 C) multiplied by the ionic valency z . For example, z = +1 for monovalent cations such as Na+ ; z = –1 for monovalent anions such as Cl− ; z = +2 for divalent cations such as Ca2+ ; and so on. For like charges, both w and F are positive and the force is repulsive, while for unlike charges they are negative and the force is attractive.

  Let us put the strength of the Coulomb interaction into perspective. For two isolated ions (e.g., Na+ and Cl– ) in contact, r

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