Types of Intermolecular Forces

Intermolecular forces are attractive or repulsive forces between atoms, molecules, and/or ions. The attractive forces compete with the randomizing factor of kinetic energy. The structure that a particular substance exhibits depends on the 

Covalent, metallic, and ionic bonds are very strong interactions. Some people consider these to be intermolecular forces. The following are weaker intermolecular forces. They appear in approximate order of decreasing strength. Even though weaker than bonds, they are nonetheless important. 

Intermolecular forces occur between molecules not within molecules. For example, covalent bonds hold a water molecule together, but intermolecular forces, not covalent bonds, attract separate water molecules to each other. 

This attraction is due to the attraction of an ion (cation or anion) with one end of a polar molecule (dipole). This type of attraction is especially important in aqueous salt solutions where the ion attracts water molecules and may form a hydrated ion (i.e., A1(H20) +). 

The ion-dipole intermolecular force is different in that two different species must be present an ion from one species and a polar molecule from a different species. 

GOAL 5 Identify and describe or explain dipole forces and hydrogen bonds. 

6 Given the structure of a molecule, or information from which it may be determined, identify the significant intermolecular forces present. 

7 Given the molecular structures of two substances, or information from which they may be obtained, compare or predict relative values of physical properties that are related to them. 

It was stated in the last section that attractive forces between particles are electrostatic in character the attractions are between positive and negative charges. But atoms and molecules are electrically neutral. How can there be electrostatic attractions The answer is that the distribution of electrical charge within the molecule is not always uniform. Some molecules are polar and some are nonpolar. In addition, some molecules are large and some are small. Molecular polarity and size both contribute to intermolecular attraction and therefore to physical properties. 

Three kinds of intermolecular forces can be traced to electrostatic attractions dipole forces, induced dipole forces, and hydrogen bonds. 

There are several types of intermolecular forces ion-dipole, dipole-dipole, hydrogen bonding, dipole-induced dipole, and dispersion forces. As we discuss these intermolecular forces (also called van der Waals forces), look at Table 12.2, which compares them with the stronger intramolecular (bonding) forces. 

Force Model Basis of Attraction Energy (kJ/mol) Example 

Like covalent radii blue quarter-circles and top numbers), van der Waals radii [red quarter-circles and bottom numbers) increase down a group and decrease across a period. The covalent radius of an element is always less than its van der Waals radius. 

CHAPTER 12 Intermolecubr Forces Liquids, Solids, and Phase Changes 

There are several types of intermolecular interactions, each of which involves electrostatic forces of some kind or other. An example is provided by the ion-ion interactions between cations and anions. Pure electrostatic (Coulombic) interactions are long-range and many, such as hydrogen bonding, are directional. Molecules can, however, be distorted by the electric fields of surrounding molecules, even if the molecules themselves are electrically neutral. 

If molecules are polar, that is, if they have a dipole moment, they may interact with each other in a head-to-tail arrangement (a dipole-dipole interaction). If one molecule is polar (with a dipole moment) and the other is nonpolar but polarizable, the polar molecule may induce a dipole in the nonpolar molecule (a dipole-induced dipole interaction). These two dipoles, one permanent and the 

Other induced, will interact in the same way as two dipoles. The strength of this interaction depends on the magnitude of the permanent dipole moment of the polar molecule, and on the polarizabiHty of the second molecule. Even if the two molecules are nonpolar, there can be attractive, low-energy molecular interactions between them. These are induced dipole-induced dipole interactions, also called London dispersion forces, in which a nonpolar molecule induces a small instantaneous dipole in another nearby polar molecule. The force F (in dynes) between two charges q and q (in electrostatic units) is expressed by Coulomb s equation  

Waals (VDW) radius is one-half the distance between adjacent nonbonded atoms ( x VDW tSs-tance), and the covalent radius is one-half the distance between bonded atoms (5 X bond length). 

CHAPfER 12 hlermolecukir Forces Liquids, Sotds, and Phase Changes 

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The term polymer is derived from the Greek words poly and meros, meaning many parts. We noted in the last section that the existence of these parts was acknowledged before the nature of the interaction which held them together was known. Today we realize that ordinary covalent bonds are the intramolecular forces which keep the polymer molecule intact. In addition, the usual type of intermolecular forces—hydrogen bonds, dipole-dipole interactions, and London forces—hold assemblies of these molecules together in the bulk state. The only thing that is remarkable about these molecules is their size, but that feature is remarkable indeed. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

Ihe boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved. The stronger the intermolecular forces, the higher the boiling point of the substance. In the remainder of this section, we examine the nature of the three different types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. 

The most common type of intermolecular force, found in all molecular substances, is referred to as a dispersion force. It is basically electrical in nature, involving an attraction between temporary or induced dipoles in adjacent molecules. To understand the origin of dispersion forces, consider Figure 9.8. 

What types of intermolecular forces are present in nitrogen, N2 Chloroform, CHCl3 Carbon dioxide, C02 Ammonia, NH3 ... 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

Identify the type of intermolecular forces in different substances. 

Real gases consist of atoms or molecules with intermolecular attractions and repulsions. Attractions have a longer range than repulsions. The compression factor is a measure of the strength and type of intermolecular forces. When Z > 1, intermolecular repulsions are dominant when Z < 2, attractions are dominant. 

Self-Test 14.12A Water adheres to glass. Predict the type of intermolecular forces that form between glass and water. 

Cl l CH2CI 12CH2OCHv (a) Draw a Lewis structure for each molecule, name it, and classify it by functional group, (b) Which molecules are isomers of each other Are any chiral If so, which ones (c) For each molecule, list the types of intermolecular forces that are present, (d) Use your answers to parts (a) and (b) to predict the relative boiling points, from lowest to highest. 

You might learn about the structure of DNA at the end of your organic chemistry course. For right now, we will be focused on problems that deal primarily with small molecules and therefore, for our purposes, we should think of H-bonding as an interaction a type of intermolecular force. 

At the opposite extreme, molecular solids contain individual molecules bound together by various combinations of dispersion forces, dipole forces, and hydrogen bonds. Conforming to like dissolves like, molecular solids dissolve readily in solvents with similar types of intermolecular forces. Nonpolar I2, for instance, is soluble in nonpolar liquids such as carbon tetrachloride (CCI4). Many organic compounds are molecular solids that dissolve in organic liquids such as cyclohexane and acetone. 

As the same types of intermolecular forces are involved, there is no qualitative difference between solute-solvent interactions and the recognition of a compound by a bio (macro) molecular compound. 

Marta and her father often go to the lake and skip stones across a pond. What type of intermolecular force creates the surface tension that allows the stones to skip ... 

Inter- and intramolecular forces (imf) are of vital importance in the quantitative description of structural effects on bioactivities and chemical properties. They can make a significant contribution to chemical reactivities and some physical properties as well. Types of intermolecular forces and their present parameterization are listed in Table 750. 

Supramolecular aggregations are commonly referred to by a variety of terms, including adduct, complex, and van der Waals molecule. In this chapter we shall primarily employ the more neutral term cluster, which may, if desired, be qualified with the type of intermolecular interaction leading to clustering (e.g., H-bonded cluster ). General and specific types of intermolecular forces are discussed in the following sections. 

Types of intermolecular forces Properties of liquids Surface tension Viscosity Capillary action Structures of solids Phase changes and diagrams... 

Each of the eight substances will exhibit London forces since they are present in everything containing electrons. London forces are only the strongest type of intermolecular force if there are no other attractions present. The most convenient method of analyzing this problem is to leave consideration of London forces to the last. 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

What is the strongest type of intermolecular force present in each of the following ... 

This intermolecular attraction occurs in all substances, but is significant only when the other types of intermolecular forces are absent. It arises from a momentary distortion of the electron cloud, with the creation of a very weak dipole. The weak dipole induces a dipole in another nonpolar molecule. This is an extremely weak interaction, but it is strong enough to allow us to liquefy nonpolar gases such as hydrogen, H2, and nitrogen, N2. If there were no intermolecular forces attracting these molecules, it would be impossible to liquefy them. 

Organic compounds that have the same functional group often have similar physical properties, such as boiling points, melting points, and solubilities. Physical properties are largely determined by intermolecular forces, the forces of attraction and repulsion between particles. Three types of intermolecular forces are introduced below. You will examine these forces further in Chapter 4. 

What types of intermolecular forces must be broken to melt solid samples of the following (a) NH3 (b) Nal (c) Fe (d) CH4... 

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