Intermolecular forces bond strength

Species (atoms, molecules, and ions) that are not chemically bonded to each other may interact with one another through intermolecular forces. The strength of intermolecular forces dictates the inherent properties of solids, liquids, and gases. Compounds with very strong intermolecular forces are normally solids at room temperature, whereas compounds with intermediate intermolecular forces are liquids, and those with extremely weak intermolecular forces are gases. The strength of these intermolecular forces explains why a solid such as sucrose melts at 185 °C, whereas ice water melts at 0 °C and molecular nitrogen boils at -196 °C. 

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

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. 

Thermal expansion — as elasticity — depends directly upon the strength of the intermolecular forces in the material. Strongly bonded materials usually expand little when heated, whereas the expansion of weak materials may be a hundred times as large. This general trend is confirmed by Table 5.1. The coefficient of thermal expansion a was found to be lower in the crosslinked polymers and higher in the less crosslinked or thermoplastic materials as observed by Nielsen [1], In addition, Table 5.1 presents the Young s moduli E of the polymers at ambient temperatures as well as the products a2E. The values of oc2E are all close to 13.1 Pa K 2 with a coefficient of variation of 1.6%. 

What Do We Need to Know Already This chapter assumes that we are familiar with the concept of energy (Section A), stoichiometry (Sections L and M), and the ideal gas law (Chapter 4). Some of the explanations refer to intermolecular forces (Sections 4.12 and 5.1-5.5). Ionic bonding (Sections 2.3-2.4) and bond strengths (Sections 2.14-2.15) are developed further in this chapter. 

As we have explained in the previous two chapters, polymers are long chains of repeat units connected by bonds. One consequence of these long chains is the high number of intermolecular forces present between polymer molecules. The character of the bonds in a polymer will partially define the strength of the intermolecular interactions between the molecules. 

The hydrogen bond is the strongest intermolecular force, typically about 1/1 Oth the strength of a covalent bond. 

Relative strength of intermolecular forces H bonds > dipole forces > dispersion forces. 

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. 

Melting points and boiling points are related to the strength of the intermolecular forces between solvent molecules, and to the molecular weight of the solvent. Dispersive forces, hydrogen bonding and permanent dipole moments all contribute. Typically, for molecules of similar mass, nonpolar compounds which... 

The effect of different bonding mechanisms, and of the primary particle size on granule strength, is shown in Figure 5.2. Four mechanisms (Capes, 1979 Sherrington and Oliver, 1981) need to be considered. First, intermolecular forces. The attractive force between particles is inversely... 

Physical properties of polymers, including solubility, are related to the strength of covalent bonds, stiffness of the segments in the polymer backbone, amount of crystallinity or amorphousness, and intermolecular forces between the polymer chains. The strength of the intermolecular forces is directly related to the CED, which is the molar energy of vaporization per unit volume. Since intermolecular attractions of solvent and solute must be overcome when a solute (here the polymer) dissolves, CED values may be used to predict solubility. 

Relative strength of intermolecular forces H bonds > dipole forces > dispersion forces. BP is directly proportional to increasing MW-dispersion forces (van der Waals force). Greater MW results in greater dispersion forces. 1 point for mentioning the intermolecular forces involved hydrogen bonds, dipole forces, dispersion forces. 

Intermolecular forces are responsible for the condensed states of matter. The particles making up solids and liquids are held together by intermolecular forces, and these forces affect a number of the physical properties of matter in these two states. Intermolecular forces are quite a bit weaker than the covalent and ionic bonds discussed in Chapter 7. The latter requires several hundred to several thousand kilojoules per mole to break. The strength of intermolecular forces are a few to tens of kilojoules per... 

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