Intermolecular forces strength

Kutzelnigg W and Maeder F 1978 Natural states of interacting systems and their use for the calculation of intermolecular forces. III. One-term approximations of oscillator strength sums and dynamic polarizabilities Chem. Phys. 35 397... 

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. 

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. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

Once equilibrium between liquid and vapor is reached, the number of molecules per unit volume in the vapor does not change with time. This means that the pressure exerted by the vapor over the liquid remains constant The pressure of vapor in equilibrium with a liquid is called the vapor pressure. This quantity is a characteristic property of a given liquid at a particular temperature. It varies from one liquid to another, depending on the strength of the intermolecular forces. At 25°C, the vapor pressure of water is 24 mm Hg that of ether, in which intermolecular forces are weaker, is 537 mm Hg. 

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%. 

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. 

Intermolecular forces are responsible for the existence of several different phases of matter. A phase is a form of matter that is uniform throughout in both chemical composition and physical state. The phases of matter include the three common physical states, solid, liquid, and gas (or vapor), introduced in Section A. Many substances have more than one solid phase, with different arrangements of their atoms or molecules. For instance, carbon has several solid phases one is the hard, brilliantly transparent diamond we value and treasure and another is the soft, slippery, black graphite we use in common pencil lead. A condensed phase means simply a solid or liquid phase. The temperature at which a gas condenses to a liquid or a solid depends on the strength of the attractive forces between its molecules. 

O 3 Predict the relative order of the boiling points of two substances from the strengths of their intermolecular forces (Examples 5.1 and 5.2). 

Describe the structure of a liquid and explain how viscosity and surface tension vary with temperature and the strength of intermolecular forces (Sections 5.6 and 5.7). 

Account for the following observations in terms of the type and strength of intermolecular forces, (a) The melting point of solid xenon is —112°C and that of solid argon is — 189°C. 

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. 

The parameter a is an indication of the strength of attractive intermolecular forces, and the parameter b is an indication of the strength of repulsive intermolecular forces. See also van der Waals equation. 

The van der Waals equation adds two correction terms to the ideal gas equation. Each correction term includes a constant that has a specific value for every gas. The first correction term, a fV, adjusts for attractive intermolecular forces. The van der Waals constant a measures the strength of intermolecular forces for the gas the stronger the forces, the larger the value of a. The second correction term, n b, adjusts for molecular sizes. The van der Waals constant b measures the size of molecules of the gas the larger the molecules, the larger the value of b. 

The fraction of molecules with enough kinetic energy to escape a liquid depends on the strength of intermolecular forces and temperature, (a) At 300 K, more bromine molecules can escape than water molecules, (b) More bromine molecules can escape at 320 K than at 300 K. 

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. 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

It has been estimated (4) that in most common solvents at room temperature two reactant molecules within a cage of solvent molecules will collide from 10 to a 1000 times before they separate. The number of collisions per encounter will reflect variations in solvent viscosity, molecular separation distances, and the strength of the pertinent intermolecular forces. High viscosities, high liquid densities, and low temperatures favor many collisions per encounter. 

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