Attracting other molecules Intermolecular forces

London force (dispersion force) This very weak type of attraction generally occurs between nonpolar covalent molecules, such as nitrogen (N2), hydrogen (H2), or methane (CH ). It results from the ebb and flow of the electron orbitals, giving a very weak and very brief charge separation around the bond. 

Dipole-dipole interaction This intermolecular force occurs when the positive end of one dipole molecule is attracted to the negative end of another dipole molecule. It s much stronger than a London force, but it s still pretty weak. 

When the 0, N, or F on one molecule attracts the hydrogen of another molecule, the dipole-dipole interaction is very strong. This strong interaction (only about 5 percent of the strength of an ordinary covalent bond but still very strong for an intermoleculcir force) is called a hydrogen bond. The hydrogen bond is the type of interaction that s present in water. 

M n a chemical reaction, substances (elements and/or com-pounds) are changed into other substances (compounds and/or elements). You can t change one element into another element in a chemical reaction — that happens in nucleeir 

A number of clues show that a chemical reaction has taken place — something new is visibly produced, a gas is created, heat is given off or taken in, and so on. 

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Consider now a gas-phase solute molecule (e.g., naphthalene) entering a liquid of like polarity. In the gas, the solute has very little interaction with other molecules, but on entering the liquid it is immediately surrounded by host molecules to which it is attracted by various intermolecular forces. These attractive interactions lower the enthalpy and chemical potential of the solute, thus generally favoring its transfer from gas to liquid. 

Intramolecular forces do not account for all attractions between particles. There are forces of attraction called intermolecular forces. The prefix inter-means "between" or "among." For example, an interview is a conversation between two people. Intermolecular forces can hold together identical particles, such as water molecules in a drop of water, or two different types of particles, such as carbon atoms in graphite and the cellulose particles in paper. The three intermolecular forces that will be discussed in this section are dispersion forces, dipole-dipole forces, and hydrogen bonds. Although some intermolecular forces are stronger than others, all intermolecular forces are weaker than intramolecular, or bonding, forces. 

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

Polar molecules attract other polar molecules through dipole-dipole intermolecular forces. Polar solutes tend to have higher solubilities in polar solvents than in nonpolar solvents. Which of the following pairs of compounds would be expected to have the higher solubility in hexafluorobenzene, Cf,I... 

In Chapter 4 we considered gases, in which intermolecular forces play only a minor role. Here, we deal with liquids and solids, in which the forces that hold molecules together are of crucial importance for determining the physical properties of bulk samples. Individual water molecules, for instance, are not wet, but bulk water is wet because water molecules are attracted to other substances and spread over their surfaces. Individual water molecules neither freeze nor boil, but bulk water does, because in the process of freezing molecules stick together and form a rigid array and in boiling they separate from one another and form a gas. 

Every gas changes into a liquid if the pressure is high enough and the temperature is low enough. The atoms or molecules of a liquid or solid stick together in a finite volume rather than expanding, as a gas does, to fill all available space. This cohesiveness comes from electrical forces of attraction between the negative electron cloud of each atom and the positive nuclei of other atoms. We describe intermolecular forces in Chapter 11. 

The attraction of the gas particles for each other tends to lessen the pressure of the gas since the attraction slightly reduces the force of the collisions of the gas particles with the container walls. The amount of attraction depends on the concentration of gas particles and the magnitude of the intermolecular force of the particles. The greater the intermolecular forces of the gas, the higher the attraction is, and the less the real pressure. Van der Waals compensated for the attractive force by the term P + an2/V2, where a is a constant for individual gases. The greater the attractive force between the molecules, the larger the value of a. 

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 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. 

Complex formation is important in photophysics. Two terms need to be described here first, an exciplex, which is an excited state complex formed between two different kinds of molecules, one that is excited and the other that is in its grown state second, an excimer, which is similar to exciplex except that the complex is formed between like molecules. Here, we will focus on excimer complexes that form between two like polymer chains or within the same polymer chain. Such complexes are often formed between two aromatic structures. Resonance interactions between aromatic structures, such as two phenyl rings in PS, give a weak intermolecular force formed from attractions between the pi-electrons of the two aromatic entities. Excimers involving such aromatic structures give strong fluorescence. 

Another characteristic dependent on the intermolecular forces is the surface tension of the liquid. Surface tension results from the unbalanced forces on molecules at the surface of a liquid. Figure 8.11 shows how surface tension results from these unbalanced forces. Consider water as the liquid in Figure 8.11. A water molecule in the interior of the liquid is surrounded on all sides by other water molecules. Attractive intermolecular forces pull the molecule equally in all directions and these forces balance out. A water molecule on the surface experiences an unbalanced force toward the interior of the fluid. This unbalanced force pulls on the surface of the water putting it under tension. This situation is similar to the tightening of the head of drum. The tension causes the surface of the water to act like a thin film. If you carefully use tweezers to place a clean needle on the surface of water, surface tension will allow the needle to float even though the needle is denser than water. 

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