Molecular solids forces

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

We have seen that the pure elements may solidify in the form of molecular solids, network solids, or metals. Compounds also may condense to molecular solids, network solids, or metallic solids. In addition, there is a new effect that does not occur with the pure elements. In a pure element the ionization energies of all atoms are identical and electrons are shared equally. In compounds, where the most stable electron distribution need not involve equal sharing, electric dipoles may result. Since two bonded atoms may have different ionization energies, the electrons may spend more time near one of the positive nuclei than near the other. This charge separation may give rise to strong intermolecular forces of a type not found in the pure elements. 

Molecular solids, 102, 301, 306 Molecular substances and van der Waals forces, 306... 

Molecular solids are assemblies of discrete molecules held in place by intermolecular forces. 

Chloromethane (CH3C1), methane, and acetic acid (CH3COOH) form molecular solids, (a) What types of forces hold these molecules in a molecular solid (b) Place the solids in order of increasing melting point. 

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

Molecular solids are aggregates of molecules bound together by intermolecular forces. Substances that are gases under normal conditions form molecular solids when they condense at low temperature. Many larger molecules have sufficient dispersion forces to exist as solids at room temperature. One example is naphthalene (Cio Hg), a white solid that melts at 80 °C. Naphthalene has a planar structure like that of benzene (see Section 10-), with a cloud of ten delocalized n electrons that lie above and below the molecular plane. Naphthalene molecules are held in the solid state by strong dispersion forces among these highly polarizable n electrons. The molecules in... 

Identify the type of solid. Solids may be network, ionic, metallic, or molecular. Different forces account for the stability of each type. 

P4 Ofi The relatively low melting point of 25 °C indicates a molecular solid. The molecular structure shows that P4 Og is a discrete molecule. Strong covalent bonding holds the atoms in each molecule together, but each molecule is attracted to the others only by dispersion forces. In this molecular solid, little energy is required to overcome dispersion forces and allow P4 Og solid to melt. 

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. 

The best solvent for a molecular solid Is one whose Intermolecular forces match the forces holding the molecules in the crystal. For a solid held together by dispersion forces, good solvents are nonpolar liquids such as carbon tetrachloride (CCI4) and cyclohexane (Cg H12) For polar solids, a polar solvent such as acetone works well. Example provides some practice in recognizing solubility types. 

The next step is to assume particular functional forms for the various terms. These can be extremely elaborate, but most are usually based on a simple, chemically intuitive model of the interactions, e.g. stretching of bonds, changes in bond and torsion angles, Coulomb forces and van der Waals intermolecular interactions. Thus, for a molecular solid comprised of discrete molecules, we might well use... 

Molecular solids have their lattices composed of molecules held in place by London forces, dipole-dipole forces, and hydrogen bonding. Solid methane and water are example of molecular solids. 

LlFSHITZ, E. M. Soviet Physics. J.E.T.P. 2 (1956) 73. Theory of molecular attractive forces between solids. 

Condensed matter can be classified by the nature of the forces that hold it together ionic solids covalent solids metallic solids molecular solids. 

Finally, some molecules possess permanent charge separations, or dipoles, such as are found in water. The general case for the interaction of any positive dipole with a negative dipole is called dipole-dipole interaction. Hydrogen bonding can be thought of as a specific type of dipole-dipole interaction. A dipolar molecule like ammonia, NH3, is able to dissolve other polar molecules, like water, due to dipole-dipole interactions. In the case of NaCl in water, the dipole-dipole interactions are so strong as to break the intermolecnlar forces within the molecular solid. 

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