Intermolecular forces sublimation

Phase changes, which convert a substance from one phase to another, have characteristic thermodynamic properties Any change from a more constrained phase to a less constrained phase increases both the enthalpy and the entropy of the substance. Recall from our description of phase changes in Chapter 11 that enthalpy increases because energy must be provided to overcome the intermolecular forces that hold the molecules in the more constrained phase. Entropy increases because the molecules are more dispersed in the less constrained phase. Thus, when a solid melts or sublimes or a liquid vaporizes, both A H and A S are positive. Figure 14-18 summarizes these features. 

The volatilization process changes the contaminant from a sohd or hquid state, where the molecules are held together by intermolecular forces, into a vapor phase. The molar heats of fusion (A//p, volatilization (AH), and sublimation (AH) are related according to the Bom-Haber cycle by... 

The apparent near constancy of the specific heats of sublimation of the nitrobenzenes suggests a similarity in their intermolecular forces, eg, a... 

In this chapter, intermolecular forces are viewed as complications and nuisances it is the molecule per se that is of interest. Therefore, unless explicitly noted to the contrary, any species of interest in this chapter is to be assumed in the (ideal) gas phase. Most organic compounds are naturally liquids or solids under the thermochemically desired conditions, much less as found by the synthetically or mechanistically inclined chemist. Corrections are naturally made by using enthalpies of vaporization (v) and of sublimation ), defined by equations la and lb ... 

Primitive considerations convince us that such secondary forces exist. For example, gases consist of disordered molecules, whether they be polyatomic like chlorine or ether vapour, or single atoms like helium or mercury vapour. But all gases, even helium, ultimately condense to liquids — and then to solids — if they are cooled and/or compressed sufficiently. When the molecules are forced into close proximity and have their kinetic energies diminished, the weak intermolecular forces are able to take control. Liquefaction results. The strength of these forces can be measured by the latent heat necessary to evaporate the liquid, or to sublime the solid. The equation,... 

During condensation, gaseous particles slow down and are overcome by the intermolecular forces at work in liquids. During sublimation and deposition, temperature and pressure conditions are so extreme that the liquid phase simply gets skipped. 

The big difference in melting points suggests a difference in type of crystal binding. The intermolecular forces in solid CO2 must be very low to be overcome by a low-temperature sublimation. CO2 is actually a molecular lattice held together only by the weak van der Waals forces between discrete CO2 molecules. Si02 is a covalent lattice with a three-dimensional network of bonds each silicon atom is bonded tetrahedrally to four oxygen atoms and each oxygen is bonded to two silicon atoms. 

The more electronegative and bulky F ions instead of the CH group in the ligand were expected to decrease intermolecular forces and increase volatility of the precursors. A typical fluorinated precursor of Ba is Ba bis-hexafluoroacetylacetonate (Ba(hfa)2). L. Wills and B. Wessels used Ba(hfa)2tetraglyme to deposit BaTi03 thin films, where they sublimed the precursor at temperatures ranging from 105 to 125°C which is much lower than for Ba(thd)2. M. Nyman and S. Desu ° have reported better volatility of Pb bis-heptafluorodimethy-... 

UFe is a molecular solid—its solid state structure contains isolated, octahedral UF molecules held together via relatively weak intermolecular forces. The intermolecular forces are very easy to break and this compound sublimes easily. On the other hand, UF3 and UF4 form network solids in which each U atom is surrounded by nine jmd eight fluorines respectively. As a consequence, UF3 and UF4have high melting points. 

For evaporative or sublimation process, correct selection of evaporation method, the evaporation source, and the evaporation temperatnre is required to surmount the attractive intermolecular forces existing within the starting material. The parameters depend primarily on the materials nsed and the film purity required. Indirect resistance heating, flash evaporation, and electron beam heating techniques are used for this purpose. 

Why does solid iodine sublime readily Use your knowledge of intermolecular forces to explain. 

Intermolecular forces are important because they dictate many of the macroscopic properties of the bulk matmal, for example the behavior of the gas and liquid phases, the crystalline structure, the elastic modulus, the ultimate strength, the heat of sublimation, and the chemical reactivity. Of course, the intermolecular forces are also responsible ultimately for the adhesion between bodies. Table 5.2 illustrates some of the properties which have been predicted from a knowledge of intermolecular forces. But the properties of gases are easiest to understand. 

Electrostatic interactions can be most simply modeled as the Coulomb interaction between partial atomic charges, while the repulsion-dispersion part is usually described by a Lennard-Jones or, more accurately, an exp-6 form, each of which contains parameters that must be fixed. High-quality empirically fitted parameter sets have been developed, where the atom-atom interactions are parameterized to reproduce the structures, sublimation enthalpies and, sometimes, further observable properties of organic molecular crystals [73,74]. Their use has been very effective in CSP. Nonempirical approaches to fitting intermolecular force fields, where the parameters are derived from quantum mechanical calculations, have occasionally been applied for CSP [75-78], but these are currently limited to small molecules, so currently lack relevance for typical pharmaceutical molecules. 

Intermolecular forces, p. 463 Intramolecular forces, p. 463 Ion-dipole forces, p. 463 Melting point, p. 495 Molar heat of fusion (A7/f s), p. 496 Molar heat of sublimation p. 497... 

Sublimation is the direct conversion of a soUd to a vapor, so the molar enthalpy of sublimation (A//sub) is the energy required to sublime 1 mole of a solid. In general, the molecules in a sohd are more tightly held together by intermolecular forces than those in a liquid, so AHsab is larger than A/fvap- If AHsub. AHyap. and AHfas could all be measured at the same temperature (such as is possible at the triple point of water), we would have... 

---