Melting points general discussion

Generally solids are divided into two clas.ses, crystalline and amorphous. The former are characterized by arrangement of particles according to a definite pattern, while the latter, as, for example, glass, do not have a crystalline structure. Most organic solid compounds are crystalline. The purification of impure crystalline compounds is usually accomplished by crystallization from an appropriate solvent. The property which is most frequently observed in order to determine the purity of a crystalline compound is the melting point, as discussed in the next experiment on page 41. 

Incorporation of another monomer into an otherwise crystalline homopolymer will generally reduce the crystallinity and lower the melting point in a continuous manner. Several theoretical models have been developed addressing the reduction of crystallinity and melting point. In discussing these models, we will designate the major comonomer as the monomer in the base homopolymer, and any others will be minor comonomers. 

The five main types of renewable polymers that can be produced from either petroleum or renewable sources are, in general, melt processable with less stability issues than those obtained from natural resources. We summarize the properties of the main types of polymers in Table 11.11 and present values of Tg and which are important to their processing. The only polymer with melt processing stability issues is PVA, which begins to degrade around 150 °C, which is similar to its melting point. A discussion of the uses, properties, and methods of processing of these polymers is summarized in the book by Rudnik (2008). All of the materials listed in Table 11.11 can be processed by means of conventional processes. 

Recognizing Cause and Effect In a crystal lattice structure, the electrons are held tightly by the ions, which are rigidly held in place by electrostatic attraction. Discuss how this characteristic explains why ionic compounds generally (a) have high melting points and (b) do not conduct electricity in the solid state. 

Since they generally have more symmetry than cis isomers, trans isomers in most cases have higher melting points and lower solubilities in inert solvents. The cis isomer usually has a higher heat of combustion, which indicates a lower thermochemical stability. Other noticeably different properties are densities, acid strengths, boiling points, and various types of spectra, but the differences are too involved to be discussed here. 

Thus far, we ve discussed the sources, production, and properties of some important metals. Some properties, such as hardness and melting point, vary considerably among metals, but other properties are characteristic of metals in general. For instance, all metals can be drawn into wires (ductility) or beaten into sheets (malleability) without breaking into pieces like glass or an ionic crystal. Furthermore, all metals have a high thermal and electrical conductivity. When you touch a metal, it feels cold because the metal efficiently conducts heat away from your hand, and when you connect a metal wire to the terminals of a battery, it conducts an electric current. 

There is obviously a particular need to develop materials which can function at high temperatures. Due to their strong covalent bonding, boron cluster compounds generally possess attractive mechanical properties as materials, e.g. stability under high temperature due to their high melting points (typically >2300 K), chemical stability, resistance to acidic conditions, and small compressibility. Furthermore, importantly, the B12 icosahedra compounds have also been found to have intrinsic low thermal conductivity, as will be discussed in detail in later sections, and which is desirable for thermoelectric applications. 

The ionic nitrates generally melt to liquids which are stable to various degrees above their melting points. These liquids can be distilled under reduced pressure.11 The covalent nitrates are generally not stable as liquids. When heated, they first sublime, frequently giving molecular vapors, and then decompose. The cation influences the stability of the anion through its ability to distort its structure in the same manner as for carbonates and sulfates, as discussed in Chapters 2 and 4. 

Wc have so far studied only perfect gases and have not taken up imperfect gases, liquids, and solids. Before we treat them, it is really necessary to understand what happens when two or more phases are in equilibrium with each other, and the familiar phenomena of melting, boiling, and the critical point and the continuity of the liquid and gaseous states. We shall now proceed to find the thermodynamic condition for the coexistence of two phases and shall apply it to a general discussion of the forms of the various thermodynamic functions for matter in all three states. 

In addition to affecting boiling points and melting points, intermolecular forces determine the solubility properties of organic compounds. The general rule is that like dissolves like. Polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. We discuss the reasons for this rule now, then apply the rule in later chapters when we discuss the solubility properties of organic compounds. 

The fact that the expected product BH3 is not obtained will be discussed in a later section. Some metals form borides containing the hexaboride group, B62. An example of this type of compound is calcium hexaboride, CaB6. In general, the structures of compounds of this type contain octahedral B62 ions in a cubic lattice with metal ions. Most hexaborides are refractory materials having melting points over 2000 °C. 

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