Lecture_5.pdf - Effect of Chemical Forces on Physical...

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Effect of Chemical Forces on Physical Properties The forces of attraction between the various ions or atoms in solids determine many of their properties Melting Points when sufficient thermal energy is supplied to a crystal to overcome the potential energy holding its atoms together - Fusion, evaporation, and sublimation result. Normally, a pure substance at constant pressure will melt at a fixed temperature, with the absorption of heat. The amount of heat absorbed is known as the heat of fusion A H f , and it is the heat required for the reaction Solid Liquid
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Δ H f is a measure of the enthalpy difference between the solid and liquid states at the melting point. The entropy difference, Δ S f between the liquid and solid is defined by Δ S f is a direct measure of the degree of disorder that arises in the system during the melting process and is by necessity positive, (since the liquid state is always more disordered than the solid)
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Factors Affecting Melting Points of Ceramics that are Predominantly Ionically Bonded Ionic charge The most important factor determaining the melting point is the strength of the bond. the greater the ionic charges, the stronger the attraction between ions, and consequently the higher the melting point. For example, both MgO and NaCl crystallize in the rock salt structure, but their melting points are, respectively, 2852 and 800°C a difference directly attributable to the fact that MgO is made up of doubly ionized ions, whereas in NaCl the ions are singly ionized
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Covalent character of the ionic bond melting points are proportional to Δ H f , increasing the covalent character of a bond tends to reduce Δ H f by stabilizing discrete units in the melt, which in turn reduces the number of bonds that have to be broken during melting, which is ultimately reflected in lower melting points. Note: covalency per se does not necessarily favor either higher or lower melting points. The important consideration depends on the melt structure; if the strong covalent bonds have to be broken in order for melting to occur, extremely high melting temperatures can result. Conversely, if the strong bonds do not have to be broken for melting, the situation can be quite different
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the covalent character of the bond increases in going from left to right, which results in changes in the structure from three- dimensional in TiO 2 , to a layered structure for CdI 2 , to a molecular lattice in the case of CO 2 .
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in order to understand the refinements in melting point trends, one needs to somewhat quantify the extent of covalency present in an ionic bond. As discuss in previous lecture, the bonds between ions were assumed to be either predominantly covalent or ionic.
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  • Fall '16
  • Unkwnow
  • Ion, Chemical bond, COVALENT CERAMICS, covalent character

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