Melting points of solids

Account for the following observations in terms of the type and strength of intermolecular forces, (a) The melting point of solid xenon is —112°C and that of solid argon is — 189°C. 

As described in Chapter JT, melting points of solids may depend on both covalent bonds and intermolecular forces, so we must explain the melting point difference with reference to the bonding differences between the two forms. 

There is another important law that follows from the classical theory of capillarity. This law was formulated by J. Thomson [16], and was based on a Clausius-Clapeyron equation and Gibbs theory, formulating the dependence of the melting point of solids on their size. The first known analytical equation by Rie [17], and Batchelor and Foster [18] (cited according to Refs. [19,20]) is... 

Thermal (A) High temperature—approach or exceed the melting point of solids autoclaving solutions. (B) Low temperature-usually below 100°C and may be conducted in a controlled humidity environment. 

The phase diagram of C02 shown in Figure 10.29 has many of the same features as that of water but differs in several interesting respects. First, the triple point is at Pt = 5.11 atm, meaning that C02 can t be a liquid below this pressure, no matter what the temperature. At 1 atm pressure, C02 is a solid below -78.5°C but a gas above this temperature. Second, the slope of the solid/liquid boundary is positive, meaning that the solid phase is favored as the pressure rises and that the melting point of solid C02 therefore increases with pressure. 

The curve BC is also traced in an alternative way. If we add ice to solid potassium iodide then we get a solution of potassium iodide in water in equilibrium with solid potassium iodide. In other words, the addition of ice to potassium iodide decreases the melting point of solid potassium iodide along BC. The curve BC is known as the solubility curve of potassium iodide, as KI is in equilibrium with its solution. The nature of the curve BC shows that the solubility of potassium iodide increases slowly with rise of temperature. At point C, a new solid phase, viz., ice begins to separate out. 

To illustrate the type of analysis that is involved we exhibit a representative set of heat capacity data in Fig. 1.20.2 for oxygen, as a plot of CP versus log T this representation is useful for the direct calculation of the entropy of oxygen from the area under the curves. Note that the element exists in three allotropic modifications in the solid state, with transition temperatures near 23.6, 43.8, and 54.4 K, the last being the melting point of solid phase I. The boiling point of liquid oxygen is near 90.1 K. An extrapolation procedure was used below 14 K. 

As boiling points of liquids and melting points of solids are indicators of molecular... 

If it were not for the existence of intermolecular attractions, condensed phases (liquids and solids) could not exist. These are the forces that hold the particles close to one another in liquids and solids. As we shall see, the effects of these attractions on melting points of solids parallel those on boiling points of liquids. High boiling points are associated with compounds that have strong intermolecular attractions. Let us consider the effects of the general types of forces that exist among ionic, covalent, and monatomic species. 

Freezing points (melting points) of solids are not as severely affected by pressure as boiling points, still the normal freezing point of a liquid is that measured at an external pressure of 760 mmHg. 

Boiling Points of Liquids and Melting Points of Solids Summary Key Terms... 

A similar argument can be made for the melting points of solids. The ease of conversion of a solid to a liquid also depends on the magnitude of the attractive forces in the solid. The situation actually becomes very complex for ionic solids because of the complexity of the crystal lattice. 

Several methods can be used to measure the melting point of solids. One of the simplest is probably to use a differential thermal analyser (DTA for short). The basic arrangement of a differential thermal analyser is simple and is shown schematically in Fig. 4.8u. The sample and an inert reference (usually alumina powder) are placed side by side in a furnace, and identical thermocouples are placed below each. The temperature of the furnace is then slowly ramped, and the difference in temperature AT = Tsampie is measured... 

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