Melting points and triple points
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص119-120
2025-11-10
66
Melting points and triple points
The temperature at which, under a specified pressure, the liquid and solid phases of a substance coexist in equilibrium is called the melting temperature. Because a sub stance melts at exactly the same temperature as it freezes, the melting temperature of a substance is the same as its freezing temperature. The freezing temperature when the pressure is 1 atm is called the normal freezing point, Tf, and its freezing point when the pressure is 1 bar is called the standard freezing point. The normal and stand ard freezing points are negligibly different for most purposes. The normal freezing point is also called the normal melting point. There is a set of conditions under which three different phases of a substance (typically solid, liquid, and vapour) all simultaneously coexist in equilibrium. These conditions are represented by the triple point, a point at which the three phase boundaries meet. The temperature at the triple point is denoted T3. The triple point of a pure substance is outside our control: it occurs at a single definite pressure and temperature characteristic of the substance. The triple point of water lies at 273.16 K and 611 Pa (6.11 mbar, 4.58 Torr), and the three phases of water (ice, liquid water, and water vapour) coexist in equilibrium at no other combination of pressure and temperature. This invariance of the triple point is the basis of its use in the definition of the thermodynamic temperature scale (Section 3.2c). As we can see from Fig. 4.1, the triple point marks the lowest pressure at which a liquid phase of a substance can exist. If (as is common) the slope of the solid–liquid phase boundary is as shown in the diagram, then the triple point also marks the lowest temperature at which the liquid can exist; the critical temperature is the upper limit. Supercritical carbon dioxide, scCO2, is the centre of attention for an increasing number of solvent-based processes. The critical temperature of CO2, 304.2 K (31.0°C) and its critical pressure, 72.9 atm, are readily accessible, it is cheap, and it can readily be re cycled. The density of scCO2 at its critical point is 0.45 g cm−3. However, the transport properties of any supercritical fluid depend strongly on its density, which in turn is sensitive to the pressure and temperature. For instance, densities may be adjusted from a gas-like 0.1 g cm−3 to a liquid-like 1.2 g cm−3. A useful rule of thumb is that the solubility of a solute is an exponential function of the density of the supercritical fluid, so small increases in pressure, particularly close to the critical point, can have very large effects on solubility. A great advantage of scCO2 is that there are no noxious residues once the solvent has been allowed to evaporate, so, coupled with its low critical temperature, scCO2 is ideally suited to food processing and the production of pharmaceuticals. It is used, for instance, to remove caffeine from coffee. The supercritical fluid is also increasingly being used for dry cleaning, which avoids the use of carcinogenic and environment ally deleterious chlorinated hydrocarbons. Supercritical CO2 has been used since the 1960s as a mobile phase in supercritical fluid chromatography (SFC), but it fell out of favour when the more convenient technique of high-performance liquid chromatography (HPLC) was introduced. However, interest in SFC has returned, and there are separations possible in SFC that cannot easily be achieved by HPLC, such as the separation of lipids and of phospholipids. Samples as small as 1 pg can be analysed. The essential advantage of SFC is that diffusion coefficients in supercritical fluids are an order of magnitude greater than in liquids, so there is less resistance to the transfer of solutes through the column, with the result that separations may be effected rapidly or with high resolution. The principal problem with scCO2, though, is that the fluid is not a very good solvent and surfactants are needed to induce many potentially interesting solutes to dissolve. Indeed, scCO2-based dry cleaning depends on the availability of cheap sur factants; so too does the use of scCO2 as a solvent for homogeneous catalysts, such as metal complexes. There appear to be two principal approaches to solving the solubilization problem. One solution is to use fluorinated and siloxane-based polymeric stabilizers, which allow polymerization reactions to proceed in scCO2. The dis advantage of these stabilizers for commercial use is their great expense. An alternative and much cheaper approach is poly(ether-carbonate) copolymers. The copolymers can be made more soluble in scCO2 by adjusting the ratio of ether and carbonate groups. The critical temperature of water is 374°C and its pressure is 218 atm. The conditions for using scH2O are therefore much more demanding than for scCO2 and the properties of the fluid are highly sensitive to pressure. Thus, as the density of scH2O decreases, the characteristics of a solution change from those of an aqueous solution through those of a non-aqueous solution and eventually to those of a gaseous solution. One consequence is that reaction mechanisms may change from those involving ions to those involving radicals.
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