Solids sublimation

Phosphorus exists as white and red phosphorus. The former allotrope may be preserved in the dark at low temperatures but otherwise reverts to the more stable red form. The white form is a waxy, translucent, crystalline, highly-toxic solid subliming at room temperature and inflaming in air at 35°C, so it is handled under water. The red form is a reddish violet crystalline solid which vaporizes if heated at atmospheric pressure and condenses to give white phosphorus. The red form ignites in air at 260°C. Both are insoluble in water, and white phosphorus can be stored beneath it. Phosphorus forms a host of compounds such as phosphine, tri- and penta-halides, tri-, tetra- and penta-oxides, oxyacids including hypophosphorous, orthophosphorous and orthophosphoric acids. 

Aeetylene is a simple asphyxiant and anaestlietie. Pure aeetylene is a eolourless, highly flammable gas with an ethereal odour. Material of eommereial purity has an odour of garlie due to the presenee of impurities sueh as phosphine. Its physieal properties are shown in Table 9.4. Aeetylene, whieh eondenses to a white solid subliming at -83°C, is soluble in its own volume of water but highly soluble in aeetone. 

Colourless ionic solid sublimes at 32.4° to unstable molecular gas (angle N-O-N -180°)... 

The reaction mixture was removed from the vessel and distilled at a pressure of 30-60 mm, and a bath temperature of 30°C to 50°C until the methanol had all been removed. The extremely viscous tarry residue remaining in the still pot was given a very crude distillation, the distillate boiling at B2°C to 1 32°C/2 mm. In an attempt to purify this distillate by a more careful distillation, 5.3 g of a liquid distilling from 53°C to 150°C/5 mm was collected. At this point, much solid sublimate was noted not only in this distillate but in the condenser of the still. 7 g of the solid sublimate was scraped out of the condenser of the still. Recrystallization of the sublimate from ethyl acetate containing a small amount of petroleum ether gave beautiful crystals melting at 175°C to 177°C (5 g). Infrared analysis confirmed that this compound was hydroquinone (9% conversion). 

Abbreviations s, solid sublimes d, solid decomposes. Under pressure. 

Sublimation is the direct conversion of a solid into its vapor. Frost disappears on a cold, dry morning as the ice sublimes directly into water vapor. Solid carbon dioxide also sublimes, which is why it is called dry ice. Each winter on Mars, solid carbon dioxide is deposited as polar frost, which sublimes when the feeble summer arrives (Fig. 6.24). The enthalpy of sublimation, AHsub, is the molar enthalpy change when a solid sublimes ... 

FIGURE 7.26 For some substances and at certain pressures, the molar Gibbs free energy of the liquid phase might never lie lower than those of the other two phases. For such substances, the liquid is never the stable phase and, at constant pressure, the solid sublimes when the temperature is raised to the point of intersection of the solid and vapor lines. 

The blue-black lustrous solid sublimes easily and forms a purple vapor. 

Sublimation Some solids sublime before they react in the gas phase. Heat transfer can be the rate-limiting step. 

Liquid carbon dioxide is usually stored under 20 bar pressure at — 18°C. Compression and cooling of the gas between the temperature limits at the triple point and the critical point will cause it to liquefy. The triple point is the pressure temperature combination at which carbon dioxide can exist simultaneously as gas, liquid and solid. Above the critical temperature point of 31°C it is impossible to liquefy the gas by increasing the pressure above the critical pressure of 73 bar. Reduction in the temperature and pressure of liquid below the triple point causes the liquid to disappear, leaving only gas and solid. (Solid carbon dioxide is also available for cryogenic work and at —78°C the solid sublimes at atmospheric pressure.)... 

The parameter V is the volume per mole, or m/n. Thus, in an isothermal process, when a mole of solid sublimates to make a perfect gas, the entropy changes with the volume according to... 

In the phase diagram, panel (a). solid C02 (Dry Ice) is in equilibrium with gaseous C02 at a temperature of —78.7°C and a pressure of 1.00 bar." The solid sublimes without turning into liquid. At any temperature above the triple point at —56.6°C, there is a pressure at which liquid and vapor coexist as separate phases. For example, at 0°C, liquid is in equilibrium with gas at 34.9 bar. Moving up the liquid-gas boundary, we see that two phases always exist until the critical point is reached at 31.3 C... 

TEREPHTHALIC ACID. CAS 100-21-01. Cf,H4(COOH)2, formula weight 166.13, crystalline solid sublimes upon heating, sp gr 1.510. The compound is almost insoluble in H2O, only slightly soluble in warm alcohol, and insoluble in ether. Terephtlialic acid (TPA) is a high-tonnage chemical, widely used in the production of synthetic materials, notably polyester fibers (poly-(ethylene terephthalate)). 

Fluorides Vanadium trifluonde VPs, green crystalline solid vanadium tetrafluoride VF4, brownish-yellow crystalline solid vanadium pentafluo-ride VF5, brownish-yellow crystalline solid, sublimes on heating. 

The drawing shows ordered particles in a solid subliming to give a gas. Formation of a gas from a solid increases molecular disorder, so AS is positive. Furthermore, because we re told that the process is nonspontaneous, AG is also positive. Because the process is favored by AS (positive) yet still nonspontaneous, AH must be unfavorable (positive). This makes sense, since conversion of a solid to a liquid or gas requires energy and is always endothermic. 

Colorless crystalline solid, sublimes at ambient temperture melts at 54°C boils at 174°C. 

Elemental iodine, I2 (solid, sublimes at 184°C), consists of violet-black rhombic crystals with a lustrous metallic appearance. More irritating to the lungs than bromine or chlorine, its general effects are similar to the effects of these elements. Exposure to iodine is limited by its low vapor pressure, compared to liquid bromine or gaseous chlorine or fluorine. 

Tert -butyllithium LiC-CH, i 3 ch3 Colorless crystalline solid subliming at 70-80°C, used as synthesis reagent... 

From the third law, entropies are always positive, requiring that the chemical potential of all phases decrease with temperature. However, because entropy is a measure of randomness, 5m gas > liq >, S , sol, and the chemical potential falls most rapidly with temperature for the gas phase and least rapidly for the solid phase. In Fig. la (drawn for a particular value of pressure), as the temperature is increased, pliq(7 ) crosses psol(T) at the melting point, and the liquid remains the most stable phase until pgas(T) crosses pliq(r) at the boiling point. In Fig. lb (drawn for a different substance or at a different pressure), pgas(7 ) falls so rapidly with temperature that it crosses psol( T) before pliq(7 ) does. As a result, liquid is never the most stable phase and, at the given pressure, the solid sublimates directly to gas. 

Zirconium tetrachloride is a white crystalline solid with specific gravity 2.803.4 The solid sublimes under atmospheric pressure at 331° [log pBin. = — (26,000/4.57T) + 12.30] but melts at 437° under its own pressure, which is about 25 atm. at this temperature.18 The vapor density shows normal behavior up to 500°, at which point dissociation is suspected.16 Electron-diffraction results show tetrahedral symmetry with the Zr-Cl distance 2.33 A.17 Todd18 has recently determined the heat capacity of zirconium tetrachloride at low temperatures (extrapolated below —222°) and calculated the usual thermodynamic constants for the compound. The melting point of hafnium tetrachloride has been estimated as 432° under its own pressure, but the solid sublimes under atmospheric pressure at 317°.12 Both metal chlorides are extremely hygroscopic. They are easily soluble in water, although extensively hydrolyzed. All high-purity samples should be desiccated as well as possible. 

For example, by employing a temperature of 250° C. and a rapid stream of fluorine, a very reactive osmium preparation was found to yield a high percentage of Osmium Oetafluoride, OsF8. This, being the most volatile of the products, is collected in a vessel cooled in a mixture of solid carbon dioxide and alcohol, under which conditions it yields a yellow, solid sublimate. It melts at 34-5° C. to a yellowish red liquid. Its vapour is colourless, has a characteristic odour and metallic taste it attacks the mucous membranes of the nose and eyes. In moist air the vapour yields a white cloud. 

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