Effective vapour pressure

In a curved surface (Figure 11.3b), the coordination of any individual molecule will be less than in a planar surface (Figure 11.3a). The coordination is less at a curved surface with the result that there is a greater tendency for the molecules to be able to leave the surface and the effective vapour pressure is higher. Thus using... 

The rate at which a chemical volatUizes from soil is controlled by simultaneous interactions between soil properties, chemical s properties and environmental conditions. Soil properties that affect volatilization include soil water content, organic matter, porosity, sorption/diffusion characteristics of the soil, etc. chemical s properties that affect volatilization include vapour pressure, solubility in water, Henry s law constant, soil adsorption coefficient, etc. and finally, environmental conditions that affect volatilization include airflow over the surface, humidity, temperature, etc. VolatUization rate from a surface deposit depends only on the rate of movement of the chemical away from the evaporating surface and its vapour pressure. In contrast, volatilization of soil-incorporated organic chemicals is controlled by their rate of movement away from the surface, their effective vapour pressure at the surface or within the soil, and their rate of movement through the soil to the vapourizing surface. 

Phosphorus(V) oxide is an extremely effective desiccating agent, reducing the vapour pressure of water over it to a negligibly small... 

At the outset it will be profitable to deal with an ideal solution possessing the following properties (i) there is no heat effect when the components are mixed (ii) there is no change in volume when the solution is formed from its components (iii) the vapour pressure of each component is equal to the vapour pressure of the pure substances multiplied by its mol fraction in the solution. The last-named property is merely an expression of Raoult s law, the vapour pressure of a substance is pro-... 

The effect of superheated steam may be illustrated by reference to baizaldehyde, which boils at 178° at 760 mm. It distils with steam at 97-9° (Pj = 703-5 mm. and pg = 56-5 mm.) and the distillate contains 32-1 per cent, of benzaldehyde by weight. If one employs steam superheated to 133°, the vapour pressure of benzaldehyde (extrapolated from the boUing point - pressure curve) is 220 mm. hence pj = 540 (water), Pg = 220 (benzaldehyde), and... 

We may now understand the nature of the change which occurs when an anhydrous salt, say copper sulphate, is shaken with a wet organic solvent, such as benzene, at about 25°. The water will first combine to form the monohydrate in accordance with equation (i), and, provided suflScient anhydrous copper sulphate is employed, the effective concentration of water in the solvent is reduced to a value equivalent to about 1 mm. of ordinary water vapour. The complete removal of water is impossible indeed, the equilibrium vapour pressures of the least hydrated tem may be taken as a rough measure of the relative efficiencies of such drying agents. If the water present is more than sufficient to convert the anhydrous copper sulphate into the monohydrate, then reaction (i) will be followed by reaction (ii), i.e., the trihydrate will be formed the water vapour then remaining will be equivalent to about 6 mm. of ordinary water vapour. Thus the monohydrate is far less effective than the anhydrous compound for the removal of water. 

As already mentioned, the choice of the supercooled liquid as reference state has been questioned by some workers who use the saturation vapour pressure of the solid, which is measured at the working temperature in the course of the isotherm determination. The effect of this alternative choice of p° on the value of a for argon adsorbed on a number of oxide samples, covering a wide range of surface areas, is clear from Table 2.11 the average value of is seen to be somewhat higher, i.e. 18 OA. ... 

If and when these difficulties can be overcome, the way will be opened to the employment of adsorptives which have a vapour pressure high enough to enable their isotherms to be measured at temperatures close to ambient. This would substantially reduce the effect of thermal leakage and with it the distortion of the isotherm in the region near saturation. 

Removal of water from gases may be by physical or chemical means, and is commonly by adsorption on to a drying agent in a low-temperature trap. The effectiveness of drying agents depends on the vapour pressure of the hydrated compound - the lower the vapour pressure the less the remaining moisture in the gas. 

At room temperature and atmospheric pressure ammonia is a colourless, alkaline gas with a pungent smell. It dissolves readily in water. Physical properties are summarized in Table 9.6. The effect of temperature on vapour pressure of anhydrous ammonia is shown in Figure 9.2. 

These statements are only true when the liquid is a pure substance, i.e., does not change in composition during evaporation. This constancy of vapour-pressure serves to distinguish pure substances from solutions. The effects of surface tension, appearing when small droplets are used, and of electrification, must also be absent (cf. 100—102). 

The effect is, however, small, because it is known that the greatest tension which can exist on an isolated conductor in air under atmospheric pressure is equal to a pressure of about 0 3 mm. of mercury this corresponds to a lowering of vapour-pressure, in the case of water, of only about 10 6 mm. of mercury. 

Kinetic, energy, 24 theory of dissipation, 87 theory of gases, 515 theory of solids, 517 theories in thermodynamics, 513 Kirchoff s equation for effect of temperature, 112, 259 equations for vapour-pressure, 179, 190, 192, 390, 412 Konowalow s theorem, 385, 407 vapour-pressure curves, 382... 

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

The effect of temperature on vapour pressure is shown in Figure 8.3. Cylinders are normally protected from over-pressurization by a fusible metal plug melting at about 85°C. 

Hydrogen sulphide is used in the preparation of metal sulphides, oil additives etc., in the purification and separation of metals, as an analytical reagent and as raw material in organic synthesis. Physical properties are summarized in Table 8.11 and effects of temperature on vapour pressure are shown in Figure 8.4. 

It is a non-flammable colourless gas which is twice as dense as air, and slightly soluble in water, forming sulphurous acid. It is readily liquefied as a gas under its own vapour pressure of about 35psig (2.4 bar) at 21°C. Figure 8.5 depicts the effect of temperature on vapour pressure Table 8.12 lists the physical properties. Cylinders tend to be protected against over-pressurization by metal plugs melting at about 85°C. 

Liquid carbon dioxide produces a colourless, dense, non-flammable vapour with a slightly pungent odour and characteristic acid taste . Physical properties are given in Table 8.5 (see also page 277). Figure 8.1 demonstrates the effect of temperature on vapour pressure. 

Propane has a characteristic natural gas odour and is basically insoluble in water. It is a simple asphyxiant but at high concentrations has an anaesthetic effect. The TLV is 2500 ppm. It is usually shipped in low-pressure cylinders as liquefied gas under its own vapour pressure of ca 109 psig at 21°C. Its pressure/temperature profile is given in Figure 9.7. 

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