The Process of Vaporization

We now turn our attention to vaporization, the process by which thermal energy can overcome intermolecular forces and produce a state change from liquid to gas. We first discuss the process of vaporization itself, then the energetics of vaporization, and finally the concepts of vapor pressure, dynamic equilibrium, and critical point. Vaporization is a common occurrence that we experience every day and even depend on to maintain proper body temperature. 

The distributions of thermal energies for the molecules in a sample of water at two different tanperatures 

A FIGURE 11.22 Meniscuses of Water and Mercury The meniscus of water is concave because water molecules are more strongly attracted to the glass wall than to one another. The meniscus of mercury is convex because mercury atoms are more strongly attracted to one another than to the glass walls. 

A RGURE 11.23 Vaporization of Water Some molecules in an open beaker have enough kinetic energy to vaporize from the surface of the liquid. 

Althongh both evaporation and condensation occur in a beaker open to the atmosphere, under normal conditions evaporation takes place at a greater rate because most of the newly evaporated molecules escape into the surrounding atmosphere and never come back. The result is a noticeable decrease in the water level within the beaker over time (nsnally several days). 

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Distillation is the separation of the constituents of a liqmd mixture via partial vaporization of the mixture and separate recovery of vapor and residue. The process of vaporization is generally of a differential nature. 

All of us are familiar with the process of vaporization, in which a liquid is converted to a gas, commonly referred to as a vapor. In an open container, evaporation continues until all the liquid is gone. If the container is closed, the situation is quite different. At first, the movement of molecules is primarily in one direction, from liquid to vapor. Here, however, the vapor molecules cannot escape from the container. Some of them collide with the surface and reenter the liquid. As time passes and the concentration of molecules in the vapor increases, so does the rate of condensation. When the rate of condensation becomes equal to the rate of vaporization, the liquid and vapor are in a state of dynamic equilibrium ... 

In discussing the phase-change problem in this chapter, the discussion has been limited to the process of vaporizing a liquid. Equally important is the process of condensing a vapor. The equations developed in Section III,C can be applied directly to this process. 

This indicates that in the process of vaporization 1 mole of liquid is converted into 2 moles of vapor. Therefore, we conclude that [Al(C2ff5)3]2 dimers are present in the liquid, but the vapor consists of monomeric Al(C2ff5)3 units. Examination of the data for [Al(Cff3)3]2 shows an entropy of vaporization of 112.61 mol 11<, which is greater than the value of 881 mol-1 K 1 predicted by Trouton s rule, but it is lower than twice the value. This value could be interpreted as corresponding to a liquid that is only partially dimerized being converted completely into monomer during vaporization. 

The heat pipe achieves its high performance through the process of vapor state heat transfer. A volatile liquid employed as the heat-transfer medium absorbs its latent heat of vaporization in the evaporator (input) area. The vapor thus formed moves to the heat output area, where condensation takes place. Energy is stored in the vapor at the input and released at the condenser. The liquid is selected to have a substantial vapor pressure, generally greater than 2.7 kPa (20 mm Hg), at the minimum desired operating temperature. The highest possible latent heat of vaporization is desirable to achieve maximum heat transfer and temperature uniformity with minimum vapor mass flow. 

The process of vaporization/condensation at constant temperature can be explained by the points D, E and F on Figure 2.1. Water either would condense or vaporize at constant temperature as the pressure approaches point E on the vapor-pressure curve. 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

The process of pressure distillation through a homogeneous membrane is based first on the common fact that the vapor pressure of any liquid can be increased by compressing it or decreased by placing it under suction, and second on the equally common fact that only pure water vapor escapes from water into vapor or air, leaving nonvolatile salts behind the phase boundary. In operating the processes of vaporization—heat transfer and diffusion across an extremely thin gap—no new phenomena or new properties of materials are required. However, the novel combination of capillary surfaces, pressure, and extremely short paths for heat and diffusion offers an opportunity for improvements in film properties and methods of construction not known before. 

Condensation refers to the formation of a liquid or solid directly from a gas. (Sometimes, forming a solid directly from a gas is called deposition.) The opposite processes are evaporation or sublimation, respectively. During all of these processes, pressure and temperature cannot be varied separately. If one is fixed, so is the other. It is common experience that water boiling at 1.0 atm pressure remains at 100°C as heat is added and its volume increases, until there is no more liquid remaining in the system. During the process of vaporization, liquid and gaseous water simultaneously exist in the system. The 1.0-atm isobar of water is shown in Fig. 7. 

On a molecular level, the change from a liquid to a gas is accompanied by a separation of the molecules—from close proximity to far apart. Only the most energetic of the liquid molecules have sufficient energy to overcome the attractive forces and go into the gas phase. Thus, the process of vaporization leaves behind the less energetic molecules. As we learned in Section 12.10, the absolute... 

Detailed discussion of the properties of Mn(C0)5N02 is outside the scope of this paper. It is of interest, however, that when heated at 65° under a vacurnn, the compound sublimes, and the product appears from its infrared spectrum to be a mixture of nitro and nitrito complexes. Some conversion of the Mn-N02 bond to Mn-ONO therefore occurs during the process of vaporization and condensation. 

Distillation is the process of vaporizing and condensing a liquid to purify or concentrate a substance or to separate a volatile substance from less volatile substances. It is the oldest method of water purification. Problems with distillation for preparing reagent water include the carryover of volatile impurities and entrapped water droplets that may contain impurities into the purified water. This will result in contamination of the distillate with volatUes, sodium, potassium, manganese, carbonates, and sulfates. As a result, water treated by distillation alone does not meet the specific conductivity requirement of type I water. 

B. Line DE. During vaporization there is no temperature change. The process of vaporization begins to occur right at point D. 

The solar condensation sequence The processes of vaporization and condensation are of major importance in the solar nebula. In order to systematize this process, Grossman (1972) used thermodynamic equilibria to calculate the composition of phases in equilibrium with a gas with cosmic element concentrations, at a pressure of 10 3 atm, and as a function of temperature. This work has subsequently been developed by others and is systematized in Lewis (2004). It is worth noting that in detail it is likely that the assumption of equilibrium conditions will not... 

The processes of vaporization and dissociation in the vapour of the adducts AlBr3,NH3 and All3,NH3 have been studied using a membrane null-manometer the latter compound undergoes ammonolysis on heating. 

By the term vaporization we are to understand certain mechanical operations by which volatile substances are separated from other fixed bodies, or from bodies which may be less volatile, by the action of beat. When a volatile liquid is separated from a less volatile liquid, by the process of vaporization, we have w t is known as evaporization. When a volatile liquid is to be collected we have what is known as distillation. When a solid is to be separated from the volatile liquid, we have what is known as desiccation, in which solid substances are deprived of moisfure. Bxcication is the process by which a solid, crystalline substance is deprived of its water of crystallization, by the aid of powerful beat. 

Capillary condensation is the process of vapor condensation in the fine... 

Development and industrialization of the process of vapor-phase Beckmann rearrangement of cyclohexanone oxime 07BCJ1280. 

The further growth of drops is caused by the process of vapor diffusion to the surfaces of the droplet. 

Why does the process of vaporization require an input of energy Why is it so important that water has a large heat of vaporization What is condensation Explain how the processes of vaporization and condensation represent an equilibrium in a closed container. Define the equilibrium vapor pressure of a liquid. Describe how this pressure arises in a closed container. Describe an experiment that demonstrates vapor pressure and enables us to measure the magnitude of that pressure. How is the magnitude of a liquid s vapor pressure related to the intermolecular forces in the liquid ... 

The discovered correlation of P. with P must be connected with the process of vaporization inside the pipe. Vaporization on the rarefaction front at a pressure close to leads to a splitting of the front of a rapid pressure drop into two. 

An alternative way to achieve atomization under temporally isothermal conditions is to place the sample on a graphite probe (see Figure 3). The probe is removed from the tube prior to the atomization stage, and then reintroduced once the tube has reached the set temperature. This system is more complex than the platform, and requires the presence of an additional aperture in the tube, which reduces sensitivity. The use of platforms and probes is very efficient in reducing interference effects, but suffers from the disadvantage that the processes of vaporization and atomization are coupled, which may sometimes require compromise conditions. 

The van t Hoff equation for the temperature dependence of equilibrium constants (Equation 10.16) is very similar in form to the Clausius-Clapeyron equation (Equation 9.4) for the temperature dependence of vapor pressure. Give an explanation for this similarity. Hint Write the process of vaporization of a liquid as an equilibrium reaction. What is the equilibrium constant )... 

Distillation is the process of vaporizing a liquid, condensing the vapor, and collecting the condensate in another container. This technique is useful for separating a liquid mixture when the components have different boiling points or when one of the components will not distill. It is one of the principal methods of purifying a liquid. Four basic distillation methods are available to the chemist simple distillation, vacuum distillation (distillation at reduced pressure), fractional distillation, and steam distillation. This technique chapter will discuss simple distillation. Vacuum distillation will be discussed in Technique 16. Fractional distillation will be discussed in Technique 15, and steam distillation will be discussed in Technique 18. 

As noted above, the process of vapor degreasing typically involves four steps , and two major movements of the parts basket. These are listed in Table 1.5. 

When the gas phase of a substance coexists in equilibrium with the liquid or solid phase, and provided T and p are not close to the critical point, the molar volume of the gas is much greater than that of the condensed phase. Thus, we may write for the processes of vaporization and sublimation... 

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