Closed container, liquid evaporation

In a chemical system, mass will flow until equilibrium is achieved. Consider, as an example, the approach to equilibrium between the liquid and gas phases of a substance. If a sample of liquid is introduced into an evacuated closed container, the liquid will begin to evaporate to form a gas phase. If one measures the pressure of this gas as a function of time, a graph similar to that shown in Figure 5.5 will be obtained.0 The pressure at first increases rapidly. After a short time period, however, it levels off at the value represented by p. ... 

Figure 5.5 Change of gas pressure with time as a liquid evaporates into a closed container. The pressure p is the equilibrium vapor pressure.

To build a molecular model of the equilibrium between a liquid and its vapor we first suppose that the liquid is introduced into an evacuated closed container. Vapor forms as molecules leave the surface of the liquid. Most evaporation takes place from the surface of the liquid because the molecules there are least strongly bound to their neighbors and can escape more easily than those in the bulk. Howevei as the number of molecules in the vapor increases, more of them become available to strike the surface of the liquid, stick to it, and become part of the liquid again. Eventually, the number of molecules returning to the liquid each second matches the number escaping (Fig. 8.2). The vapor is now condensing as fast as the liquid is vaporizing, and so the equilibrium is dynamic in the sense introduced in Section 7.11 ... 

FIGURE 8.2 When a liquid and its vapor are in dynamic equilibrium inside a closed container, the rate of condensation is equal to the rate of evaporation. 

At 25 °C, water is ordinarily a liquid. However, even at 25 °C, water evaporates. In a closed container at 25 °C, water evaporates enough to get a 27 torr water vapor pressure in its container. The pressure of the gaseous water is called its vapor pressure at that temperature. At different tempera-... 

A dynamic equilibrium is a situation in which two (or more) opposing processes occur at the same rate so that no net change occurs. This is the kind of equilibrium that is established between two physical states of matter, e.g., between a liquid and its vapor, in which the rate of evaporation is equal to the rate of condensation in a closed container ... 

The transition from the liquid to the gaseous state is called evaporation or vaporization. The reverse is referred to as condensation or, in terms of rainfall, precipitation. If heated to 100°C in a closed container at 1 atm pressure, the two phases of water will coexist in the equilibrium given in Eq. 2.4. 

All substances in the form of liquids (and even many solids) possess a type of molecular motion that results in the escape of molecules from their surface in the form of vapor when they are not confined. When a liquid is left in an open container at room temperature, its molecules evaporate. When the liquid is confined in a partially full container that is dosed, the molecules will continue to escape from the surface however, because they cannot escape from the closed container, some of the molecules will return to liquid. Within a short time, an equilibrium will be achieved between the number of molecules escaping from the surface and those returning to the surface of the liquid. 

Water evaporates at all temperatures above 0 °C. Thus, water in a open container evaporates, and in time no liquid water molecules remain in the container. However, water in a closed container does not exhaust, although evaporation takes place continuously. 

Evaporation of liquid water forms water vapor which is a gas in the closed container. After a while, water vapor molecules start collisions with each other and with the water s surface then they turn into water. Therefore, evaporation and condensation are a reversible process in a closed container. Reversible processes are represented by Irriversible processes are represented by... 

The conversion of a liquid to a vapor takes place in a visible way when the liquid boils, but it takes place under other conditions as well. Let s imagine the two experiments illustrated in Figure 10.11. In experiment (a), we place a liquid in an open container in experiment (b), we place the liquid in a closed container connected to a mercury manometer (Section 9.1). After a certain amount of time has passed, the liquid in the first container has evaporated, while the liquid in the second container remains but the pressure has risen. At equilibrium and at a constant temperature, the pressure has a constant value called the vapor pressure of the liquid. 

FIGURE 10.11 Liquids after sitting for a length of time in (a) an open container and (b) a closed container. The liquid in the open container has evaporated, but the liquid in the closed container has brought about a rise in pressure. 

Molecules that enter the vapor phase in an open container can escape the liquid and drift away until the liquid evaporates entirely, but molecules in a closed container are trapped. As more and more molecules pass from the liquid to the vapor, chances increase that random motion will cause some of them to return occasionally to the liquid. Ultimately, the number of molecules returning to the liquid and the number escaping become equal, at which point a dynamic equilibrium exists. Although individual molecules are constantly passing back and forth from one phase to the other, the total numbers of molecules in both liquid and vapor phases remain constant. 

We re already familiar with the concept of equilibrium from our study of the evaporation of liquids (Section 10.5). When a liquid evaporates in a closed container, it soon gives rise to a constant vapor pressure because of a dynamic equilibrium in which the number of molecules leaving the liquid equals the number returning from the vapor. Chemical reactions behave similarly. They can occur in both forward and reverse directions, and when the rates of the forward and reverse reactions become equal, the concentrations of reactants and products remain constant. 

Recently, Long et al. [32] reported the results of a series of experiments in which they studied RDX decomposition in open and closed ( pierced ) containers to monitor the kinetics as a function of the extent of reaction. Heating RDX in a closed container causes decomposition to occur in the liquid phase for which they found Ea 47.8 kcal/mol, in accord with the accepted value for N-N bond fission. They also determined Ea 23.9 kcal/mol for evaporation, a value well below the energy for most chemical decomposition reactions. 

If the population of the state of lower energy is much larger than that of the upper one (which is often the case), the value of n is essentially a constant. Such a situation exists, for example, when a small amount of liquid evaporates in a closed container, leaving a considerable amount of the liquid phase. Therefore, we can write Eq. (4.2) as... 

Suppose all the liquid water in a closed container evaporates. Is it certain that the pressure of the water vapor in the container is equal to the vapxjr pressure of water ... 

When a liquid is placed in a closed container, the amount of liquid at first decreases but eventually becomes constant. The decrease occurs because there is an initial net transfer of molecules from the liquid to the vapor phase (Fig. 16.44). However, as the number of vapor molecules increases, so does the rate of return of these molecules to the liquid. The process by which vapor molecules re-form a liquid is called condensation. Eventually, enough vapor molecules are present above the liquid so that the rate of condensation equals the rate of evaporation (see Fig. 16.45). At this point no further net change occurs in the amount of liquid or vapor because the two opposite processes exactly balance each other the system is at equilibrium. Note that this system is highly dynamic on the molecular level—molecules are constantly escaping from and entering the liquid at a high rate. However, there is no net change, because the two opposite processes just balance each other. 

A closed container of water is a two-phase system in which molecules of water are in a gas phase in the space above the liquid phase. Moving randomly above the liquid, some of these molecules strike the walls and some go back into the liquid, as shown in Figure 21. An equilibrium, in which the rate of evaporation equals the rate of condensation, is soon created. The molecules in the gas exert pressure when they strike the walls of the container. The pressure exerted by the molecules of a gas, or vapor, phase in equilibrium with a liquid is called the vapor pressure. You can define boiling point as the temperature at which the vapor pressure equals the external pressure. 

Figure 13-24 compares evaporation in an open container with evaporation in a closed container. If water is in an open container, all the molecules will eventually evaporate. The time it takes for them to evaporate depends on the amount of water and the available energy. How does temperature affect the rate of evaporation In a partially filled, closed container, the situation is different. Water vapor collects above the liquid and exerts pressure on the surface of the liquid. The pressure exerted by a vapor over a liquid is called vapor pressure. How would a rise in temperature affect vapor pressure ... 

To protect your children from metallic mercury, teach them not to play with shiny, silver liquids. Schoolteachers (particularly science teachers) and school staff need to know about students fascination with metallic mercury. Teachers and school staff should teach children about the dangers of getting sick from playing with mercury, and they should keep metallic mercury in a safe and secured area (such as a closed container in a locked storage room) so that children do not have access to it without the supervision of a teacher. Metallic mercury evaporates slowly, and if it is not stored in a closed container, children may breathe toxic mercury vapors. 

Figure 13-10 (a) Liquid continuously evaporates from an open vessel, (b) Equilibrium between liquid and vapor is established in a closed container in which molecules return to the liquid at the same rate as they leave it. (c) A bottle in which liquid-vapor equilibrium has been established. Note that droplets have condensed. 

A molecule in the vapor may strike the liquid surface and be captured there. This process, the reverse of evaporation, is called condensation. As evaporation occurs in a closed container, the volume of liquid decreases and the number of gas molecules above the surface increases. Because more gas phase molecules can collide with the surface, the rate of condensation increases. The system composed of the liquid and gas molecules of the same substance eventually achieves a dynamic equilibrium in which the rate of evaporation equals the rate of condensation in the closed container. 

At 25 °C, water is ordinarily a liquid. However, in a closed container even at 25 °C, water evaporates to get a 24 torr water vapor pressure in its container. The pressure of the gaseous water is called its vapor pressure at that temperature. At different temperatures, it evaporates to different extents to give different vapor pressures. As long as there is liquid water present, however, the vapor pressure above pure water depends on the temperature alone. Only the nature of the liquid and the temperature affect the vapor pressure the volume of the container does not affect its final pressure. The water vapor mixes with any other gas(es) present, and the mixture is governed by Dalton s law of partial pressure, just as any other gas mixture is. 

When a rain puddle dries up, the water molecules leave the liquid state and become water vapor. Finally, the puddle is gone. However, if liquid water is in a closed container, only some of the water evaporates. Figure 10.20 describes how a liquid in a closed container comes to equilibrium with its vapor. 

The partial pressure of H20 above liquid water in a closed container at 25°C will build up to this value. If the cover is removed so that this pressure cannot be maintained, the system will cease to be at equilibrium and the water will evaporate. This temperature corresponds of course to the boding point of water. The normal boiling point of a liquid is the temperature at which the partial pressure of its vapor is 1 atm. The only way to heat water above its normal boding point is to do so in a closed container that can withstand the increased vapor pressure. Thus a pressure cooker that operates at 120°C must be designed to withstand an internal pressure of at least 2 atm. 

Eventually the attendant is reaching the balls quickly enough to return them to the bin just as quickly as the kids can throw them out. At this point, the process continues at a frantic pace but with no net change in the numbers of balls in the bin and on the floor. This is like the dynamic equilibrium reached between the rates of evaporation and condensation of liquid and vapor in a closed container. When the rates are equal, there is no net change in the amount of liquid or vapor in the system. 

WTien a liquid is placed in a closed container and begins to evaporate, the vapor exerts a pressure against the container s walls (and against the surface of the liquid). When this system reaches a dynamic equilibrium between the rates of evaporation and condensation, the amount of vapor above the liquid remains constant. According to the ideal gas equation, the pressure of the vapor (or gas) is dependent on the vapor s concentration (moles divided by volume or nIV) and its temperature ... 

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