Condensation, Evaporation, and Dynamic Equilibrium

Over the past weeks, you have seen numerous examples of how chemistry can deepen your understanding of everyday phenomena. In this chapter, we revisit the topic of liquids and the changes they undergo, in order to explain some of the things you might experience on an unusually warm spring morning. 

7 47 A.M. You get out of hed and take a quick shower. As you step out of the shower, you shiver with cold, even though the day is already warm. 

7 51 A.M. You turn toward the bathroom mirror to comb your hair, but it s so steamed up you can hardly see yourself What causes water to collect on the mirror s surface  

7 54 A.M. The water that dripped onto the floor has almost dried up now, but the water that collects in the bottom of your toothbrush cup never seems to disappear. Why  

8 01 A.M. Drops of cologne land on the counter. Why do they evaporate so much more quickly than water, and even more quickly in the heat from your blow dryer  

---

Chapter 14 J Liquids Condensation, Evaporation, and Dynamic Equilibrium... 

The vapour pressure increases as soon as the container is sealed, but the rate of increase slows down after a few seconds because, although water molecules continue to evaporate, some water molecules also condense back to form liquid water. After a few more seconds the vapour pressure levels off and reaches a maximum called the saturated vapour pressure - so-called because the air is saturated with water vapour at that temperature. For pure water at 20 °C, the saturated vapour pressure is 2.33 kPa. Even at this stage, molecules continue to evaporate and condense, but a dynamic equilibrium has been achieved in which the number of molecules evaporating per second equals the number of molecules which are condensing per second (Fig. 10.11(b)). For this reason, saturated vapour pressure is often called equilibrium vapour pressure. We represent the equilibrium by the equation... 

The BET treatment is based on a kinetic model of the adsorption process put forward more than sixty years ago by Langmuir, in which the surface of the solid was regarded as an array of adsorption sites. A state of dynamic equilibrium was postulated in which the rate at which molecules arriving from the gas phrase and condensing on to bare sites is equal to the rate at which molecules evaporate from occupied sites. 

Vapor pressure is an important property of liquids, and to a much lesser extent, of solids. If a liquid is allowed to evaporate in a confined space, tlie pressure of Uie vapor phase increases as Uie amount of vapor increases. If Uiere is sufficient liquid present, Uie pressure in Uie vapor space eventually comes to equal exacUy Uie pressure exerted by the liquid at its own surface. At Uiis point, a dynamic equilibrium exists in wliich vaporization and condensation take place at equal rates and Uie pressure in Uie vapor space remains constant. The pressure exerted at equilibrium is called Uie vapor pressure of the liquid. Solids, like liquids, also exert a vapor pressure. EvaporaUon of solids (sublimaUon) is noUccable only for Uie few solids characterized by appreciable vapor pressures. 

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 ... 

Rate of evaporation = rate of condensation The dynamic equilibrium between liquid water and its vapor is denoted H20(l) H20(g)... 

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. 

The easiest of the colligative properties to visualize is the effect of solute molecules on the vapor pressure exerted by a liquid. In a closed system, the solvent and its vapor reach dynamic equilibrium at a partial pressure of solvent equal to the vapor pressure. At this pressure, the rate of condensation of solvent vapor equals the rate of evaporation from the liquid. 

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 ... 

Following mono-layer uptake, further increase in pressure results in multi-layer adsorption of N2. For this part of the isotherm, condensation-evaporation equilibrium is assumed to take place, instead of adsorption-desorption equilibrium for each individual layer other than the first layer. This dynamic equiUbria for the first and higher layers and some simplifying assumptions form the basis for the B ET treatment of the multi-layer adsorption isotherm. A lengthy derivation leads to the BET relation between adsorbed volume of N2 and relative pressure. Here relative pressure is defined as the ratio of the equilibrium pressure to the... 

So, (once again) let s imagine a glass of water at a constant temperature. Let s also put some plastic wrap tightly over the top of the glass, so that the water cannot evaporate out into the surrounding room and escape our system. Some of the water molecules in the liquid sample will vaporize into the gas phase, and some of the gaseous water molecules will condense into the liquid phase. Eventually, the system will reach a state of dynamic equilibrium. 

Sharma, B (1998) Equilibrium and Dynamics of Evaporating or Condensing Thin Fluid Domains Thin Film Stability and Heterogeneous Nucleation, ACS J. Langmuir, vol. 14, pp. 4915-4928. 

Phase equilibrium is a dynamic process that is quite different from the static equilibrium achieved as a marble rolls to a stop after being spun into a bowl. In the equilibrium between liquid water and water vapor, the partial pressure levels off, not because evaporation and condensation stop, but because at equilibrium their rates become the same. The properties of a system at equilibrium are independent of the direction from which equilibrium is approached, a conclusion that can be drawn by observing the behavior of the liquid-vapor system. If we inject enough water vapor into the empty flask so that initially the pressure of the vapor is above the vapor pressure of liquid water, PvaplHiO)) then liquid water will condense until the same equilibrium vapor pressure is achieved (0.03126 atm at 25°C). Of course, if we do not use enough water vapor to exceed a pressure of 0.03126 atm, all the water will remain in the vapor phase and two-phase equilibrium will not be reached. 

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. 

The two opposing rates are not zero, but are equal to each other—hence we call this dynamic, rather than static, equilibrium. Even though evaporation and condensation are both continuously occurring, no net change occurs because the rates are equal. 

---