Phase change sublimation

Movement across a boundary line corresponds to a phase change. The arrows on the figure show six different phase changes sublimation and its reverse, deposition melting and its reverse, freezing and vaporization and its reverse, condensation. 

The thermal decomposition of a solid, which necessarily (on the above definition) incorporates a chemical step, is sometimes associated with the physical transformations to which passing reference was made above melting, sublimation, and recrystallization. Aspects of the relationships between physical transitions and decomposition reactions of solids are discussed in a book by Budnikov and Ginstling [1]. Since, in general, phase changes exert significant influence upon concurrent or subsequent chemical processes, it is appropriate to preface the main survey of the latter phenomena with a brief account of those features of melting, sublimation, and recrystallization which are relevant to the consideration of thermal decomposition reactions. 

Fusion and vaporization are the most familiar phase changes, but sublimation is also common. Sublimation is a phase change in which a solid converts directly to a vapor without passing through the liquid phase. Dry ice (solid CO2) sublimes at 195 K with A ii/subl — 25.2 kJ/mol. Mothballs contain naphthalene (Cio Hg,... 

Phase changes, which convert a substance from one phase to another, have characteristic thermodynamic properties Any change from a more constrained phase to a less constrained phase increases both the enthalpy and the entropy of the substance. Recall from our description of phase changes in Chapter 11 that enthalpy increases because energy must be provided to overcome the intermolecular forces that hold the molecules in the more constrained phase. Entropy increases because the molecules are more dispersed in the less constrained phase. Thus, when a solid melts or sublimes or a liquid vaporizes, both A H and A S are positive. Figure 14-18 summarizes these features. 

These energies relate to bond rearrangement in gaseous molecules, but calculations are often performed for reactions of condensed phases, by combining the enthalpies of vaporization, sublimation, etc. We can calculate a value without further correction if a crude value of AHr is sufficient, or we do not know the enthalpies of phase changes. 

Dry ice is carbon dioxide (CO2) in its solid phase. We call it dry because it is wholly liquid-free at such solid CO2 looks similar to normal ice (solid water), but it melts without leaving a puddle. We say it sublimes, i.e. undergoes a phase change involving direct conversion from solid to gas, without liquid forming as an intermediate phase. CC>2(i) can only be formed at extreme pressures. 

The term sublimation strictly refers to the phase change solid -> vapour, with no intervention of a liquid phase. In industrial applications, however, the term usually includes the reverse process of condensation or desublimation solid -> vapour -> solid. In practice, it is sometimes desirable to vaporise a substance from the liquid state and hence the... 

This temperature is called the normal boiling point of water. If the container were to be opened, some of the gas molecules would escape. To replace the missing water, the phase change represented by Eq. 2.4 would be driven toward the products until all of the liquid water evaporated. The direct transition from the solid to the gaseous phase is termed sublimation. Ice will sublime under arid conditions, especially in polar climates. 

At the right combination of pressure and temperature, matter can move directly from a solid to a gas, or vapor. This type of phase change is called sublimation, and it s the kind of phase change responsible for the white mist that emanates from dry ice, the common name for solid carbon dioxide. Movement in the opposite direction, from gas directly into solid phase, is called deposition. 

At temperatures of about 4000°K., the free energy of formation of acetylene from its elements approaches zero, and the equilibrium yield of acetylene is appreciable. The system is complicated, however, by other reactions and phase changes which occur at these high temperatures. Carbon sublimes at about 4000°K., various species of carbon Ci, C2, and Ca are formed, and dissociation of molecular hydrogen occurs. 

Freeze drying is used to remove water from heat-sensitive substances at low temperature by the process of sublimation, where water is removed via a phase change from a solid to a vapor without passing through a liquid state. This takes place below the triple point of water (Fig. 1), at approximately 0°C and 4.5 mm of mercury (Hg). In addition, when freeze drying is carried out properly, the freeze-dried solid has a relatively high specific surface area, which promotes rapid, complete reconstitution. 

Sublimation is the process by which a solid changes into a gas without going through the liquid phase. It is the phase change responsible for making snow disappear without melting. 

Snow sublimation happens particularly often in the western United States, where warm, dry winds often blow after an intense cold spell. When these warm, dry winds blow over an area covered in snow, the snow sublimates directly to a gas, skipping the liquid phase entirely. In some areas, this wind is known as the Chinook Wind (Chinook is a Native American word that means snow eater. 8) Although sublimation plays a less vital role in the planets water cycle than some other phase changes, such as evaporation and condensation, it still serves to move water around Earth. 

Like sublimation, deposition plays a lesser role in the water cycle than some other phase changes, but it is no less important to the overall process. Deposition moves gaseous water in the air into the planets water cycle. 

State-of-matter changes, or phase changes, usually depend on the surrounding temperature and pressure. Evaporation, condensation, sublimation, and deposition are examples of common phase changes that often happen naturally on Earth. Chemists can also produce phase changes by manipulating temperatures and pressures in controlled environments. 

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