Endothermic process vaporization

A process that releases heat into the surroundings is called an exothermic process. Most common chemical reactions—and all combustions, such as those that power transport and heating—are exothermic (Fig. 6.8). Less familiar are chemical reactions that absorb heat from the surroundings. A process that absorbs heat is called an endothermic process (Fig. 6.9). A number of common physical processes are endothermic. For instance, vaporization is endothermic, because heat must be supplied to drive molecules of a liquid apart from one another. The dissolution of ammonium nitrate in water is endothermic in fact, this process is used in instant cold packs for sports injuries. 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

This is a form of Henry s law, where the Henry s law constant is simply the product of the vapor pressure of solute 2 multiplied by the energy correction term exp(Ae/ 7 ). For oleophilic substances, which do not like water, mixing is an endothermic process and the term Ae/kT is positive, so that Henry s law constant is much higher than the vapor pressure of the pure substance. 

This is a strongly endothermic process, but it becomes possible at high temperature due to a favorable entropy change - formation of the random vapor state from solid reactants. Such reactions provide another reason for the lower flame temperatures achieved when organic binders are added to oxidizer/metal mixtures [3]. 

Feed to the FCC unit is mixed with hot catalyst and steam in a reactor line called a riser. The ratio of catalyst oil feed can typically range from 4 1 to 9 1 by weight. Overall, FCC is an endothermic process. Heat provided by the hot, circulating catalyst is the prime source of energy driving the FCC process. In the riser, vaporized oil is cracked catalytically in less than two seconds. The vapors and catalyst flow out of the riser and into the reactor. At this point, most cracking reactions have occurred. 

If you analyze the four spontaneous endothermic processes mentioned previously, you ll see that each involves an increase in the randomness of the system. When ice melts, for example, randomness increases because the highly ordered crystalline arrangement of tightly held water molecules collapses and the molecules become free to move about in the liquid. When liquid water vaporizes, randomness further increases because the molecules can now move independently in the much larger volume of the gas. In general, processes that convert a solid to a liquid or a liquid to a gas involve an increase in randomness and thus an increase in entropy (Figure 17.3). 

Some chemical changes are also endothermic processes. Figure 3 shows a chemical reaction that occurs when barium hydroxide and ammonium nitrate are mixed. Notice in Figure 3 that these two solids form a liquid, slushlike product. Also, notice the ice crystals that form on the surface of the beaker. As barium hydroxide and ammonium nitrate react, energy is absorbed from the beaker s surroundings. As a result, the beaker feels colder because the reaction absorbs energy as heat from your hand. Water vapor in the air freezes on the surface of the beaker, providing evidence that the reaction is endothermic. 

What happens in the reverse processes, when water vapor condenses to hq-uid water or liquid water freezes to ice The same amounts of energy are released in these exothermic processes as are absorbed in the endothermic processes of vaporization and melting. Thus, the molar enthalpy (heat) of condensation (A//gojjd) the molar enthalpy of vaporization have the same numerical value but opposite signs. Similarly, the molar enthalpy (heat) of solidification (A/Zg iid) and the molar enthalpy of fusion have the same numerical value but differ in sign. 

C is correct. Condensation must occur to form liquid water. Condensation is an exothermic process, so the formation of liquid water should be more exothermic than the formation of water vapor. The standard enthalpy of formation of water vapor will not be an endothermic process, so D is wrong. 

The L vov sublimation model. The applications (outlined in Sections 2.4.5. and 2.4.6.) of the Hertz-Knudsen-Langmuir vaporization model to decompositions by L vov et al. [94] are based on the assumption that decomposition involves an initial sublimation step, followed by condensation of the less volatile products. Because sublimation is an endothermic process, the condensation process would need to make a significant energetic contribution for the decomposition to be exothermic overall. 

Case 1 If the intermolecnlar forces between A and B molecules are weaker than those between A molecules and between B molecules, then there is a greater tendency for these molecules to leave the solution than in the case of an ideal solution. Consequently, the vapor pressure of the solution is greater than the sum of the vapor pressures as predicted by Raoult s law for the same concentration. This behavior gives rise to the positive deviation [Fignre 12.9(a)]. In this case, the heat of solution is positive (that is, mixing is an endothermic process). 

As heat is taken from your skin to vaporize the water, you cool down. The heat required to vaporize one mole of a liquid is called its molar enthalpy (heat) of vaporization (AHvap). Similarly, if you want a glass of cold water, you might drop an ice cube into it. The water cools as it provides the heat to melt the ice. The heat required to melt one mole of a solid substance is called its molar enthalpy (heat) of fusion (AHfus). Because vaporizing a liquid and melting a solid are endothermic processes, their AH values are positive. Standard molar enthalpies of vaporization and fusion for five common compounds are shown in Table 15.4. 

O Reading Check Categorize condensation, solidification, vaporization, and fusion as exothermic or endothermic processes. 

Syngas can be produced in the endothermic process of methane oxidation by water vapor, often referred to as the steam reforming of methane ... 

AH° is positive because this is an endothermic process. We also expect AS° to be positive becanse this is a liquid vapor phase change. AG° is positive because we are at a temperature that is below the boiling point of benzene (80.1°C). 

Evaporation and condensation are opposite processes. Evaporation is an endothermic process condensation is an exothermic process. Evaporation requires an input of energy to provide the increased kinetic energy possessed by the molecules when they are in the gaseous state. It occurs when the molecules in a liquid are moving fast enough and randomly enough that molecules are able to escape from the surface of the liquid and enter the vapor phase. 

The enthalpy change for a reaction depends on the states of the reactants and products. If the product in Equation 5.18 were H20(g) instead of H20(/), AH would be —802 kJ instead of —890 kJ. Less heat would be available for transfer to the surroundings because the enthalpy of H20(g) is greater than that of H20(Z). One way to see this is to imagine that the product is initially liquid water. The liquid water must be converted to water vapor, and the conversion of 2 mol H20(() to 2 mol H20(g) is an endothermic process that absorbs 88 kJ ... 

The feedstock picture further diversified in 1920 with the commercialization of ethanol dehydrogenation to generate acetaldehyde. (The process was conducted in the vapor phase at 260-290°C using copper-chromite catalysts.) While the process was known as early as 1886 the development of adequate catalysts for the endothermic process would take nearly 35 years. Subsequent oxidation to acetic acid provided an additional source of acetic acid. These technologies would largely stay in place with only minor modification until the 1950 s. A summary of the chemical routes to the various acetyls in 1920 is shown in Figure 1. 

The heat input required to convert the solid ice at 0°C to liquid water at 0°C is found by multiplying the mass of ice (5.00 g) by the heat of fusion of ice (80 cal/g) to give 400 cal. This gives 5.00 grams of liquid water at 0°C whose temperature must be increased to 100°C before vaporization can occur. The heat required to increase the temperature is found by multiplying the mass (5.00 g) times the heat capacity of water (1.00 cal/g-°C) times the total temperature change (100°C). The temperature change requires a total of 500 cal. Lastly, the heat required to convert the 5.00 g of water at 100°C to 5.00 g of steam at 100°C is simply the mass (5.00 g) times the heat of vaporization (540 cal/g). The vaporization requires 2,700 cal. Thus, the total heat input required (an endothermic process) is 400 cal plus 500 cal plus 2,700 cal or 3,600 cal. 

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