Endothermic/exothermic process

When a solute is dissolved in a solvent, heat change generally occurs. A dissolution process may be exothermic or endothermic. Exothermic processes emit energy as heat. Endothermic processes absorb energy as heat. Temperature rises in an exothermic process, but falls in an endothermic one. When lithium chloride (LiCl) dissolves in water, the solution gets warmer and the temperature goes up. We can say that the dissolution of lithium chloride is exothermic. (Figure 6). 

Let us consider the formation of sodium chloride from its elements. An energy (enthalpy) diagram (called a Born-Haber cycle) for the reaction of sodium and chlorine is given in Figure 3.7. (As in the energy diagram for the formation of hydrogen chloride, an upward arrow represents an endothermic process and a downward arrow an exothermic process.)... 

Endothermic and exothermic processes. On the left the icicle melts as heat is absorbed by the ice, an endothermic process. On the right, steam rises from boiling water, an exothermic process. 

There have been few attempts to classify decomposition reactions of solids. Gamer [64] made only the broad distinction between endothermic processes (which are often reversible and include dissociation of crystalline hydrates and carbonates) and exothermic processes (which are usually... 

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. 

The change in enthalpy of a system is equal to the heat supplied to the system at constant pressure. For an endothermic process, AH > 0 for an exothermic process, AH < 0. 

FIGURE 7.17 (a) In an exothermic process, heat escapes into the surroundings and increases their entropy, (b) In an endothermic process, the entropy of the surroundings decreases. The red arrows represent the transfer of heat between system and surroundings, and the green arrows indicate the entropy change of the surroundings. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

Qs = heat generated in the system. If heat is evolved (exothermic processes) Qs is taken as positive, and if heat is absorbed (endothermic processes) it is taken as negative. Qp = process heat added to the system to maintain required system temperature. 

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]. 

An endothermic process absorbs heat energy from its surroundings an exothermic process releases heat energy to its surroundings. If a reaction is endothermic in one direction, it is exothermic in the opposite direction. For example, the melting of 1 mole of ice water is an endothermic process requiring 6.02 kJ of heat ... 

Likewise, if control is lost of an endothermic reaction process, such as by a heating control valve opening too far or by a steam leak directly into the reaction mass, a degradation reaction or other secondary or side reaction may be initiated that can be exothermic and can lead to a thermal runaway. 

Figure 3.8 Born-Haber cycle constructed to obtain the lattice enthalpy A//(E, lce) of sodium chloride. All arrows pointing up represent endothermic processes and arrows pointing down represent exothermic processes (the figure is not drawn to scale)...

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

In a plot of the difference in temperature between a sample and a reference material vs. the temperature of the surroundings, why does an endothermic process produce a negative peak while an exothermic process produces a positive peak. 

The difference is Ts- Tr, in which Ts is the temperature of the sample and TR is the temperature of a reference material. This difference is negative when an endothermic process occurs because Ts lags behind TR due to the fact that the sample absorbs heat from the environment while its temperature remains the same. The opposite is true for an exothermic process. 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

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