Endothermic and exothermic

The shock-modified composite nickel-aluminide particles showed behavior in the DTA experiment qualitatively different from that of the mixed-powder system. The composite particles showed essentially the same behavior as the starting mixture. As shown in Fig. 8.5 no preinitiation event was observed, and temperatures for endothermic and exothermic events corresponded with the unshocked powder. The observations of a preinitiation event in the shock-modified mixed powders, the lack of such an event in the composite powders, and EDX (electron dispersive x-ray analysis) observations of substantial mixing of shock-modified powders as shown in Fig. 8.6 clearly show the first-order influence of mixing in shock-induced solid state chemistry. 

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. 

Thermal analysis No test Same as USP stds. endotherm and exotherm units — 6° (HDPE), and 8° (LDPE) Same as USP stds. endo and exo limits—9((PET), none for PETG, glass transition within 4° (PET), 6° (PETG)... 

Figure 21.8(b) shows that stagnrg can also be used to climb the equilibrium barrier in adiabatic operation for an endothermic reaction. The differences between endothermic and exothermic systems are ... 

A comparison of Figures 11.15 and 11.16 with 11.17 and 11.18 will show the peaks for endotherms and exotherms reversed in direction for DTA relative to DSC. This is a commonly met presentation but as there is no agreed convention, different authors may use different presentations. Confusion can arise unless care is exercised in the interpretation of the thermograms. 

Endothermic and exothermic physical and chemical changes produce results in DSC. Melting is just one such change. Other examples would include chemical reactions that are either endothermic or exothermic. Physical changes would include deformations that do not involve melting. 

DTA-TG data for LiBH4 MgH2 0.3 1 is shown in figure 2. The data shows clearly the weight losses due to decomposition of the sample and heat flow due to endothermic and exothermic reactions during heating to 600°C. 

On the other hand, when a reaction results in a net release of energy, it is called an exothermic reaction. In an exothermic reaction, more energy is released to form bonds than is used to break bonds. Therefore, energy is released. Figure 5.3 shows the relationship between bond breaking, bond formation, and endothermic and exothermic reactions. 

To avoid confusion it should be emphasized that the terms "endothermic and exothermic adsorption are defined here relative to the sign of the energy change (but not the sign of the heat of adsorption). 

The shapes of these curves is plotted in Figure 6-13 for endothermic and exothermic reactions. If AH > 0, then the shape of the X(T) curve is nearly unchanged because the equilibrium conversion is lower at low temperatures, but if AH < 0, then X(T) increases with T initially but then decreases at high T as the reversibihty of the reaction causes X to decrease. However, the multiplicity behavior is essentially unchanged with reversible reactions. 

Differential Scanning Calorimetry (DSC). A DuPont 990 thermal analyzer equipped with a DSC cell was employed to record the endothermic and exothermic reactions which occurred during temperature-programmed (10°C/min) heating of the polymer samples. Sample weights were 15 mg, and the ambient atmosphere was either prepurified nitrogen or line air. 

The sensitivity of expls is a characteristic of great importance and can be correlated with the rate of deton. Perfect crysts and other nearly perfect elastic materials are the most sensitive, while liquids or colloids (plastic, fluid or hard) resist initiation and also have tendency to damp out-the wave of deton. The sensitivities of endothermic and exothermic compds are different and this causes them... 

Burn-rate modifiers probably affect most of these combustion steps, that is, the endothermic and exothermic reactions and heat losses. Rastogi et al. have shown that burn rate, surface temperature, flame temperature and rate of decomposition are enhanced in case of catalyzed propellants while these are lowered in case of burn-rate retarders. This may be due to heat produced in catalytic reactions in the former case whereas bum rates are reduced on account of endothermicity of the condensed phase reactions on the propellant surface in the case of retarders. It is also reported that carbonates of copper and chromium are better catalysts... 

Dissociative chemisorption energies calculated by density functional theory for various molecules on a number of stepped transition metal surfaces. All values are given in eV per molecule. Positive and negative values signify endothermal and exothermal chemisorption reactions, respectively. 

DTA is used in this study to understand the endothermic and exothermic phenomena resulting from desorption, decomposition and combustion of water and surfactant molecules occluded in the framework of samples A and B. Both samples have these common DTA features (Figure 3) an endothermic peak below 100 °C (apparently due to the evaporation of physically adsorbed water), an endothermic peak below 300 °C (attributed to the removal of lattice water and the decomposition of surfactant molecules), and a strong exothermic peak at around 335 °C (attributed to the combustion of surfactant molecules in air). The DTA results distinctly show that the surfactant molecules are occluded in almost identical positions within silica framework of samples A and B. 

This question is about endothermic and exothermic reactions. 

The first array-based technique was designed specifically to study reactions on solid phase catalysts as IR thermography.9,19 This approach utilizes IR sensitive FPA detectors to measure the temperature of catalysts under reaction conditions. This approach has the advantages of a theoretical high thermal sensitivity, typically several tens of millikelvin, and the ability to study both endothermic and exothermic reactions. The main disadvantage of this approach, however, is the lack of chemical information. It must be assumed that the temperature change is associated entirely with the desired reaction pathway. The presence of unexpected side reactions will not be detected in this approach, as long as they have similar thermal behavior as the reaction under study. 

Integrated reactors One type of integrated reactor is micro structured heat exchanger/reactor concepts, which may work as cross- or counter-flow reactors. Another type couples endothermic and exothermic reactions in two separate flow paths normally operated in the co-current mode. Both reactor types are designed as prototype components of future fuel processors for mobile applications. 

Kolios, G., Frauhammer J., Eigenberger G., Efficient reactor concepts for coupling of endothermic and exothermic reactions, Chem. Eng. Sci. 2002, 57,1505-1510. 

A nuclear reaction may involve either the absorption or release of energy hence, the terms endothermal and exothermal are applicable. 

Experiments run in the air. Data in parentheses give area in mm2 under peak recalculated on 100.00-mg samples. Shoulders are denoted by "s". En and Ex denote endothermic and exothermic effects, respectively. The range of temperature in which mass indicated is lost is given in parentheses. The height of peak (X 10"2 mg) is given in parentheses. Shoulders are denoted by s. ... 

The energy diagrams of an endothermic and exothermic reaction are compared below. 

Endothermic and Exothermic Reactions Evaluated using the AGsystem < 0 (Equation (14.11)) Criterion... 

Fig. 1.2. Schematic flow configurations of heat-integrated processes for coupling endothermic and exothermic reactions, (a) Countercurrent flow of process streams, (b) Cocurrent flow of the process streams in the reactor stages and heat recovery in separate circuits.

The above considerations indicate that, independent of implementation details, the space-time yield of endothermic reactions could be significantly enhanced by shifting the reaction site to the heat-exchanging surfaces. This intention has led to the production of a large variety of multifunctional reactor concepts for coupling endothermic and exothermic reactions. In the following section the state of the art in this area will be discussed for selected examples. 

In the asymmetric mode of operation the endothermic and exothermic reactions take place separately either in different compartments (recuperative mode) or at different time intervals (regenerative mode). The attractiveness of the asymmetric mode is related to the fact that the separation of the process streams allows an individual tuning of the operating conditions for the endothermic and the exothermic subsystem. 

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