Endothermic and exothermic changes

Monitoring of the temperature difference between a sample and an inert reference as they are heated uniformly. Endothermic or exothermic changes in the sample lead to characteristic deviations in temperature, which can be used for qualitative and quantitative analyses. 

Sample and an inert reference heated separately, with the power supply to the sample heater variable so that the temperature difference can be maintained at zero even when endothermic or exothermic changes occur. The difference in power supplied to the two heaters is monitored as the analytical signal (AE). Alternatively, the differential heat flow to sample and standard when they are heated from the same source is monitored. 

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. 

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 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. 

When gum formation proceeds, the minimum temperature in the catalyst bed decreases with time. This could be explained by a shift in the reaction mechanism so more endothermic reaction steps are prevailing. The decrease in the bed temperature speeds up the deactivation by gum formation. This aspect of gum formation is also seen on the temperature profiles in Figure 9. Calculations with a heterogenous reactor model have shown that the decreasing minimum catalyst bed temperature could also be explained by a change of the effectiveness factors for the reactions. The radial poisoning profiles in the catalyst pellets influence the complex interaction between pore diffusion and reaction rates and this results in a shift in the overall balance between endothermic and exothermic reactions. 

Appreciate the ideas of energy changes in reactions, and use the terms endothermic and exothermic . 

A sample is continuously heated at a constant rate (e.g. 10 C min ) while two changes are recorded (1) the temperature difference between an inert compound and the sample with a thermocouple (differential thermo analysis DTA) and (2) the weight loss measured with a balance (thermogravimetry TGA) (Mackenzie, 1957 Smykatz-Kloss, 1974). With DTA, information is obtained about endothermic and exothermic phase transformations (see Fig. 1-2), whereas with TGA adsorbed water and structural OH can be measured. 

DSC is a precise method of measuring the endothermic and exothermic behaviors of sample materials. Unlike the earlier version of the thermal analyzer, the differential thermometric analyzer (DTA) measures the temperature difference between two cells heating in the same furnace. The power-compensated DSC uses two independent furnaces, one for the sample and one for the reference. When an exothermic or endothermic change occurs in the sample materials, energy is applied to or removed from one or both furnaces to compensate for energy changes in the sample. This means that the system directly measures energy flows to or from a sample at all times. 

Another important conclusion is obtained from the coupled DSC/NMR experiments. The water content of the Nafion membranes strongly depends on the temperature. Therefore the analysis of a possible water phase separation cannot be done with experiments involving temperature changes like DSC. This is pretty different from what is obtained with y-alumina which represent a relative fixed and non temperature dependent hydrophobic matrix. The endothermic and exothermic peaks observed during heating and cooling runs of the water-Nafion systems may be interpreted in two ways... 

In general, thermal changes such as endothermic and exothermic transitions can be followed either by raising or lowering the sample temperature. Andrade and his coworkers obtained DSC thermograms of synthetic hydrogels by cooling the samples (20). 

The storage of hydrogen in solid-solution materials is characterized by endothermic and exothermic phase change reactions. The thermodynamic nature of these reactions requires the full characterization of influential material properties to enable the optimization of heat and mass transfer within the system. Additionally, the thermodynamic nature of the materials will define the containment technologies required to withstand the operational pressures and temperatures. 

The applications of DTA and DSC to inorganic compounds are similar to those discussed for organic compounds. Endothermic and exothermic peaks are caused by phase transitions (melting, boiling, polymorphic changes), dehydration, dissociation, isomerization, oxidation-reduction reactions, and so on. These applications are summarized in Figure 7.19. A... 

The enthalpic changes which occur in organic compounds are considerably less complex than those for organic polymers. However, they may exhibit various polymorphic changes which can be detected by DTA and DSC. The main sources of endothermic and exothermic enthalpic changes in organic compounds are fusion, vaporization, solid-solid transitions, sublimation, dehydration, decomposition, and combustion. 

The resultant thermogram is similar to a DTA trace but more accurate and reliable. Endothermic changes are recorded as heat input into the sample, and exothermic changes as heat input into the reference. The area of the peaks is an exact measure of heat input involved. Differences in heat capacity or thermal conductivity do not affect the results. From the data, accurate quantitative analytical results can be obtained. 

Do not confuse the terms endothermic and exotherm/c with endergonic and exergonic. Endothermic and exothermic refer only to heat changes endergonic and exergonic refer to energy changes that contain both heat (enthalpy) and order (entropy) components. 

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