State endothermic changes

They may react, decompose or change state endothermically, thus absorbing heat. 

Q.9.16 The energy of a system is easily measured by thermal techniques. When the state of a system and a change in the state observables is temperature dependent, an exothermic or endothermic change can be measured. For the following list of events indicate whether a physical change is endothermic or exothermic. 

A slightly different approach to hyphenation of instruments is simultaneous analysis, where two or more methods are applied to the same sample. An example of this approach is the commercially available DSC-Raman system from PerkinElmer, Inc. (Menard et al. 2010). In this system, a three-axis positioning adapter with optics allows accurate placement of the Raman probe over the sample in the DSC. Raman spectra can be collected as the DSC experiment progresses. The authors give an example of the ability to positively identify three solid polymorphs of acetaminophen. The combination of Raman and DSC gives the precise temperatures at which the solid-state conversions from one polymorph to the next occur. The DSC provides the temperature data and the information about exothermic and endothermic changes in the material, while the polymorphs are identified from their Raman spectra. Raman spectroscopy is discussed in Chapter 4. 

Entropy Changes in the Surroundings Entropy Change and the Equilibrium State Spontaneous Exothermic and Endothermic Changes... 

Changes of State Solids, liquids, and gases can undergo various changes of state. The changes shown in green are exothermic, and those shown in blue are endothermic. 

In a system with alternate spin states, a change of spin state may occur during the reaction. As shown in Fig. 15.3, this can play a role in the thermodynamics of the reaction. Assume the reagent spin state. A, leads to an excited spin state of the product, B this can even be an endothermic, unfavorable process, as shown here. If this reaction pathway intersects the corresponding curve for the other spin state, crossover is expected to give not B but B. The path is now A —> 1 —> 2 B, and the reaction is only thermodynamically favorable thanks to the accessibility of the alternate spin state. 

At the transition temperature r , the specific heat, Cp, mea ired with a Perkin-Ebner DSC-2 shows an endothermic change of slope as shown in Figure 26. In addition, a small drop in is observed at T, The slope change could be reproduced with a Mettler2000 calorimeter. Since the weight fraction of the polymer main chains amounts only to 25% we tend to attribute the transition to the mesogenic side chains or to the system as a whole. If this assumption holds true, the transition could be regarded as a precursor of the transition into the nematic state at lower temperatures, This may be taken as a hint toward an intramolecular nature for this process. 

We see that the total change in entropy is a positive quantity for both these spontaneous processes, even though one process is exothermic and the other is endothermic. When this type of calculation is carried out for other processes, the same result is always obtained. For any spontaneous process, the total change of entropy is a positive quantity. Thus, this new state function of entropy provides a thermod3mamic criterion for spontaneity, which is summarized in the second law of thermodynamics ... 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

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