Enthalpy change explanation

This important formula, which can be derived more formally from the laws of thermodynamics, applies when any change takes place at constant pressure and temperature. Notice that, for a given enthalpy change of the system (that is, a given output of heat), the entropy of the surroundings increases more if their temperature is low than if it is high (Fig. 7.16). The explanation is the sneeze in the street analogy mentioned in Section 7.2. Because AH is independent of path, Eq. 10 is applicable whether the process occurs reversibly or irreversibly. 

There has been a tendency to view the lanthanide elements as having nearly identical chemistry In recent times this view has been criticized.75 Standard enthalpy changes for three reactions are plotted in Figure 14.17. How do you account for the dramatic differences shown in plots (a) and (b)7 Can you provide an explanation for the bumps" at Eu and Yb in plot (c) ... 

The L terms are partial molal enthalpies defined by Equation 9. Another method which calculates the enthalpy change for adding 1 mole of monomer to the micelle has also been used. Additional explanation of these methods can be found in the literature (1,5). 

Since the IP of hydrogen is a constant equal to 313.6 kcal/mol [16], the enthalpy change A// , depends basically on the difference between the bond dissociation energy D iA—H), and the electron affinity EA(A). For many years the experimental way of obtaining AH for a compound was from dissociation energies and electron affinity measurements. For an interesting explanation of details of such procedures, see Bartmess and Mclver [17] and Lias and Bartmess [18]. A more recent and comprehensive discussion of the various experimental techniques available for measuring these properties can be found in the NIST webbook [18]. 

The enthalpy change, measured calorimetrically, of micellization is generally small (7,8). A realistic, if simplistic, explanation is that the energy lost in reducing the... 

The real breakthrough in terms of kinetic theory was published in 1973 by Aniansson and Wall [80, 81], who provided much more applicable kinetic equations for stepwise micelle formation using a polydisperse model. In a substantial paper two years later they were able to predict the first-order rate constants for the dis-sociation/association of surfactant ions to and from micelles (and hence residence times/lifetimes of surfactant monomers within micelles) [82]. They found values for the association and dissociation of surfactants into/from micelles (Ar and k , respectively) for sodium dodecyl sulfate (SDS) as 1 x 10 s and 1.2 x 10 mok s". Their kinetic model still remains essentially unchanged as a basis for the kinetics of micellar formation and breakdown. Modifications made to existing theory also allowed them to offer a significant thermodynamic explanation for the low enthalpy change upon micellization. 

What could be the cause of such a large difference in thermodynamic stability After all, the number of Ni -N coordinate-covalent bonds is six in both the products of these two reactions, so the enthalpy changes (Ai/) involved when these bonds are formed should be fairly similar. That seems to leave entropy as the major explanation for the effect. Indeed, the rationale for the chelate effect can be understood in two ways, both related to the relative probabilities that the two reactions will occur. First, consider the number of reactants and products in the two cases. As written more explicitly in Equations (6.11) and (6.12), it is apparent that the number of ions and molecules scattered throughout the water structure in the first reaction stays the same (seven in both the reactants and the products). In the second reaction, however, three ethylenediamine molecules replace six water molecules in the coordination sphere, and the number of particles scattered at random throughout the aqueous solution increases from four to seven. The larger number of particles distributed randomly in the solution represents a state of higher probability or higher entropy for the products of the second reaction. Therefore, the second reaction is favored over the first due to this entropy effect. 

The substance from which all nitrogen compounds are ultimately derived, NiCg), is unusually stable. One explanation of the limited reactivity of the N2 molecule is based on its electronic structure. As discussed in Chapter 10, the bond between the two N atoms in N2 is a triple covalent bond, which is unusually strong and difficult to break. In thermochemical terms, the enthalpy change associated with breaking the bonds in one mole of N2 molecules is very high—the dissociation reaction is highly endothermic. 

An increased yield stress is required [62] in order to reverse the unfavourable conformations of the molecular chains that develop during annealing. This explanation is supported by the energy changes observed in annealed polymers. The enthalpy difference, as determined by DSC was AH = 1.8 J/g (Sect. 4.2), whereas the additional work required for yielding in an annealed sample was... 

Many workers have offered the opinion that the isokinetic relationship is confined to reactions in condensed phase (6, 122) or, more specially, may be attributed to solvation effects (13, 21, 37, 43, 56, 112, 116, 124, 126-130) which affect both enthalpy and entropy in the same direction. The most developed theories are based on a model of the half-specific quasi-crystalline solvation (129, 130), or of the nonideal conformal solutions (126). Other explanations have been given in terms of vibrational frequencies involving solute and solvent (13, 124), temperature dependence of solvent fluidity in the quasi-crystalline model (40), or changes of enthalpy and entropy to produce a hole in the solvent (87). 

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