Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Phase transitions enthalpy

If, in going from 0 K to T, a substance undergoes phase changes (fusion, vaporization, etc) at and Tg with molar enthalpies of transition AHy, one can write... [Pg.369]

Phase transitions at which the entropy and enthalpy are discontinuous are called first-order transitions because it is the first derivatives of the free energy that are disconthuious. (The molar volume V= (d(i/d p) j is also discontinuous.) Phase transitions at which these derivatives are continuous but second derivatives of G... [Pg.612]

This method is used to locate phase transitions via measurements of the endothennic enthalpy of phase transition. Details of the teclmique are provided elsewhere [25, 58]. Typically, the enthalpy change associated with transitions between liquid crystal phases or from a liquid crystal phase to the isotropic phase is much smaller than the melting enthalpy. Nevertheless, it is possible to locate such transitions with a commercial DSC, since typical enthalpies are... [Pg.2554]

The enthalpy over a temperature range that includes phase transitions, melting, and vaporization, is represented by ... [Pg.535]

Figure 4.3b is a schematic representation of the behavior of S and V in the vicinity of T . Although both the crystal and liquid phases have the same value of G at T , this is not the case for S and V (or for the enthalpy H). Since these latter variables can be written as first derivatives of G and show discontinuities at the transition point, the fusion process is called a first-order transition. Vaporization and other familiar phase transitions are also first-order transitions. The behavior of V at Tg in Fig. 4.1 shows that the glass transition is not a first-order transition. One of the objectives of this chapter is to gain a better understanding of what else it might be. We shall return to this in Sec. 4.8. [Pg.207]

An example of the determination of activation enthalpies is shown in Figs. 11 and 12. A valuable indication for associating the correct minimum with the ionic conductivity is the migration effect of the minimum with the temperature (Fig. 11) and the linear dependence in the cr(T versus 1/T plot (Fig. 12). However, the linearity may be disturbed by phase transitions, crystallization processes, chemical reactions with the electrodes, or the influence of the electronic leads. [Pg.546]

Graph the above data in the form Cp,m/T against T2 to test the validity of the Debye low-temperature heat capacity relationship [equation (4.4)] and find a value for the constant in the equation, (b) The heat capacity study also revealed that quinoline undergoes equilibrium phase transitions, with enthalpies as follows ... [Pg.198]

Fig. 30. DSC traces showing the phase transition of the model membrane in its monomeric and polymeric form. Note the difference in the enthalpies of the transition monomer AH = 56 J/g, polymer AH = 26 J/g... Fig. 30. DSC traces showing the phase transition of the model membrane in its monomeric and polymeric form. Note the difference in the enthalpies of the transition monomer AH = 56 J/g, polymer AH = 26 J/g...
When a phase transition takes place at constant pressure, as is most common, the heat transfer due to the phase change is the change in enthalpy of the substance. [Pg.356]

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... [Pg.142]

When the free enthalpy of reaction AG for the transformation of the structure of a compound to any other structure is positive, then this structure is thermodynamically stable. Since AG depends on the transition enthalpy AH and the transition entropy AS, and AH and AS in turn depend on pressure and temperature, a structure can be stable only within a certain range of pressures and temperatures. By variation of the pressure and/or the temperature, AG will eventually become negative relative to some other structure and a phase transition will occur. This may be a phase transition from a solid to another solid modification, or it may be a transition to another aggregate state. [Pg.30]

If one of these quantities experiences a discontinuous change, i.e. if AS 0 or AV 0, then the phase transition is called a first-order transition according to Ehrenfest. It is accompanied by the exchange of conversion enthalpy AH = TAS with the surroundings. [Pg.32]

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... [Pg.238]

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. [Pg.73]

KEY TERMS phase transition heat capacity enthalpy of fusion... [Pg.108]

The solubilising effect of PEO chains was also observed by micro calorimetry. The transition temperatures were 33.5 and 34.3 °C for PNIPAM-g-PEO-6 and PNIPAM-g-PEO-7, respectively. The enthalpy associated with the transition was higher in the case of PNIPAM-g-PEO-6 (2.5 kj mor1 of repeating NI-PAM unit) compared with that of aqueous PNIPAM-g-PEO-7 (1.3 kj mol-1). For both copolymers, it was significantly lower than the enthalpy of the phase transition of linear PNIPAM in water (approximately 7 kj mol-1). [Pg.43]

An enormous number of phase transitions are known to occur in common solid compounds. For example, silver nitrate undergoes a displacive phase transition from an orthorhombic form to a hexagonal form at a temperature of approximately 162°C that has a enthalpy of 1.85 kj/mol. In many cases, the nature of these transitions are known, but in other cases there is some uncertainty. Moreover, there is frequently disagreement among the values reported for the transition temperatures and enthalpies. Even fewer phase transitions have been studied from the standpoint of kinetics, although it is known that a large number of these transformations follow an Avrami rate law. There is another complicating feature of phase transitions that we will now consider. [Pg.273]


See other pages where Phase transitions enthalpy is mentioned: [Pg.26]    [Pg.131]    [Pg.933]    [Pg.26]    [Pg.131]    [Pg.933]    [Pg.612]    [Pg.1902]    [Pg.1906]    [Pg.2554]    [Pg.193]    [Pg.228]    [Pg.306]    [Pg.85]    [Pg.160]    [Pg.165]    [Pg.178]    [Pg.427]    [Pg.190]    [Pg.229]    [Pg.62]    [Pg.373]    [Pg.536]    [Pg.540]    [Pg.61]    [Pg.38]    [Pg.238]    [Pg.63]    [Pg.67]    [Pg.26]    [Pg.108]    [Pg.69]    [Pg.264]    [Pg.82]    [Pg.117]    [Pg.119]   
See also in sourсe #XX -- [ Pg.263 ]

See also in sourсe #XX -- [ Pg.129 ]




SEARCH



Enthalpy Changes For Phase Transitions

Enthalpy of phase transition

Hydration enthalpy phase transition

Phase transition temperatures and enthalpies

Phase transitions enthalpy of fusion

Phase transitions enthalpy of vaporization

Phase transitions, standard enthalpies

Phases enthalpy

Solvation enthalpy phase transition

Standard Enthalpies of Phase Transition

Transition enthalpy

© 2024 chempedia.info