Molar transition 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... 

The temperature dependence of reaction enthalpies can be determined from the heat capacity of the reactants and products. When a substance is heated from T to T2 at a particular pressurep, assuming no phase transition is taking place, its molar enthalpy change from AHm (T]) to AHm (T2) is... 

It follows that the standard molar entropy of the transition can be derived from the measured standard molar enthalpy of transition through the relationship... 

Atrs m change in standard molar enthalpy of a phase transition... 

Since surface pressure is a free energy term, the energies and entropies of first-order phase transitions in the monolayer state may be calculated from the temperature dependence of the ir-A curve using the two-dimensional analog of the Clausius-Clapeyron equation (59), where AH is the molar enthalpy change at temperature T and AA is the net change in molar area ... 

Two different crystalline forms of benzothiophene have been described [18]. The one that is the stable form at low temperatures is labeled I and the other II. Calorimetric measurements down to 12 K have been made with each crystalline form. At the normal transition temperature, 261.6 K, the molar enthalpy of transition (I = II) is 3010 J moI . Some additional thermodynamic data obtained by these investigators are given in Table 11.6. Is Crystal II a perfect crystal at 0 K ... 

Elemental sulfur occurs in several forms, with rhombic sulfur being the most stable under normal conditions and monoclinic sulfur slightly less stable. The standard enthalpies of combustion of the two forms to sulfur dioxide are —296.83 and —297.16 kj-mol 1, respectively. Calculate the change in molar enthalpy for the rhombic - monoclinic transition. 

The molar enthalpy for the transition from a solid to a supercooled liquid is not a constant with respect to temperature. The molar heat capacities of the solid and supercooled liquid forms of the solute inLuence its magnitude at temperatures below the melting point. It is frequently assumed that either the molar heat capacity of the solid at constant prestiiiippnd the molar heat capacity of its liquid form at constant pressure,pi, are nearly constant or that they change at the same rate with a change in temperature. In either case, the molar differential heat capacity, deLned as... 

O Hare, P.A.G., Lewis, B.M., Susman, S. and Volin, K.J. (1990) Standard molar enthalpies of formation and transition at the temperature 298.15 K and other thermodynamic properties of the crystalline and vitreous forms of arsenic sesquiselenide (As2S3). Dissociation enthalpies of As-Se bonds. Journal of Chemical Thermodynamics, 22, 1191-206. 

The molar enthalpy of fusion (AEP J is the heat necessary to convert one mole of a solid into a liquid at its normal melting point. The molar enthalpy of vaporization (AH°vap) is the heat required to convert one mole of a liquid to a gas at its normal boiling point. When melting or vaporization occurs at constant pressure, it is acceptable to use heat instead of enthalpy. This is because heat and change in enthalpy are equal to each other under constant pressure conditions. The interested student should consult any physical chemistry textbook for more details. Both AHfm and AHyap are inherently endothermic, and represent an amount of energy that must be added to the sample in order for the phase transition to occur. The heat of fusion represents the amount of energy necessary to overcome the intermolecular forces to the point that the molecules can start to move around each other. The heat of vaporization represents the amount of energy necessary to overcome all intermolecular forces so that the molecules can escape into the gas phase. 

As seen from Fig. 6 at 305°C the values of partial molar enthalpy for different runs of hydrogen desorption have a large deviation probably because of a proximity to critical temperature, thus it is difficult to determine the phase boundaries. The critical temperature for the existence of ZrMn2 hydride phase estimated by different authors is 277-327 °C [8] and 318°C [15], The plot of the AHdes. -C could be divided into three parts the hydrogen a-solid solution region (0pi transition (0.6p2 transition (1.0[Pg.353]

Figure 49. Dependence of enthalpy of activation for the hydrolysis of t-butyl chloride on the mole fraction of water in ethyl alcohol + water mixtures (curve b), together with dependence of partial molar enthalpies of initial (curve a) and transition states (curve c) (Arnett etal., 1965).

Using the molar enthalpy of transition A/ftransition = T(Svap -. S Nq). we get the Clapeyron equation... 

Forster cycle Indirect method of determination of excited state equilibria, such as pK values, based on ground state thermodynamics and electronic transition energies. This cycle considers only the difference in molar enthalpy change (AAH) of reaction of ground and excited states, neglecting the difference in molar entropy change of reaction of those states (AAS). 

We prepared polymers containing the mesogenic structure 25 of Table 1, both in the homopolymer form and as copolymers containing polysiloxane and decamethylene flexible spacers, and these polymers were compared in their properties to the earlier polymers prepared with a polymethylene spacer only The polymer with mesogenic structure 32 and the siloxane spacer showed a low molar enthalpy for the melt transition, while the copolymer and the polymer with the polymethylene spacer showed quite high melt enthalpies. 

A proper description of electronic defects in terms of simple point defect chemistry is even more complicated as the d electrons of the transition metals and their compounds are intermediate between localized and delocalized behaviour. Recent analysis of the redox thermodynamics of Lao.8Sro,2Co03. based upon data from coulometric titration measurements supports itinerant behaviour of the electronic charge carriers in this compound [172]. The analysis was based on the partial molar enthalpy and entropy of the oxygen incorporation reaction, which can be evaluated from changes in emf with temperature at different oxygen (non-)stoichiometries. The experimental value of the partial molar entropy (free formation entropy) of oxygen incorporation, Asq, could be... 

The difference Hi a Hs a represents the change in the enthalpy at the transition of 1 mol of the pure component considered from the solid to the liquid state, which is thus the molar enthalpy of fusion Dfusf/A and Eq. (3.18) has the final form... 

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