Enthalpy estimation

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

Write the balanced chemical equation for the complete fluorination of methane to tetrafluoromethane. Using bond enthalpies, estimate the enthalpy of this reaction. The corresponding reaction using chlorine is much less exothermic. To what can this difference be attributed ... 

Table 4.5. Average values (kJ/mol) of the metal—metal bond enthalpies estimated from metal carbonyls (Mco) and from bulk metals (Mmet) (adapted from Mingos and Wales (1990)).

This number of enthalpies estimated from infrared frequency shifts agree with these parameters. 

Neglecting the effect of pressure on enthalpy, estimate the rate at which heat must be supplied. 

This example uses the Lee-Kesler generalized correlation for the reduced enthalpy estimations (see Tables F5-F8) in a throttling process. The reduced properties lead to enthalpies... 

One hundred g-moles per hour of liquid n-hexane at 25 C and 7 bar is vaporized and heated to 300 C at constant pressure. Neglecting the effect of pressure on enthalpy, estimate the rate at which heal must be supplied. 

Values of Cp and enthalpy estimated by this model have been checked recentlyagainst experimental values for a coal slag and it can be seen from Figure 7 that estimated values lie within 2% of the experimental results and that the enthalpy varies with temperature as predicted in Figure 6. Enthalpies estimated for liquid coal slags and ashes with the aid of this model were always found to lie within 5% of the experimental values. However it should be noted that the experimental enthalpy values obtained in the temperature range between Tg] and Tg j have little significance since these refer to samples with different and undefined crystallinities. ... 

Af//° (USe2, (3, 298.15 K) < - 359 kJ-mol". No value is selected because the P-USe2 phase was only found in small quantities as a by-product. The enthalpy estimated in [70BAS/SMI] was accepted in [92GRE/FUG] and there considered to apply to a-USe2. 

These arguments lend some support to the claim that enthalpy estimates are correct to within 2 kcal./mole and entropy estimates are correct to within 6 cal./mole deg. In fact, even deviations of these magnitudes appear to be unlikely. 

The TPD of both Cat 1 and Cat 2 could be described by the quasiequilibrium model with two adsorption states. The TPD curves of Cat 2 were best described by two significant adsorption states whereas those of Cat 1 were best described by one major and one minor adsorption state. The parameters of the minor adsorption state were poorly identified and the pre-exponential factor was therefore fixed to match the maximum AS. The estimated parameters with their 95 % confidence intervals are reported in Tables 2 and 3. The enthalpy estimates for state 1 were nearly the same for Cat 1 and Cat 2, whereas those for state 2 differed. Figure 2 presents the model solution and the TPD data of Cat 2. Compared to the previous results obtained by applying the differential reactor model the fit and the number of required states remained the same despite the reactor model. 

The enthalpy of formation of NiTeo.775(cr) has been estimated from emf measurements in the 7 + NiTe2 c(cr) two-phase field [92BAL/DIC]. Neither of these standard entropy or standard enthalpy estimations (AfT/" (NiTeo.775, cr, 298.15 K) = - (41.7 4.4) kJ mol ) is based directly on measurements of the NiTeo.775(cr) phase, so no recommendation can be given in this review. This is not of any particular importance here as the 71 phase is of peripheral interest only. 

Table 5.5 Comparison in accuracy between different methods for enthalpy estimation ...

The first program with the ability of generating reactions was written by T. F. Brownscombe in his thesis. He listed the 4- and 6-centred pericyclic reactions and used enthalpy estimates as selection critera. Stevens, R. V., Brownscombe, T. F. Thesis, Rice Unviversity, Houston, TX 1972 Diss. Abstr. Int B 1973, 34 (3). 

All possible combinations of ground- and excited-state NF radicals, which are consistent with the potential energy functions for the various electronic states of N2 and NF and consistent with the rules of spin and orbital angular momentum conservation were discussed. The corresponding reaction enthalpies estimated on the basis of D(N-F) = 70 kcal/mol are given in the following table (the processes that form molecular fluorine (AH in parentheses) are improbable because near four-center encounters are necessary) [5] ... 

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