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Enthalpy direct method

Calorimetric, electrochemical and vapour pressure methods are treated separately. The different techniques are to a large extent complementary. In general, enthalpy and entropy are measured most accurately by calorimetry, while electrochemical and vapour pressure techniques represent efficient direct methods for determination of activities and Gibbs energies. [Pg.309]

In fact, we know of no analogous reaction calorimetric study of the hydrolysis or alkylation of any enamine, although this would appear to be a satisfactorily accurate and direct method of deriving enthalpies of formation of such species. [Pg.263]

No direct methods, but enthalpies of adsorption and/or immersion give some Information. [Pg.42]

The direct method for enthalpy determination is calorimetry [55-67]. This technique enables the heat of surface reactions to be measured in more complicated situations, e.g. in the pH regions outside the pzc region, and also in the case of (specific) adsorption of species other than p.d. ions. However, the measured heat is a sum of the contributions of all reactions taking place at the interface, so that in interpretation of data one meets the problem of distinguishing between the different contributions. An additional problem is to account for electrostatic effects. On the other haxid, one can always express the results as the enthalpy per amount of adsorbed species or per surface charge. When doing so the... [Pg.870]

The Direct Method. This method of measuring AHf works for compounds that can be readily synthesized from their elements. Suppose we want to know the enthalpy of formation of carbon dioxide. We must measure the enthalpy of the reaction when carbon (graphite) and molecular oxygen in their standard states are converted to carbon dioxide in its standard state ... [Pg.217]

Direct methods. To measure the evaporation enthalpy of volatile substances, several types of adiabatic equipment were developed. They require a large amount of highly purified substance. The accuracy of results depends to a large degree on the design complexity of the equipment and the degree of precision of the measurement. For highly volatile substances, direct methods of determination employ calorimeters. ... [Pg.131]

Knowing the standard enthalpies of formation of reactants and products enables us to calculate the enthalpy of a reaction. We will discuss ways to determine these quantities either by the direct method from the elements or by the indirect method, which is based on Hess s law of heat summation. (6.6)... [Pg.229]

In a direct comparison with the enthalpy ratio used in the enthalpy-based method, in the work-based method, the ratio of the availability of the steam at lower pressure to the availability of the steam at the generation pressure is used ... [Pg.372]

From the above discussion, the importance of the knowledge of AH and AS, and therefore AG for polymerization reactions, is obvious. The enthalpy and entropy of some polymerization reactions are given in Table 5. The experimental methods for their determination have been reviewed (141,149,150). For the determination of the entropy, the direct method is based on equation 50. From a plot of the equilibrium concentration of the monomer vs l/T, both AH and AS can be computed. Another method involves the estimation of AS from kinetic frequency factors (146). [Pg.8453]

Scott et al. have developed a technique for obtaining rate constants and activation enthalpies directly by a calorimetric method, measuring the change of temperature with time after the reagents are mixed in a reactor immersed in a constant-temperature environment. Their new data on the Fe -Sn reaction in chloride media agree well with earlier work. ... [Pg.14]

As we noted earlier, it is not always possible to synthesize a pure substance directly, with no secondary reactions, from its elementary substances, and it is then not possible to determine an enthalpy of formation by the direct method. This obstacle is often circumvented by considering the enthalpies of combustion... [Pg.116]

Reaction calorimetry is the experimental determination of the enthalpy changes accompanying chemical reactions by direct methods using calorimeters. It is the principal means by which enthalpies of formation of pure chemical compounds are determined. With the exception of certain binary compounds, chiefly oxides, it is impractical to measure the enthalpy of formation of a compound from its elements directly, and it is necessary to determine the enthalpy of a reaction involving the compound in which the enthalpies of the other reactants and products are all known, and then to apply Hess s law. Occasionally, enthalpies of formation can be derived from the study of equilibria (as measured by the e.m.f.s. of electrochemical cells, dissociation pressures, etc.) by means of second- or third-law methods, or from electron impact experiments, but such indirect approaches are outside the scope of the present review, which is confined to the discussion of experimental procedures used in direct calorimetric methods. [Pg.95]

Property Type State Parameters Variables Method [HMSDllll] HEAT OF MIXING AND SOLUTION Two-component system, single-phase liquid or two-phase liquid-liquid (LL) Pure component 1, liquid Pure component 2, liquid r/K, Temperature Xj/-, Mole fraction of component 1 //E/J mol-, Molar excess enthalpy Direct low-pressure calorimetric measurement of at variable Xj and constant T ... [Pg.3764]

Ether enthalpy/kj moM of ether (displacement method) (direct method)... [Pg.408]

The reaction inverse to 10 was studied by direct methods and it was found that 10 = 2- lO cm /(mol s). The equilibrium rate constant K, as it is known, is expressed through the change in entropy A5 and enthalpy Mi T) in the reaction for the reactants and products in the standard state... [Pg.111]

In their study of krypton hydration, Durell and Wallqvist also reported a calculation of the enthalpy of hydration evaluated by the direct method of Eq. [31]." Both constant volume and constant pressure enthalpies were determined by varying the volume of the krypton solution. Their results are displayed in Table 1. The enthalpy of hydration in the constant volume case (—6.3 1.3 kcal/mol) is significantly more exothermic than in the constant pressure case (—3.4 1.3 kcal/mol). The latter number agrees very well with the experimental value of —3.3 kcal/mol, also obtained at constant pressure. The calculated enthalpies of solvation were decomposed into solute-water and water-water (solvent reorganization) terms. The solute-water contribution is comparable and favorable (—5.4 kcal/mol) in both the constant volume and constant pressure calculations. The solvent reorganization term, in contrast, shows a large ensemble dependence. In the constant-pressure case, the solvent reorganization term has a value of 2.0 1.3 kcal/mol. The overall favorable enthalpy of hydration of krypton at constant pressure therefore results from the solute-water attractions rather than from a... [Pg.64]

The direct application of the enthalpy balance method to growth reactions such as Equation (13) requires knowledge of the enthalpy of combustion for the cell mass. It is a major difficulty to obtain a clean sample of animal cell biomass... [Pg.603]

Enthalpies are evaluated directly using the methods presented in Chapter 5 and Appendices A and B. [Pg.295]

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. [Pg.77]

A variant on this procedure produces a first approximation to the molecular mechanics (MM) heat paiameters (Chapters 4 and 5) for C—C and C—H. Instead of atomization energies, the enthalpies of formation of propane and butane (—25.02 and —30.02 kcal mol ) are put directly into the b vector. The results (2.51 kcal mol and —3.76 kcal mol ) are not very good approximations to the heat parameters actually used (2.45 kcal mol and —4.59 kcal mol ) because of other factors to be taken up later, but the calculation illustrates the method and there is rough agreement. [Pg.57]

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... [Pg.294]

Equations 20.176 and 20.179 emphasise the essentially thermodynamic nature of the standard equilibrium e.m.f. of a cell or the standard equilibrium potential of a half-reaction E, which may be evaluated directly from e.m.f. meeisurements of a reversible cell or indirectly from AG , which in turn must be evaluated from the enthalpy of the reaction and the entropies of the species involved (see equation 20.147). Thus for the equilibrium Cu -)-2e Cu, the standard electrode potential u2+/cu> hence can be determined by an e.m.f. method by harnessing the reaction... [Pg.1229]

The time derivative is zero at steady state, but it is included so that the method of false transients can be used. The computational procedure in Section 4.3.2 applies directly when the energy balance is given by Equation (5.28). The same basic procedure can be used for Equation (5.25). The enthalpy rather than the temperature is marched ahead as the dependent variable, and then Tout is calculated from Hout after each time step. [Pg.167]


See other pages where Enthalpy direct method is mentioned: [Pg.19]    [Pg.293]    [Pg.16]    [Pg.243]    [Pg.474]    [Pg.243]    [Pg.98]    [Pg.270]    [Pg.238]    [Pg.322]    [Pg.91]    [Pg.477]    [Pg.265]    [Pg.236]    [Pg.1961]    [Pg.130]    [Pg.195]    [Pg.404]    [Pg.85]    [Pg.28]    [Pg.120]    [Pg.241]   


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