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Reaction enthalpies methods

This is the situation exploited by the so-called isolation method to detennine the order of the reaction with respect to each species (see chapter B2.1). It should be stressed that the rate coefficient k in (A3,4,10) depends upon the definition of the in the stoichiometric equation. It is a conventionally defined quantity to within multiplication of the stoichiometric equation by an arbitrary factor (similar to reaction enthalpy). [Pg.763]

There is also no significant influence of statistic thermodynamical calculations on the reaction parameters. That can be seen in the Tables 3 and 4. In Table 4 the calculated reaction enthalpies and free reaction enthalpies are faced with experimental values estimated by means of thermochemical methods. [Pg.187]

The processes (11) and (12) have been studied by means of the MINDO/3 method optimizing the geometry with respect to the cartesian coordinates of the model compound atoms. The reaction enthalpies AH , calculated for the reactions (11) and (12) are to be seen in Table 11. [Pg.199]

The solvent influence, calculated with the Huron-Claverie method, reverses the qualitative graduation of the reaction enthalpies of the propagation steps as the chain length increases, in comparison to the gas phase. The same results were obtained using the same model system by Basilevski et al.125), while using a fundamentally different model for the solvent influence. [Pg.218]

In Ref.125) the calculation of an activation barrier for reaction (21) in the gas phase is considered to be an error of the MINDO/3 method and the process is assumed to be activationless. But in respect to the medium effect a barrier of 54 k J mol-1 is obtain-ed which agrees again with the results from Huron-Claverie calculations. Bertran et al. calculated the influence of the solvation on the electrophilic attack of a proton 133) or a methyl cation 134,135) on ethene using a MINDO/3 supermolecule model. Smaller reaction enthalpies also result in solution than in the gas phase in addition to the appearance (H+ + ethene) or the increase (CH 4 + ethene) of an activation barrier1361. [Pg.218]

Decreasing reaction enthalpies and an increase in the activation barriers are calculated in the gas phase as the chain length increases. In solution the activation barriers are higher and the reaction enthalpies increase along with chain length. The calculation of activation barriers don t seem to be an error of the MINDO/3 method. [Pg.222]

Kinetic studies of chemical equilibrium (Reaction 4) have provided very accurate thermodynamic information about the series Me3 SiH +i (with n having values from 0 to 3). ° In particular, the rate constants 4 and k, obtained by time-resolved experiments, allow the determination of the reaction enthalpy (AHr) either by second or third law method. In Table 2 the DHfRsSi-H) values obtained by Equation (5) are reported. [Pg.121]

Practically, all the hydrocarbons have BDE of C—H bonds higher than 300 kJ mol-1, and this method of calculation can be used for them. The enthalpy of reaction was calculated as AH= D(R—H)—220 (kJ mol-1). The weakest bonds participate in this reaction. The pre-exponential factor depends on the reaction enthalpy value for the reactions with high enthalpy [18]. [Pg.167]

The reaction enthalpy is known as a very important factor that determines the reactivity of reactants in free radical abstraction reactions [71]. The IPM method helps to calculate the increment of AEfi that enthalpy determines in the activation energy of the individual reaction. This increment can be estimated within the scope of IPM through the comparison of activation energy Ee of the chosen reaction and activation energy of the thermoneutral reaction Ee0 (see Equation [6.18] in Chapter 6). This increment was calculated for several reactions of different peroxyl radicals with ethers (Table 7.19). [Pg.318]

The influence of the reaction enthalpy on the activation energy can be estimated using the IPM method (see Chapter 6). The parameters of free radical reactions with phenols are collected in Table 15.2. They are different for sterically nonhindered (AriOH) and sterically hindered (Ar2OH) phenols. The values of coefficients a, b, and zero-point vibration energy of the phenolic O—H bond are the following a = 1.014, b = 4.665 x 1011 (kJ mol )l/2 m 1, and 0.5ALy = 21.5 kJ mol-1 [33],... [Pg.514]

FIGURE 16.1 The dependence of activation energy E on reaction enthalpy A He for reaction of hydrogen atom abstraction by aminyl radical from the C—H bond of alkylperoxyl radical and O—H bond of hydroperoxyl radical calculated by IPM method (see Chapter 6). The points fix the reactions with minimum and maximum enthalpy among known aromatic aminyl radicals. [Pg.572]

The chemistry of carbenes in solution hits been extensively studied over the past few decades.1-5 Although our understanding of their chemistry is often derived from product analyses, mechanistic details are often dependent on thermodynamic and kinetic data. Kinetic data can often be obtained either directly or indirectly from time-resolved spectroscopic methods however, thermochemical data is much less readily obtained. Reaction enthalpies are most commonly estimated from calculations, Benson group additivities,6 or other indirect methods. [Pg.253]

The Benson group contribution method, and more recent methodologies, allow the computation of heat of hydrogenation reactions, even for large molecules (note that Benson method gives the reaction enthalpy assuming each species to be a perfect gas ). Software and database (e.g., NIST) are also available. [Pg.1522]

Method of Craven [50] This average bond energy summation method (ABES) is a simplification of the method described by Sanderson [55]. The reaction enthalpy is calculated by subtracting the total bond energies present before the reaction from the total bond energies of the products. [Pg.34]

Table 24. Metal-metal bond enthalpies (kJ mol 1) in binuclear metal carbonyl complexes determined by electron impact and reaction kinetic methods... Table 24. Metal-metal bond enthalpies (kJ mol 1) in binuclear metal carbonyl complexes determined by electron impact and reaction kinetic methods...
The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... [Pg.175]

Titration calorimetry involves the measurement of heat evolved while adding a titrant. This technique is well established for determining reaction enthalpies in homogeneous solution (see refs. 15 and 16 for general reviews of the method) but has been used far less often to measure adsorption enthalpies in heterogeneous suspensions. Instead, adsorption studies have relied mainly on the... [Pg.143]

Why did we prefer to use the cycle in figure 2.1 instead of the easier method after equation 2.21 Simply because we had considered that the best data for the standard enthalpies of formation of pure ethanol and acetic acid are those recommended in Pedley s tables [15]. The values (-277.6 kJ mol-1 and -484.3 kJ mol-1, respectively) are both about 1 kJ mol-1 less negative than those in the NBS Tables, and their difference nearly cancels when the reaction enthalpy is calculated. But of course, we are seldom so lucky. Using data from different databases may lead to much larger discrepancies. [Pg.18]

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. [Pg.22]

Most of the methods for estimating reaction enthalpies are applicable only to the gas phase. Solvation enthalpy data are thus particularly important because they allow gas-phase estimates to be extended to reactions in solution—which is the most common medium for reactions of practical interest. However, solvation enthalpies are not very abundant and must often be estimated. Unfortunately, this can be a difficult exercise, especially when A is a solid, because sublimation enthalpies are scarce and hard to estimate. Thus, ASU, H°(A) is usually the unknown term in equation 2.44. The solution enthalpy term, Asi 7/°(A), is generally small and can often be predicted—or determined with a calorimeter. [Pg.26]

Besides the second law method, there is another way of extracting reaction enthalpies from gas-phase equilibrium constants. This alternative involves the determination of a single value of an equilibrium constant at a given temperature and the calculation of the reaction entropy at the same temperature. From equations 2.54 and 2.55, we obtain... [Pg.36]

It is generally agreed that the third law method yields more accurate values than the second law method because it does not require any assumption regarding the temperature variation of the reaction enthalpy and entropy. The usual procedure to obtain third law data is to calculate the reaction enthalpy and entropy for each experimental value of Kp and take the average of all the values derived for a given temperature. [Pg.37]

The modified Arrhenius method yields more accurate results for Ea than the linear plot because it does not include the assumption that this parameter is constant with the temperature. Nevertheless, the linear plot is widely adopted because for many reactions, the variation of Ea with T is small. Also, linear plots are more suitable than nonlinear plots to handle low-precision data. In either case, the procedure to derive the activation enthalpies and the reaction enthalpies is as described. [Pg.41]

The method is thus identical to the one described for gas-phase reactions. Thus, the activation energies of the forward and reverse reactions can be obtained at a temperature T from either simple or modified Arrhenius plots, and their difference is equal to the reaction enthalpy at the same temperature. Note, however, that equation 3.39 is valid for any elementary reaction in solution, whatever the molec-ularity, whereas in the case of gas-phase reactions, the equivalent expression depends on the reaction molecularity (see equations 3.19 and 3.22). [Pg.44]

The experimental methods designed to investigate the energetics of gas-phase ions have been another important source of thermochemical data, particularly throughout the past two or three decades [9,10]. In this chapter, we discuss the main quantities that are measured experimentally and lead to reaction enthalpy values. [Pg.47]

A final word of caution regarding the use of single-temperature equilibrium constants. Although this is a rather expeditious method to derive reaction enthalpies, the obtained values may be quite inaccurate. For instance, a small 10 J K I mol-1 error in the estimated Ar,Vy. yields a 3 kJ mol-1 error in Ar at 298.15 K. [Pg.218]

Table 6.25 Comparison of experimental reaction enthalpies at 0 K (kJ/mol) for the addition of methyl radical to alkenes CH2=CXY with those calculated3 with the wavefunction-based electronic structure methods. Table 6.25 Comparison of experimental reaction enthalpies at 0 K (kJ/mol) for the addition of methyl radical to alkenes CH2=CXY with those calculated3 with the wavefunction-based electronic structure methods.
The addition of radicals to alkenes is used to assess the performance of various levels of theory in the prediction of radical reaction enthalpies. Results for the addition of methyl radical to ethylene (Table 6.24) [41] show that the higher-level methods perform well in predicting the reaction enthalpy values range from -105.6 to -111.5 kJ/mol compared with the corrected experimental value of -113.1 kJ/mol. The AMI method greatly overestimates the exothermicity while the UB3LYP/6-311+G(3df,2p) level of theory, which performs well for the reaction barrier, significantly underestimates the exothermicity. The RB3LYP values... [Pg.191]


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