Enthalpies of dissociation

Measurement of the properties of H-bonded systems over a range of temperatures leads to experimental values of AG, AH and AS for H-bond formation, and these data have been supplemented in recent years by increasingly reliable ab initio quantum-mechanical calculations. Some typical values for the enthalpy of dissociation of H-bonded pairs in the gas phase are in Table 3.9. 

The ease of dissociation of the X2 molecules follows closely the values of the enthalpy of dissociation since the entropy change for the reaction is almost independent of X. Thus F2 at 1 atm pressure is 1% dissociated into atoms at 765°C but a temperature of 975°C is required to achieve the same degree of dissociation for CI2 thereafter, the required temperature drops to 775°C for Br2 and 575°C for I2 (see also next section for atomic halogens). 

Taylor [648] has shown that the deceleratory decomposition of HgO is satisfactorily described by the contracting volume equation [eqn. (7), n = 3], Calculated values of E (162—201 kJ mole rise with increasing crystallite size and are somewhat greater than the enthalpy of dissociation (160 kJ mole 1). Since estimated values of A are consistent with the predictions of the Polanyi—Wigner equation, eqn. (19), it is concluded that breakdown involves the detachment of individual molecules rather than the unzipping of the long zig-zag polymeric —Hg—O— chains which constitute the reactant lattice. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

The enthalpy of dissociation of hydrogen bonds, A//nBomi, is a measure of their strength. Explain the trend seen in the data for the following pure substances, which were measured in the gas phase ... 

As regards enthalpy of dissociation A//het(R-R ) and entropy of dissociation A5het(R-R )> only a few data are available (Table 4). Bordwell (1988 ... 

The lattice enthalpy, Aiatt//m, is the molar enthalpy change accompanying the formation of a gas of ions from the solid. Since the reaction involves lattice disruption the lattice enthalpy is always large and positive. Aatom//m and Adiss//m are the enthalpies of atomization (or sublimation) of the solid, M(s), and the enthalpy of dissociation (or atomization) of the gaseous element, X2(g). The enthalpy of ionization is termed electron gain enthalpy, Aeg//m, for the anion and ionization enthalpy, Ajon//m, for the cation. 

Whichever value of this Rh-Cl bond enthalpy is adopted, it is rather lower than either D (Rh-Cl) in RhCl3 or D (RhCl2-Cl), as Table 12b shows. Recent measurements of the enthalpy of dissociation of [Rh(CO)2Cl]2 in solution to form the coordinatively unsaturated species Rh(CO)2Cl, gave 8 s) a value of 94 kJ mol-1. 

The electrochemical standard free enthalpy, of dissociation of the surface acid or base sites consists of the chemical standard free enthalpy, AG°, an electrostatic energy, eA, and an interaction energy, m0, for the adsorption coverage in the Frumkin adsorption model is the potential across the compact layer, 0 is the adsorption coverage, and m is the Frumkin parameter [Frumkin, 1925] ... 

The distinction between these two terms may be more evident if described in terms of a simple example, the C-H bond. The enthalpy of dissociation of the C-H bond depends on the nature of the molecular species from which the H atom is being separated. For example, in the methane molecule... 

This value of the C-H bond enthalpy does not correspond to the enthalpy of dissociation of the carbon-hydrogen bond in methane, as represented in Equation (4.35). 

Handa, Y.P. (1986a). Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodynamics, 18 (10), 915-921. 

Kang, S.-P. Lee, H. (2001). Enthalpies of dissociation of clathrate hydrates of carbon dioxide, nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran). J. Chem. Thermodynamics, 33 (5), 513-521. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

This all explains the nature of failures of the DFT based methods in those cases, when correlations substantially come into play as in e.g. d -iron (II) ferrocene molecule. Here the errors even of advanced DFT methods become catastrophic. For example, in Ref. [86] the calculated enthalpies of dissociation of ferrocene to the free Fe + ion and two Cp anions (Cp = C5H5 - cyclopentadienyl) depending on the functional used appear to be by 3-4 eV smaller than the experimental value. The reason is transparent... 

The difference in the two values for enthalpy of dissociation of 56 + 42 = 98 kJ mol-1 is equivalent to a difference in pKa of 17 units in favour of HCI being the stronger acid. The factors that contribute to reducing the value for HCN are its high H—CN dissociation enthalpy, its relatively large negative electron attachment enthalpy, and its relatively less negative value for the enthalpy of hydration of the ion. 

Secondly we apply the auxiliary constraint on AHM at x =, T = TAC given by Eq. (16). Converting AHM (mole species) given by Eq. (94) to AHM (gram-atom components) via Eq. (44) and inserting this into Eq. (16) gives an equation for the enthalpy of dissociation of , 4, and cas in terms of the other coefficients and known thermodynamic quantities ... 

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