Enthalpy for dissociation

When the combination X,Yor X, Y is of the capto-dative type, as is the case for an alkoxy and an ester group, the enthalpy of bond dissociation is 10-15 kcal lower than when all four groups are electron-attracting. When the capto-dative combination CN/NR2 occupies both the X, Y and the X, Y positions, the enthalpy for dissociation of the C(3)—C(4) bond is less than lOkcal/mol. ... 

The terms AHam and AHax constitute the enthalpies of atomization of the element and the nonmetal respectively, AHei is the ionization enthalpy of the metal, AHae is the ionization enthalpy of the nonmetal, and U0 is the lattice energy. For gaseous diatomic nonmetals, AHax is the enthalpy for dissociation (i.e., bond energy... 

When N2F4 was dissolved in nonpolar solvents such as perfluoro-2,3-dimethylhexane, 2,2-di-chloro-3-chloroperfluorobutane, and various Kel-F oils, the NF2 radical was easily detected at 25°C by its characteristic ESR spectrum [21]. Pure liquid N2F4 is weakly dissociated at 25°C (0.0045%). For comparison, a value of 0.022% was measured in the vapor phase at 25°C and 13 atm [22]. The temperature-dependent intensity of the NF2 ESR triplet that was measured between -30 and +25°C in liquid N2F4 yielded a reaction enthalpy for dissociation of 19.8 1.3 kcal/mol [22]. 

The kinetics of formation and/or dissociation of the mono-complex of Ni with isoquinoline have been measured in a variety of solvents (Table 1). Neither the dissociation rate constant nor the overall equilibrium constant for complex formation correlates with Gutmann s donicity scale for solvents, but the activation enthalpy for dissociation does show a linear relationship with solvent donicity. This, conclude the authors, can be rationalized from a mechanistic point of view by assuming that the solvent molecules co-ordinated to the metal ion help to stabilize the transition state. A similar correlation between A/f J donicity (Table 1) is found ... 

On the basis of these data, he concluded that both in PP and butene polymers, virtually all backbone bonds have lower enthalpy for dissociation than any of the... 

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). 

Activation energy values for the recombination of the products of carbonate decompositions are generally low and so it is expected that values of E will be close to the dissociation enthalpy. Such correlations are not always readily discerned, however, since there is ambiguity in what is to be regarded as a mole of activated complex . If the reaction is shown experimentally to be readily reversible, the assumption may be made that Et = ntAH and the value of nt may be an indication of the number of reactant molecules participating in activated complex formation. Kinetic parameters for dissociation reactions of a number of carbonates have been shown to be consistent with the predictions of the Polanyi—Wigner equation [eqn. (19)]. 

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 ... 

TABLE 5. Bond dissociation enthalpies for some organotin compounds 3 (kJmol-1)... 

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. 

Some of the few complete sets of enthalpies for binary fluorides are collected in Tables XVIII and XIX. The inversion in dissociation heats for copper and beryllium fluorides can be associated with the closed-shell configurations of Be2+ and Cu+. The alternations in bond energies... 

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

We conclude this section by giving, for the sake of clarity, the most general definition of mean bond dissociation enthalpy. For any molecule AYmXn, where A is a central atom and X and Y are any mono- or polyatomic groups, the A-X... 

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