Requirement to dissociate

The same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

It will be seen that most of the dissociation constants in Table 9 lie between 10-3 and 10-11. It is of interest to know how much work is required to dissociate any of these molecules or molecular ions, transferring a proton to a distant water molecule. Using (91) in the form... 

The significantly lower dediazoniation rate of the l/f-3,5-dimethylpyrazole-4-di-azonium ion (8.22) compared with that of the benzenediazonium ion was the central subject of an MNDO study by Brint et al. (1985). The diazonium ion 8.22 has been recovered unchanged after heating for 3 h at 100 °C in aqueous hydrochloric acid. It is not completely decomposed after a similar treatment for 48 h (Reilly and Madden, 1925). Brint et al. calculated the heats of formation of this diazonium ion and of the corresponding heteroaryl cation 8.23 (Scheme 8-16). They found that the values of A//f for the diazonium ion 8.22 and for the benzenediazonium ion are almost identical, whereas that for the cation 8.23 is much greater. The energy required to dissociate the pyrazolediazonium ion is therefore nearly twice that required for the benzenediazonium ion (A//f = 329 and 194 kJ mol-1, respectively). 

Other Ion Affinities Binding affinities for many different types of ions to neutrals are defined analogously to hydride affinity, as the 298 K enthalpy required to dissociate the complexed species. The ion can be a cation or an anion. Conversely, ion affinities can be described in terms of the dissociation. 

The initiation reaction is the dissociation of molecular bromine rather than the dissociation of molecular hydrogen because the energy necessary to dissociate the latter is much greater than that required to dissociate the former. This fact is evident from a consideration of the standard enthalpies of reaction. 

It is important to recognize that the mechanism indicated by the figures is quite different from one corresponding to an initial complete dissociation of the YZ molecule followed by combination of the X and Y atoms. This latter mechanism corresponds to movement from one valley at J up to the plateau at N and back down to the second valley at M. The activation energy for the latter mechanism would be equal to the energy required to dissociate the YZ molecule. By moving along the valley as in the mechanism indicated by the dashed line in the... 

This equation sums up the heats of formation of all of the products from a reaction, and then subtracts from that value the heat required to dissociate all of the starting materials into their elements. The difference between these two values is the net heat change, or heat of reaction. The heats of formation of a number of materials of interest to the high-energy chemist may be found in Table 2.7. All values given are for a reaction occurring at 25°C (298 K). 

The energy required to dissociate the acetone molecule into two methyl radicals and a molecule of carbon monoxide may be calculated from thermocheinical data and amounts to 89 kcal. mol.-1. It would be energetically possible, therefore, for a photodecomposition of type B to occur,... 

DH°[A—B] of a specific A—B bond may be expressed as the enthalpy required to dissociate a bonded A—B(g) species into gaseous fragments ... 

Photochemistry. The Hg(3P,) sensitized photolysis ofC2H2 has produced benzene, hydrogen, and polymer [Shida et al. (872)]. Since the Hg(3Fi) state does not have sufficient energy required to dissociate the H—C2H bond, the products must be produced by reactions of an electronically excited C2H2. The photolysis of acetylene at 1849 A has produced hydrogen, ethylene, vinylacetylene, diacetylene, benzene, and solid polymers [Tsukada and Shida (979), Zelikoff and Aschenbrand (1082)]. 

Structurally, a more specific definition would be that a molecule is a local assembly of atomic nuclei and electrons in a state of dynamic siahility. The cohesive forces are electrostatic, but. in addition, relatively small electromagnetic interactions may occur between (he spin and orbital motions of (he electrons, especially in the neighborhood of heavy nuclei. The intcrnuclear separations are of the order of 1-2 x 10" meter, and the energies required to dissociate a stable molecule into smaller fragments... 

Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolyti-cally is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is 84 kcal mole-1 (Table 4-6), dissociation to a methyl cation and a chloride ion requires about 227 kcal mole-1 ... 

The second illustration indicates that less energy is required to dissociate structures with both cavities filled, than those with one cavity filled. Tse (1994) suggests that collisions of a guest with the cavity wall weakens interactions between the hydrogen bonds, which is also reflected in a high value of thermal expansion. 

Step 2 Calculate the energy required to dissociate hydrate... 

The energy required to dissociate a proton from gas-phase cationic species, BH+, is called the proton affinity of the base B, PA(B), and is measure of both the acidity of BH+ and the basicity of B. 

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