Exoergic reaction

The case = 60° is again the simplest. Energy disposition in the products depends on the relative energy but not on the phase of the reactants. From the value of 0 the ratio E jE may be calculated these are included in Table 10.2. In the range 30° 0 60° the products have more vibrational energy than the reactants and the AB formed dissociates if EJ D. 

Changing initial conditions affect both reactivity and energy disposition. For fixed E, increasing Ei corresponds to decreasing 0. For = 60° we find from (10.11) and Table 10.2 that when E EJ3 all trajectories are reactive. For E EJ3 the reaction probability drops until, when Ef = 3E, no reaction occurs. With a further increase of E, the reaction probability rises again. For other choices of product energy disposition as well as reactivity depend upon both phase and 0.  

A good account of more realistic treatments of reaction dynamics is given by J. C. Polanyi, Acc. Chem. Res. 5, 161 (1972). 

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Fig. 3. One-dimensional barrier along the coordinate of an exoergic reaction. Qi(E), Q i(E), QiiE), Q liE) are the turning points, coo and CO initial well and upside-down barrier frequencies, Vo the barrier height, — AE the reaction heat. Classically accessible regions are 1, 3, tunneling region 2.

The energetics of the Eley-Rideal reaction (A E —230 kJ mol-1) are well established.42 Here, the highly exoergic reaction forming gas-phase HC1 was probed by time-of-flight velocity measurements,39,41 scattering angular distributions,39,41 and state-selective laser spectroscopy.39-41... 

The condition for a spontaneous reaction is that the change in the Gibbs free energy must be negative (dG < 0) - a so-called exoergic reaction. 

Fig. 19 shows that with increasing temperature, formation of 1, 5, and 7 benefit entropically and become substantially exoergic reactions. Formation of 3 and 4a exhibit little entropic benefit. As shown in Fig. 20, the reaction barriers similarly... 

Figure 8.1 Graph of (a) G( ), (b) A(i ) for an exoergic reaction (AG° < 0), showing the equilibrium point of minimum Gibbs free energy (triangle) at eq (dotted line). In (a), the direction of spontaneous reaction ( rolling ball ) is shown both for initial 0.15 (where 4 < 0) and 0.85 (where A > 0), both tending toward equilibrium at eq — 0.60 (where A = 0).

Note that the sign convention used in nuclear chemistry and physics that assigns a positive Q value for exoergic reactions is opposite to that used in chemistry where exoergic reactions have negative values of AH and AE. 

Exoergic Reactions (Electronically, Vibrationaliy, and Rotationaily Excited States)... 

When potential surfaces are available, quasiclassical trajectory calculations (first introduced by Karplus, et al.496) become possible. Such calculations are the theorist s analogue of experiments and have been quite successful in simulating molecular reactive collisions.497 Opacity functions, excitation functions, and thermally averaged rate coefficients may be computed using such treatments. Since initial conditions may be varied in these calculations, state-to-state cross sections can be obtained, and problems such as vibrational specificity in the energy release of an exoergic reaction and vibrational selectivity in the energy requirement of an endo-... 

In a typical case, the barrier widths in heavy-particle tunneling reactions correspond to transfer distances that are much smaller than that for hydrogen transfer and are not usually realized at van der Waals interreactant spacings in solids. Therefore, chemical conversions associated with heavy-particle tunneling are rare, often occurring in exoergic reactions where d is much smaller than the geometric transfer distance. A few examples of these reactions are cited in Section 9.2. 

The more exoergic reaction Ba + NzO has a smaller reaction cross section ( 90 A2 or 27 A2) [347, 351] and crossed-molecular beams studies [349] show that the BaO product is backward-scattered with a large amount of internal excitation ((Fr) < 0.20). Laser-fluorescence measurements [348] of the BaO(X Z+) product for the reaction in the presence of an argon buffer gas, find population of vibrational states up to v = 32. The relative populations have a characteristic temperature of 600 K for v = 0—4 and 3600 K for v = 5—32 with evidence of non-thermal population of v — 13—16. This study also observes population of A n and a 3II states of BaO with v = 0—4. A molecular beam study of Ba + N20 with laser-induced fluorescence detection indicates that the BaO( X) product is formed with a very high rotational temperature. 

This electron transfer is likely to occur when Ma and X2 are still far apart ( "- 7-8 A) with subsequent dissociation of X ", and this appears to be the reason that the most exoergic reaction, resulting in formation of two MX molecules, does not occur to any appreciable extent. The cross sections for formation of M and MX in this class of reaction are roughly two orders of magnitude below that for reaction (131). This appears to be also true of the process leading to chemionization which requires the occurrence of a second electron jump. 

The dynamics of the reactions of 0( P) with cyclohexane, cyclohexene, and cyclohexa-1,4-diene have been studied by measurement of the product OH(X II) internal state distributions in a molecular beam/LIF apparatus. The rotational state distributions were found to be similar for all three reactions and consistent with small (1—3%) partitioning of the available energy, indicating that H-abstraction occurs only when the O atom is collinear with the C-H bond under attack. Comparisons with model predictions suggested that some of the extra energy available in the more exoergic reactions between 0( P) and the unsaturated hydrocarbons is released into internal excitation of the hydrocarbon radical product, resulting in only a modest increase in OH vibrational excitation. 

A long-range electron transfer is possible in this reaction, as in alkali metal atom reactions. However, the resulting electron-transfer complex Ba NO does not correlate to the ground-state products BaO which has the structure Ba +0. Moreover, the NOJ ion is stable and its dissociation into NO 4- 0 is endoergic. Hence the Ba "NOj complex may survive for many rotational periods despite the availability of a very exoergic reaction channel. This is expected to dissociate after the transfer of the second valence electron of barium, which is probably hindered by an energy barrier. 

The first photochemical study of this reaction was carried out in 1969 by Oldershaw and Porter [104], who photolyzed static N2O/HI samples at different wavelengths, and used final product analyses to deduce reaction probability versus photolysis wavelength. This provided clear evidence of a substantial entrance channel barrier (i.e., 4400cm ) for the highly exoergic reaction (4a), which was later confirmed and quantified by Marshall et al. [40,41], who carried out experimental rate constant versus temperature measurements as well as ab initio calculations of the stationary points on the potential surface. Oldershaw and Porter were also able to discern the appearance of reaction (4c) with an apparent threshold of 13,500 1400cm, in accord with the thermochemistry, as well as our observations, as discussed below. 

E.E.Nikitin and L.Yu.Rusin, Statistical distribution functions of products of exoergic reactions, Khimiya Vysokhikh Energii 2, 124 (1975)... 

Fullerton and Moran (1971) considered the role of dispersion and short range forces in reactions of He+ + N2, in the context of the phase-space model, and also (1972) reactions of C+ with 02 and N2. The same authors (Moran and Fullerton, 1971) discussed collision-induced dissociation of excited 02 and NO+ ions, and in another paper (1972) rotational excitation of N + produced in thermal reactions A + + N2 -> N + + A, with A a noble-gas atom. Phase-space results in the last paper compared favourably with experiment, with some discrepancies observed for the moderately exoergic reactions. 

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