Enthalpies of ozonation

The enthalpy of ozone decomposition AH=Do2—o = 107kJ mol-1. The most probable reaction of initiation by ozone in solution is the abstraction reaction [133] ... 

Direct calorimetric measurements of two ozonides have been reported. Both the enthalpy of combustion of the ozonides and the direct enthalpies of ozonation of the precursor olefin were measured. The first species to be studied was the purported ozonide of A °octalin (l,2,3,4,5,6,7,8-octahydronaphthalene) . It is doubtful that the product of the octalin ozonation reaction would be the molozonide formed by direct addition with no subsequent rearrangement (i.e. ll,12,13-trioxabicyclo[4.4.3]tridecane) but perhaps even less likely is the rearranged and hence normal ozonide, the ll,12,13-trioxa[4.4.2.1]paddlane. From the published enthalpy of combustion of —5628 kJmoU, we derive an enthalpy of formation of this species, whatever it is, of —593.7 kJmoU. ... 

Enthalpies of ozonation, 165 Enthalpies of reactions acyl peroxides, 162-3 arene endoperoxides, 166-7 tert-butyl perbenzoate ETR reactions, 909-11... 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

Ozone is a highly endothermic substance. Its enthalpy of formation reaches 2.96 KJ/g, which makes it very unstable thermodynamically by CHETAH criterion Ci. In the liquid or solid state, it detonates spontaneously. 

The reaction favours the formation of ozone with a significant equilibrium constant. Appendix C also lists the enthalpies of formation and the standard enthalpy of the reaction ArH° can be calculated. The answer for the enthalpy calculation is ArH° = —106.47 kJ mol, showing this to be an exothermic reaction, liberating heat. The entropy change at 298 K can also be calculated because ArG° = ArH° — T ArS°, so ArS° = 25.4 Jmol-1 K-1, indicating an increase in the entropy of the reaction as it proceeds by creating one molecule from two. 

Some elements exist in more than one form under standard conditions. For example, carbon can exist as either graphite or diamond, as shown in Figure 5.16. Graphite is defined as the standard state of carbon. Therefore, the standard enthalpy of formation of graphite carbon is 0 kj/mol. The standard enthalpy of formation of diamond is 1.9 kj/mol. Another example is oxygen, 02(g). Oxygen also exists in the form of ozone,... 

The second ozonide is that from fraw -stilbene, i.e. the 3,5-diphenyl trioxolane derivative. The ozonation experiment was done on both the cis and trans olefins resulting in reaction enthalpies of —471.5 and —428.9 kJmoU, respectively. The difference between these two values is ca 43 kJ moU, which is close to the difference between the two olefins enthalpies of formation (one as liquid and the other as solid), 46 2 kJ moU. This result vindicates the original authors suggestion of using the ozonation enthalpies as a method of determining the difference between enthalpies of formation of a Z/ pair of olefins. 

Bond energies can be used in the discussion of the structure of molecules, For example, in 1933 it was pointed out27 that the enthalpy of formation expected for ozone from molecular oxygen would be — 77.9 kcal/mole (or less if a correction were made for strain in the... 

The addition of ozone to ethyne leads initially to formation of 1,2,3-trioxolene 7 with an estimated activation enthalpy of 9.6 kcal mol-1 at 298 K (cf. experiment, 10.2 kcal mol-1) and a reaction enthalpy of —55.5 kcal mol-1 <2001CPL268, 2001JA6127>. The vibrational frequencies were calculated for structure 7 by the PM3 method <1997PCA2471>. The intermediate 7 rapidly decomposes into a number of other intermediate products (see Section 6.05.4.1). 

The above data show that three pathways exist for the reaction of ozone with ethylene eoordinated and non-eoordinated addition in the singlet state and a reaction in the triplet state. The latter pathway is hardly possible for the thermal reaction the former two pathways exist in eompetition. To assess the effieieney of each pathway, we calculated the corresponding reaction rate eonstants. For this pmpose, a MOLTRAN program and results of the quantum ehemieal ealeulations were used to calculate the enthalpies (AH") and entropies (AS") of activation of the two reaction pathways. From the data obtained, the eorresponding rate constants k were ealeulated in terms of the standard transient state theoiy ... 

Olefins. Here, the reaetion of asymmetrieal addition of ozone prevails. The reaction (2) share is less than 0.1%. The reaetion (2) enthalpy for these olelms is -10 h- +6 kJ/mof thus, the reaetion of asymmetrieal addition prevails. The conclusion agrees with the data of quantum-ehemieal ealeulations for ethylene and conclusions of many works on the reaetions with olefins [1-3]. 

Diphenylethylenes. In reaetions of ozone with diphenylethylenes, the reaction (2) prevails. The enthalpies of these reaetions are low enough (they vaiy over the range -55 -84 kJ/moT ). 

Estimate the standard enthalpy of formation of dichlorodifluoromethane, CCl2F2(g) (Fig. 12.15). This compound is also known as Freon-12 and has been used as a refrigerant because of its low reactivity and high volatility. It and other related chlorofluorocar-bons (CFCs) are being phased out because of their role in depleting the ozone layer in the outer atmosphere, as discussed in Section 20.5. 

The compound CF3CITCI2 (with a C—C bond) has been proposed as a substitute for CCI3F and CCI2F2 because it decomposes more quickly in the atmosphere and is much less liable to reduce the concentration of ozone in the stratosphere. Use the atomization enthalpies and average bond enthalpies from Table 12.3 to estimate the standard enthalpy of formation (AH°) of CF3CITCI2 in the gas phase. 

Table 6.3 lists the standard enthalpies of formation for a number of elements and compounds. (Eor a more complete list of values, see Appendix 3.) By convention, the standard enthalpy of formation of any element in its most stable form is zero. Take oxygen as an example. Molecular oxygen (O2) is more stable than the other allotropic form of oxygen, ozone (O3), at 1 atm and 25°C. Thus we can write AHfiOf) = 0. but AHf (Os) + 0. Similarly, graphite is a more stable allotropic form of... 

Using the data in Table A-V, calculate the average bond enthalpy of the oxygen-oxygen bond in ozone. 

Temperature Effeet on Ozone Stability. Based on enthalpy released during the ozone decomposition (6-63), calculate the relative O3 concentration in air that can lead to a doubling of initial room temperature of the air (heating to a temperature of about 600 K). Estimate the characteristic time of ozone decomposition (6-81) at this relative ozone concentration in air, and at this temperature (about 600 K). 

In the problem, we are given AH° for the reaction, and we can look up the 0=0 bond enthalpy in Table 9.4 of the text. Solving for the average bond enthalpy in ozone,... 

The standard enthalpy of formation (from Appendix 3 of the text) for ozone ... 

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