Fluorine, dissociation reaction

Heats of formation for a complete set of Group VILA fluorides are unavailable, but a set of xenon fluoride cations, isoelectronic with iodine fluorides, exhibits the alternating pattern expected for odd- and even-electron molecules. The original energy-level diagram for stepwise fluorine dissociation is shown in Fig. 5. The tabulated values were derived from the ionization energies of XeF and the threshold values for XeFJ — XeF, - + F, where n is even (27), together with heats of formation obtained by reaction calorimetry (137). 

The rate coefficient of the dissociative attachment (5-181) is very high (k = 3 10 cm /s), and it actually has no activation energy. This process dominates the fluorine dissociation in non-thermal plasma and makes it fast, but it is not very energy elfective because of an electron loss in the reaction (see Table 5-2, and explanation of a similar phenomenon in Section 5.10.5). Lower values of the dissociation energy cost and, hence, higher energy... 

The contribution of the energy factor to chain branching was demonstrated later for several fluorination reactions, e.g. for H + F2 -> HF -j- F + 414 kJ. The heat of this process exceeds that of molecular fluorine dissociation by a factor of about 2.5 this seems to ensure the possibility of utilizing the liberated energy for the dissociation of F2 molecules. 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

A simple equihbrium calculation reveals that, at 25°C and atmospheric pressure, fluorine is less than 1% dissociated, whereas at 325°C an estimated 4.6% dissociation of molecular fluorine is calculated. Obviously, less than 1% of the coUisions occurring at RT would result in reaction if step la were the only important initiation step. At 325°C the fluorine atom initiation step should become more important. From the viewpoint of energy control, as shown in Table 1, it would be advantageous to have step lb predominate over step 2a and promote attack by molecular rather than atomic fluorine. Ambient or lower temperatures keep the atomic fluorine concentration low. 

Chemical removal of surface material is produced through standard bond-breaking reactions. Typically chlorofluorocarbons (CECs) have been used, eg, CECl, CE2CI2, CE Cl, CE4, CHE, C2C1E. Eor example, CE dissociates into E atoms and fluorinated fragments of CE in a plasma ... 

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

The bond dissociation energy of fluoromethane is 115 kcal mol , which is much higher than the other halides (C-Cl, C-Br and C-1, respectively 84, 72 and 58 kcal mol ) [6], Due to its strength, the carbon-fluorine (C-F) bond is one of the most challenging bonds to activate [7], A variety of C-F bond activation reactions have been carried out with different organometallic complexes [8], Among them, nickel [9] and ruthenium complexes have proven to proceed selectively under mild conditions [10],... 

In this respect XeF2 is similar to molecular fluorine. The relatively low oxidizing ability and reactivity ofXeF2 in the same type of chemical reactions is explained by its greater dissociation energy. The activation energy of the dissociation (XeF2-> XeF + F) is 190-210 kJ/mol. The XeF radicals are very weakly bound (EXcF = 58-80 kJ/mol) and can be an effective source ofF atoms.3... 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

In later experiments with improved techniques the initial partial dissociation of Fj, CI2 etc. by the r/discharge could be replaced by addition of NO to the F2 flow, thus providing atomic fluorine by the reaction... 

The unsaturated gain was 8 %/cm, and 12 % of the chemical energy involved in the above reaction could be converted to laser radiation. The atomic fluorine is produced by thermal dissociation of SF at about 2500 °K. 

---