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Concept
Difluorure de krypton
Résumé
Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. It was the first compound of krypton discovered. It is a volatile, colourless solid at room temperature. The structure of the KrF2 molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acids to form salts of the KrF+ and KrF3+ cations.
The atomization energy of KrF2 (KrF2(g) → Kr(g) + F2(g)) is 21.9 kcal/mol, giving an average Kr–F bond energy of only 11 kcal/mol, the weakest of any isolable fluoride. In comparison, difluorine is held together by a bond of 36 kcal/mol. Consequently, KrF2 is a good source of the extremely reactive and oxidizing atomic fluorine. It is thermally unstable, with a decomposition rate of 10% per hour at room temperature. Krypton difluoride is endothermic, with a heat of formation of 14.4 ± 0.8 kcal/mol measured at 93 °C.
Krypton difluoride can be synthesized using many different methods including electrical discharge, photoionization, hot wire, and proton bombardment. The product can be stored at −78 °C without decomposition.
Electric discharge was the first method used to make krypton difluoride. It was also used in the only experiment ever reported to produce krypton tetrafluoride, although the identification of krypton tetrafluoride was later shown to be mistaken. The electrical discharge method involves having 1:1 to 2:1 mixtures of F2 to Kr at a pressure of 40 to 60 torr and then arcing large amounts of energy between it. Rates of almost 0.25 g/h can be achieved. The problem with this method is that it is unreliable with respect to yield.
Using proton bombardment for the production of KrF2 has a maximum production rate of about 1 g/h. This is achieved by bombarding mixtures of Kr and F2 with a proton beam operating at an energy level of 10 MeV and at a temperature of about 133 K. It is a fast method of producing relatively large amounts of KrF2, but requires a source of high-energy protons, which usually would come from a cyclotron.
The successful photochemical synthesis of krypton difluoride was first reported by Lucia V.
Source officielle
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