| Krypton difluoride | |
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Krypton(II) fluoride |
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Other names
Krypton fluoride |
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| Identifiers | |
| CAS number | 13773-81-4 |
| PubChem | 83721 |
| ChemSpider | 75543 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | F2Kr |
| Molar mass | 121.79 g mol−1 |
| Exact mass | 121.908313037 g mol−1 |
| Appearance | Colourless crystals (solid) |
| Density | 3.24 g cm−3 (solid) |
| Solubility in water | Reacts |
| Structure | |
| Crystal structure | Body-centered tetragonal[1] |
| Space group | P42/mnm, No. 136 |
| Lattice constant | a = 0.4585 nm, c = 0.5827 nm |
| Molecular shape | Linear |
| Dipole moment | 0 D |
| Related compounds | |
| Related compounds | Xenon difluoride |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. Together with krypton tetrafluoride it was one of the first compounds of krypton discovered.[2] It is a volatile, colourless solid. 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 Kr2F3+ cations.[3]
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Krypton difluoride can be synthesized using many different methods including electrical discharge, photochemical, hot wire, and proton bombardment. It can also be prepared by irradiating krypton with ultraviolet rays in a fluorine-argon gas mixture at liquid helium temperature. The product can be stored at −78 °C without decomposition.[4]
The first method used to make krypton difluoride and the only one ever reported to produce krypton tetrafluoride was the electrical discharge method. 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.[3][5]
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 that is 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 α-particles which usually would come from a cyclotron.[6][3]
The photochemical process for the production of KrF2 involves the use of UV light and can produce under ideal circumstances 1.22 g/h. The ideal wavelengths to use are in the range of 303–313 nm. It is important to note that harder UV radiation is detrimental to the production of KrF2. In order to avoid the harder wavelengths, simply using Pyrex glass or Vycor or quartz will significantly increase yield because they all block harder UV light. In a series of experiments performed by S. A Kinkead et al., is was shown that a quartz insert (UV cut off of 170 nm) produced on average 158 mg/h, Vycor 7913 (UV cut off of 210 nm) produced on average 204 mg/h and Pyrex 7740 (UV cut off of 280 nm) produced on average 507 mg/h. It is clear from these results that higher energy ultra violet light reduces the yield significantly. The ideal circumstances for the production KrF2 by a photochemical process appear to occur when krypton is a solid and fluorine is a liquid which occur at 77K. The biggest problem with this method is that is requires the handling of liquid F2 and the potential of it being released if it becomes over pressurized.[3][5]
The hot wire method for the production of KrF2 involves having the krypton in a solid state with a hot wire running a few centimeters away from it as fluorine gas is then run past the wire. The wire has a large current, causing it to reach temperatures around 680 °C. This causes the fluorine gas to split into its radicals which then can react with the solid krypton. Under ideal conditions, it has been known to reach a maximum yield of 6 g/h. In order to achieve optimal yields the gap between the wire and the solid krypton should be 1 cm, giving rise to a temperature gradient of about 900 °C/cm. The only major downside to this method is the amount of electricity that has to be passed through the wire thus making it dangerous if not properly set up.[5][3]
Krypton difluoride can exist in one of two possible crystallographic morphologies: α-phase and β-phase. β-KrF2 generally exists at above −80 °C, while the α-KrF2 is more stable at lower temperatures.[3] The unit cell of α-KrF2 is body-centred tetragonal.
Krypton difluoride is primarily a powerful oxidising and fluorinating agent. It can oxidise gold to its highest-known oxidation state, +5:[7]
KrF+
AuF−
6 decomposes at 60°C into gold(V) fluoride and krypton and fluorine gases:[8]
KrF2 can also directly oxidise xenon to xenon hexafluoride:[7]
KrF2 is used to synthesize the highly reactive BrF+
6 cation.[4] KrF2 reacts with SbF5 to form the salt KrF+
SbF−
6; the KrF+
cation is capable of oxidising both BrF5 and ClF5 to BrF+
6 and ClF+
6, respectively.[9]