Krypton difluoride, KrF
2 is a chemical compound of
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is of ...
and
fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
. It was the first
compound of krypton discovered. It is a
volatile, colourless solid at room temperature. The structure of the KrF
2 molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong
Lewis acids to form salts of the KrF
+ and Kr
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s.
The atomization energy of KrF
2 (KrF
2(g) → Kr
(g) + 2F
(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, KrF
2 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.
Synthesis
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.
Electrical discharge
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 F
2 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.
Proton bombardment
Using proton bombardment for the production of KrF
2 has a maximum production rate of about 1 g/h. This is achieved by bombarding mixtures of Kr and F
2 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 KrF
2, but requires a source of α-particles, which usually would come from a
cyclotron.
Photochemical
The successful photochemical synthesis of krypton difluoride was first reported by
Lucia V. Streng in 1963. It was next reported in 1975 by J. Slivnik.
The photochemical process for the production of KrF
2 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. Harder UV radiation is detrimental to the production of KrF
2. 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., it 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 ultraviolet light reduces the yield significantly. The ideal circumstances for the production KrF
2 by a photochemical process appear to occur when krypton is a solid and fluorine is a liquid, which occur at 77 K. The biggest problem with this method is that it requires the handling of liquid F
2 and the potential of it being released if it becomes overpressurized.
Hot wire
The hot wire method for the production of KrF
2 uses 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. A major downside to this method is the amount of electricity that has to be passed through the wire. It is dangerous if not properly set up.
Structure
Krypton difluoride can exist in one of two possible crystallographic morphologies: α-phase and β-phase. β-KrF
2 generally exists at above −80 °C, while α-KrF
2 is more stable at lower temperatures.
The unit cell of α-KrF
2 is body-centred tetragonal.
Chemistry
Krypton difluoride is primarily a powerful oxidising and fluorinating agent: for example, it can oxidise
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
to its highest-known oxidation state, +5. It is more powerful even than elemental fluorine due to the even lower bond energy of Kr–F compared to F–F, with a redox potential of +3.5 V for the KrF
2/Kr couple, making it the most powerful known oxidising agent, though could be even stronger:
: 7 (g) + 2 Au (s) → 2 KrFAuF (s) + 5 Kr (g)
KrFAuF decomposes at 60 °C into
gold(V) fluoride and krypton and fluorine gases:
: KrFAuF → (s) + Kr (g) + (g)
can also directly oxidise
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
to
xenon hexafluoride:
: 3 + Xe → + 3 Kr
is used to synthesize the highly reactive BrF cation.
reacts with to form the salt KrFSbF; the KrF cation is capable of oxidising both
and
to BrF and ClF, respectively.
is able to oxidise
silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
to its +3
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
, reacting with elemental silver or with
to produce
.
Irradiation of a crystal of KrF
2 at 77 K with γ-rays leads to the formation of the krypton monofluoride radical, KrF•, a violet-colored species that was identified by its
ESR spectrum. The radical, trapped in the crystal lattice, is stable indefinitely at 77 K but decomposes at 120 K.
See also
*
Krypton fluoride laser
References
General reading
*
External links
NIST Chemistry WebBook: krypton difluoride
{{fluorine compounds
Fluorides
Nonmetal halides
Krypton compounds