InterviewSolution
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InterviewSolution
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Fluorine is prepared by the electrolysis ofA. molten cryoliteB. fused `KF`C. anhydrous `HF`D. a solution of `KHF_(2)` in `HF` |
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Answer» Correct Answer - 4 Difluorine is still produced by the Moissan electrochemical method,a process devised `129` years ago. Here fluorine is obtained by treating `CaF_(2)` with concentrated `H_(2)SO_(4)` to give an aqueous mitxture of `HF`. This is distilled yielding anhydros liquid `HF`. Then a cooled solution of `KHF_(2)` in anhydrous `HF` is electrolyzed ,giving `F_(2)` and `H_(2)`. `CaF_(2)+H_(2)SO_(4)rarrCaSO_(4)+2HF` `KF+HFrarrK[HF_(2)]` `HF+K[HF_(2)]overset("electrolyse")rarrH_(2)+F_(2)` `KHF_(2)rarrKF+HF, KF rarr K^(+)+F^(-)` Cathhode, `K^(+)+e^(-)rarrK` `K+HFrarrKF+H` `2HrarrH_(2)` Anode, `F^(-)rarrF+e^(-)` `2FrarrF_(2)` There are many differents in ibtainibng fluorine (i) `HF` is corrosive and etches glass and also causes very painful skin wounds. These arise partly by dehydraing the tissue and partly from th eacidic nature of `HF`. The wounds are slow to heal beacuse `F^(-)` ions remove `Ca^(2+)` ionsf rom the tissues (ii) Gaseous `HF` is also very toxic (`3p p m)` compared with `HCN(10p p m)` (iii) Anhydrogen `HF` is only slightly ionized and is therefore a poor conductor of electricity .Thus a mixture of `KF` and `HF` is electrolyzed to increases the conductively. Moissan used a solution of `KF` in `HF` with a mole ratio of `1:13` but modern methods use a mole ratio of `1:2` of `KF:HF` so that the vapour pressure of toxic and corrosive `HF` is much lower. this mixture melts at about `72^(@)C`, a much easier temperature to maintain .Note that `KF` and `HF` react to form the acid salt `K^(+)[F-H...F]^(-)` (iv) Water must be rigorously excluded otherwise the `F_(2)` produced will oxidize it to `O_(2)` (v) `H_(2)(g)` liberated at the cathode must be separated from the `F_(2)` liberated at the abode by a diaphragm (made of teflon), otherwise they will react explosively (vi) `F_(2)` is extermely teactiveIt catches fire with traces of grease or with crystanlline silicon. Glass and most metals are attacked. It is difficult to find suitable materials from which we could make the reaction vessel. Moissan used a Pt U- tube beause `Pt` is very unreactive but it is very expensive .`Cu` or Monel metal `(Cu/Ni" alloy")` are now used instead, because they cost less. A protective fluoride film forms on the surface of the metal and slows down further attack. (vii) The cathods are made of steel ,the anodes are carbon,and teflon is used for electrical insulation. Graphite anodes must be not be used beacuse graphite reacts with `F_(2)` froming graphite compounds `CF`. In htese `F` atoms progressively invade the space between the sheets of graphite, focusing them aprt and buckleing them. Thsi gradully stops the fraphite from conducting. Thsu current needed increase generating more heat and eventually an explosion may occur .To avoid this ungraphitized carbon is used. It is made from powered coke compacted and impregnated with `Cu`. Recently a chemical method for the prepartion of fluorine has beeen develped. This involves the following reaction: `K_(2)MnF_(6)+2SbF_(5)rarr2KSbF_(6)+MnF_(3)+(1)/(2)F_(2)` In this reaction, the stronger Lewis acid `SbF_(5)` displaces the waker one,`MnF_(5)` from its salt. `MnF_(4)` is unstable and readily decomposes to give `MnF_(3)` and fluorine. |
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