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what is the electrolyte used in the electolytic method of preparation orf flourine
Answers
Answer:
Because of extreme reactivity fluorine does not occur in the free state. It occurs as the fluorides (F–) of certain metals such as Calcium and Aluminium.
Its major minerals are
Fluorspar – CaF2, Cryolite – Na3AlF6 and Fluorapatite -3Ca(PO4)2 , CaF2.
Difficulties in the Isolation of Fluorine:
The following difficulties were Involved in the isolation of fluorine.
Its oxidation is not possible.
It exists as fluoride (F–) in its minerals. The preparation of it from a fluoride is an oxidation reaction.
2 F– → F2 + 2e–
It is the most electronegative element and itself is a very strong oxidising agent. F2 is a more powerful oxidising agent than O2. Therefore oxidation of fluoride to fluorine could not be carried out by chemical oxidising agents. Therefore, no oxidising agent was able to separate fluorine from hydrogen.
When hydrofluoric acid is heated with oxidising agents, like MnO2, KmnO4, K2Cr2O7 no fluorine is obtained. This is because of the High affinity of fluorine for hydrogen.
H-F bond Is considerably strong due to small bond length and high polarity.
It cannot be prepared by electrolysis of HF.
Electrolysis of hydrofluoric acid solution gave ozonised oxygen at the anode, instead of fluorine. This is due to the action of F2 on water.
2H2O + 2F2 → 4HF + O2
3H2O + 3F2 → 6HF + O3
Then the electrolysis of anhydrous hydrogen fluoride was attempted. It was found to be a poor conductor (non-conductor) of electricity. HF is highly volatile and poisonous.
Proper apparatus for preparation and storage was not available.
As F2 is extremely reactive, it attacks glass, platinum, carbon and other materials commonly used for the construction of the apparatus of electrolysis.
Aqueous HF is an active solvent. It attacks the glass and various metals. Thus the main need was to find a suitable apparatus end suitable electrolyte.
Preparation of Fluorine:
Dennis Method:
Principle:
Pure, dry and anhydrous potassium hydrogen fluoride (KHF2) is electrolyzed in the molten state at 523 K when hydrogen is liberated at the cathode while F2 is liberated at the anode.
Diagram:
Dennis Cell (Apparatus):
This is a heavy V-shaped copper tube. F2 reacts with copper forming a protective layer of copper fluoride CuF2. This protects the cell from further attack by fluorine. The V-shaped tube prevents fluorine liberated at the anode and hydrogen at the cathode from coming in contact with each other.
Electrodes: The electrodes are made of graphite. The copper tube is closed with copper caps through which graphite electrodes are fitted. Copper caps are cemented to and Insulated from the copper tube by bakelite cement.
Electrolyte: Pure, dry and anhydrous KHF2, in a molten condition. Anhydrous HF Is added periodically.
Temperature: 523 K
Heating unit: The copper tube is covered from outside, with asbestos cement insulator of current but conductor of heat resistance wire for electrical heating legging – to prevent loss of heat by radiation.
Outlets: Two outlets are provided for hydrogen and fluorine in the upper portions of the two arms.
Current and Voltage: A current of 5 amperes and at 12 volts is employed.
Reactions:
2KHF2 → 2KF + 2HF
2KF → 2K+ + 2 F–
(HF acts as an ionising liquid in which KF ionises. KF is a carrier of current)
At Anode
2F– → F2 + 2e–
At Cathode
2K + 2e– → 2K
2K + 2HF → 2KF + H2 ↑
As only HF is used up in the process, it is added periodically.
Drawbacks of Dennis Method:
F2 liberated at anode is usually contaminated with HF and CF4
The graphite electrodes are continuously eaten up by fluorine and hence electrodes have to be periodically replaced.
Molten electrolyte froths or foams. This foam or froth rises or creeps up the slanted V-shaped tube and chokes the outlets. This leads to explosion due to the mixing of H2 and F2.
Current efficiency in this method is low i.e. about 30%.
At the high temperature (523 K) at which electrolysis is carried out, the attack of fluorine on the cell is vigorous.
Note:
Graphite is not suitable electrode at anode because it combines with fluorine formed and produces CF4. Hence it is necessary to replace graphite electrode time to time. The reaction is as follows
C + 2F2 → CF4