Fluorine

In this article, we shall study halogen member fluorine. Fluorine is positioned in Group 17 and the second period of the periodic table. It is highly reactive.

Occurrence:

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 oxidizing agent.  F2 is a more powerful oxidizing agent than O2.  Therefore oxidation of fluoride to fluorine could not be carried out by chemical oxidizing agents. Therefore, no oxidizing agent was able to separate fluorine from hydrogen.
• When hydrofluoric acid is heated with oxidizing 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 the electrolysis of HF. Electrolysis of hydrofluoric acid solution gave ozonized 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 ionizing liquid in which KF ionizes.  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 the 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 an 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

(Carbon)        (Carbon tetrafluoride)

Whitlaw Gray’s Method:

Principle:

Pure, dry and anhydrous potassium hydrogen fluoride is electrolyzed in the molten state at 523 K. When hydrogen is liberated at the cathode while F₂ is liberated at the anode.

Diagram:

Whytlaw-Gray Cell (Apparatus):

The cell construction is modified to overcome some disadvantages of the ‘Dennis method”. Three main modifications are The cylindrical vessel, the vertical walls of this vessel minimize the creeping of the electrolyte. The diaphragm between electrodes prevents the mixing of Hydrogen and Fluorine. Blocking of exit tubes is avoided since they are with a large bore.

Electrodes: The cell consists of a rectangular copper vessel wound for electrical heating.  This copper vessel act as a cathode. A pure thick graphite rod suspended at the centre serves as an anode. Anode and cathode are separated by a diaphragm.  The anode is cemented to and insulated from the diaphragm cell by a paste of fluorspar CaF₂. and water glass Na₂SiO₃. The diaphragm is closed at the bottom and it is laterally perforated below the level of the electrolyte.  The upper solid non-porous part leads to a delivery tube (with large bore) for F2, gas.

Electrolyte: Pure, dry and anhydrous KHF₂, in a molten condition.  Anhydrous HF Is added periodically.

Temperature: 523 K

Heating unit: The copper vessel is covered from outside, with asbestos cement insulator of current but conductor of heat. A resistance wire for electrical heating and lagging – to prevent loss of heat by radiation.

Outlets: The outer copper vessel has an outlet for hydrogen and Inner copper diaphragm vessel has an outlet for fluorine gas. The outlets have large bore.

Current and Voltage: A current of 12 to 15 amperes and at 15 volts is employed.

Reactions:

2KHF₂ → 2KF       +    2HF

2KF  →    2K⁺  +    2 F^–

(HF acts as 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 s used up in the process, it is added periodically.

Purification:

F₂ gas obtained by Dennis method contains two impurities  (a)  HF and (b) CF₄. To purify fluorine, the gas is passed through copper U tubes containing dry NaF which absorbs HF vapours to form sodium hydrogen fluoride.

NaF_(s)     + HF_(g)   →    NaHF₂

Carbon tetrafluoride is condensed by cooling the gas in a trap cooled in liquid oxygen (90 K). Only CF₄ gets condensed and is removed. Fluorine (BP 86 K) remains uncondensed and is collected in pure form.

Collection and Storage:

Fluorine gas is heavier than air. It is collected by upward displacement of air in copper cylinders. Fluorine is then compressed and stored In Cu-Ni alloy cylinders.

The Advantages of Whytlaw Gray’s Method over Denis Method:

• Fluorine liberated at the anode and hydrogen at the cathode are prevented from coming In contact with each other by the use of diaphragm cells.
• The walls of the electrolytic cell are vertical and broad outlets provided at the top of the containers for fluorine and hydrogen gas.  The molten electrolyte cannot choke the outlets.
• The current efficiency is as high as 80%.
• This process is continuous.

Properties of Fluorine:

Physical properties of Fluorine:

• Colour: Fluorine is a pale yellow gas.
• Odour: It has an Irritating pungent odour.
• Nature: It Is highly poisonous and has a corrosive action on skin.
• Density: It Is heavier than air.
• Boiling point: 86 K

Chemical Reactions of Fluorine:

Fluorine is one of the most chemically active elements known since it reacts with almost all elements with the exception of nitrogen, oxygen, helium, and argon.

Reaction with Water:

Fluorine decomposes water at ordinary temperature forming hydrofluoric acid and liberating oxygen and ozone.

2H₂O  +   2F₂  → 4HF +   O2

3H₂O  +   3F₂  → 6HF +   O3

Reaction with cold dilute sodium hydroxide:

Cold dilute (2%) aqueous solution of NaOH gives sodium fluoride and oxygen difluoride.

2 NaOH +   2F₂  →   2NaF  + H20 +  OF₂

Sodium fluoride             Oxygen difluoride

Reaction with hot concentrated sodium hydroxide:

Hot concentrated, aqueous solution of NaOH gives sodium fluoride and oxygen.

4 NaOH + 2F₂  →  4NaF  + 2H₂0 +  2O₂

Reaction with Metals:

• With Sodium: Sodium burns spontaneously in fluorine to form sodium fluoride.

2Na   +  F2  →      2NaF

• With Magnesium: Magnesium reacts with fluorine on worming to form magnesium fluoride.

Mg   +   F₂    →     MgF₂

• With Copper: Copper forms a thin layer of CuF₂, on its surface which resists a further attack of fluorine.

Cu   +   F₂   →     CuF₂

• With Mercury: Mercury reacts with fluorine to give mercury fluoride

Hg   +   F₂   →     HgF₂

Reaction with Nonmetals:

• With Hydrogen: Fluorine reacts with Hydrogen with explosive violence even in dark and even at a low temperature as low as 63 K to form hydrogen fluoride.

H₂ +  F₂ → 2HF

• With Boron : 

2B  +  3F₂   →  2 BF₃  (Boron trifluoride)

• With Carbon :

C    +   2F₂ →   CF₄ (Carbon tetrafluoride)

• With Silicon (Quartz):  

Si   +   2F₂ →   SiF₄ (Silicon tetrafluoride)

• With Phosphorous : 

2P  +  3F₂ → 2PF₃  (Phosphorus trifluoride)  and 

2P +    5F₂ →   2PF₅ (Phosphorus pentafluoride)

• With Sulphur: 

S   +  3F2  →    SF₆  (Sulphur hexafluoride)

With chlorine Sulphur forms SCl4  and with fluorine, it forms SF₆.

• Fluorine has the highest electronegativity and the smallest atomic size, due to which it is the strongest oxidizing agent. Hence it brings out the maximum oxidation state of any element.
• Hence with fluorine, every element shows a higher oxidation state. Hence with fluorine Sulphur shows its highest oxidation state of +6 while chlorine shows the +4 oxidation state.

Action of hydrocarbon (Methane): 

Because of strong affinity of fluorine for hydrogen, fluorine decomposes hydrocarbons forming hydrogen fluoride and leaving behind carbon. Excess of fluorine react with carbon to form gaseous carbon tetrafluoride.

CH₄ + 2F₂ → 4 HF +  C

Oxidising properties:

• Potassium Chlorate (KClO₃):

KClO₃   +   F₂ +   H₂O  →  2HF + KClO₄  (Potassium perchlorate)

• Potassium Sulphate (K₂SO₄)

K₂SO₄   +   F₂ →  2KF +  K₂S₂O₈ (Potassium persulphate)

• Potassium Carbonate (K2CO3) Potash ash:

K2CO₃   +   F₂ →   2KF +  K₂C₂O₆  (Potassium percarbonate)

lnterhalogen Compounds of Fluorine:

Binary compounds of fluorine with other halogens are called interhalogen compounds of fluorine. In these compounds, fluorine shows -1 oxidation state.  The other halogens show positive oxidation states such as +1, +3, +5, +7.

Fluorine can form four different types of binary compounds (fluorides) with other halogen and they are as follows.

Type	XF	XF3	XF5	XF7
With Cl	ClF	ClF3	–	–
With Br	BrF	BrF3	BrF5-	–
With I	–	–	IF5	IF7

Action of Fluorine on Chlorine:

When chlorine and fluorine are made to react with each other in equal volumes, chlorine monofluoride is formed

Cl₂ +   F₂ →   2CIF  (Chlorine monofluoride)

When chlorine and excess fluorine are made to react, chlorine trifluoride is formed

Cl₂ +  3 F₂  →  2CIF₃  (Chlorine trifluoride)

Action of Fluorine on Bromine:

When fluorine is made to react with bromine diluted with nitrogen, bromine monofluoride is formed

Br₂ + 3F₂     →         2BrF₃    (Bromine trifluoride)

When excess fluorine is made to react with bromine, bromine pentafluoride is formed.

Br₂ +  5 F₂ →  2BrF₅  (Bromine pentafluoride)

Action of Fluorine on Iodine:

When excess fluorine is made to react with iodine, iodine pentafluoride is formed

I₂ +   5F₂   →    2IF₅  (Iodine pentafluoride)

When much excess fluorine is made to react with iodine, iodine heptafluoride is formed

I₂ +  7 F₂ →      IF₇    (Iodine heptafluoride)

Uses of Fluorine:

• Freon: It is used in the preparation of dichloro-difluoro-methane, CCl₂F₂, known as ‘Freon’.  Freon is used in refrigeration and air-conditioning.
• Teflon: It is used to prepare Teflon. Teflon is polymerized tetrafluoroethylene. Teflon is a chemically inert and electrical insulator. It is used for coating frying pans, cooking pots, and reaction vessels.
• UF₆: It is used for the preparation of UF₆.  UF₆ is helpful in the separation of isotopes of Uranium.
• SF₆: It is used for the preparation of SF₆.  SF₆ is used in the vulcanization of rubber and for high voltage insulation.
• Rocket Fuel: In combination with hydrazine (N₂H₄), fluorine is used as rocket fuel.

Prevention of Tooth Decay or Dental Decay:

The tooth enamel is composed of Ca₅(OH)(PO₄)₃ which slowly converts to Ca₅F(PO₄)₃ due to the regular brushing of teeth with the fluoride toothpaste. (The fluoride ion is added in the form of soluble SnF₂. or sodium mono-fluoro-phosphate to toothpaste).

The fluoro compound Ca₅F(PO₄)₃  resists the attack of germs and mild organic acids associated with foodstuff and prevents tooth decay.