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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Fluorine and Hydrogen Fluoride Page 1

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1.

Introduction

 

Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At) are collectively known as halogens.

 

2.

Position in the Periodic Table

 

Halogens are placed in Group VIIA of the long form of periodic table. They are placed just before the inert gases. Their electronic configuration is ns2np5. The electronic configuration of Fluorine is 1s22s22p5.
The last electron enters the p-orbital, hence halogens are included under the p-block elements.

 

3.

General Properties of Halogens

 

  • Halogens possess seven electrons in their outermost orbital. Hence, they show monovalency.
  • The atomic radii and the ionic radii increase from fluorine to iodine. With increase in atomic radii, the electronegativity and ionisation potential decrease.
  • Fluorine shows only -1 oxidation state, while all other halogens show +1, +3, +5, +7 and -1 oxidation states.
  • The electron affinity and bond dissociation energy decrease from chlorine to iodine.
  • The electron affinity of fluorine is less than chlorine, while its bond dissociation energy is less than chlorine and bromine. This is due to its small atomic size and greater electron-electron repulsion.
  • Fluorine and chlorine are gases, bromine is a reddish-brown liquid and iodine is a violet solid.
  • Fluorine is a pale yellow gas, while chlorine is a greenish-yellow gas.

 

4.

Fluorine

 

Occurrence
Fluorine does not occur in a
free state but in the combined form as fluorides or mixed fluorides of some metals. The minerals of fluorine are:
1. Fluorspar or fluorite: CaF2 (calcium fluoride)
2. Cryolite: 3NaF.AlF3 or Na3AlF6 (sodium aluminium fluoride)
3. Fluorapatite: CaF2.3Ca3 (PO4)2 (mixed calcium phosphate and fluoride).

 

5.

Preparation of Fluorine

 

Fluorine can be prepared by the electrolytic method using two type of cells:

i. Dennis Method (Using Dennis Cell):
The cell used is a V-shaped copper tube with copper caps. Two graphite electrodes are inserted in the two arms of the cell. There are two outlets provided for H2 and F2 in the upper portion of the arms of the cell.
Dry and pure potassium hydrogen fluoride (KHF2) is electrolysed at 532 K. A current of about 4.5 amperes at 12 volts is passed through the molten mass. H2 is liberated at the cathode and F2 at the anode.
The F2 obtained is contaminated with CF4 and HF vapours. F2 is purified by passing the contaminated gas through U tubes containing dry sodium fluoride (NaF) to remove HF.
NaF + HF
NaHF2
The gas, when passed through a trap cooled in liquid oxygen, liquefies CF4. Pure F2 is stored in cylinders made of Cu-Ni alloy.

Drawbacks of the Dennis Method:
1. Graphite electrodes react with F2 to form carbon tetrafluoride. Hence, they are not suitable for use as anodes.
2. F2 liberated at the anode is contaminated with HF formed by partial sublimation of KHF2 and CF4.
3. Fused KHF2 creeps over the edge of the cell, choking the exit tubes.
4. Greater electric current is required as the current efficiency is low.
5. H2 and F2 gases can mix, causing an explosion.

Reactions
Dissociation:

KHF2 KF + HF
KF
K+ + F-

At Cathode:
2K+ + 2e
- 2K
2K + 2HF
2KF + H2

At Anode:
2F
- 2F + 2e -
2F F2

ii. Whytlaw-Gray Method:
The cell consists of a cylindrical copper vessel that acts as the cathode. A graphite anode is fixed in the center of a coaxial copper tube, which is suspended in the copper vessel.
KHF2 is electrolysed at 523 K. A current of 12-15 amperes is passed at 15 volts. H2 is liberated at the cathode and F2 at the anode. F2 is contaminated with HF and CF4. HF is removed by passing the contaminated gas through NaF absorption tubes and CF4 by passing the contaminated gas through liquid O2. Pure F2 is then stored in suitable cylinders.
Advantages of the Whytlaw-Gray Method over Dennis Method:
1. Since the anode and the cathode are separated by a perforated diaphragm, mixing of H2 and F2 is prevented.
2. Current efficiency is about 80%.
3. The exit tubes are wide hence they do not get blocked.
4. The cell is cylindrical, hence KHF2 is kept at a low level to prevent its creeping.

Reactions:
Dissociation:

KHF2 KF + HF
KF
K+ + F-


At Cathode:
2K+ + 2e
- 2K
2K + 2HF
2KF + H2

At Anode:
2F
- 2F + 2e-
2F
F2

 

6. Physical Properties of Fluorine

 

1. Fluorine is a pale yellow gas.
2. It has a pungent and irritating odour.
3. It is poisonous and has a corrosive action on the skin.
4. It is a powerful oxidising agent.
5. It dissolves in water, liberating ozonised oxygen.
6. It is 1.3 times heavier than air.
7. It is a non-combustible gas.
8. It has
-1 oxidation state in its compounds.
9. Its M.P. is
-220C and B.P. is - 188C.

 

7.

Chemical Properties of Fluorine

 

Reaction with Water:

 

H2O + 2F2 4HF + O2
3H2O + 3F2
6 HF + O3

 

 

ozone

 

 

8.

Reaction with Alkalies

 

(a) With Dilute Alkali:
2NaOH + 2F2 2NaF + OF2 + H2O

 

 

oxygen difluoride

 

 

(b) With Concentrated Alkali:
4NaOH + 2F2 4NaF + O2 + 2H2O

 

9.

Reaction with Metals

 

2Na + F2 2NaF
Mg + F2
MgF2
Cu + F2
CuF2
2Fe + 3F2
2FeF3

 

10.

Reaction with Non-metals

 

H2 + F2 2HF
2B + 3F2
2BF3
C + 2F2
CF4
S + 3F2
SF6
2P + 5F2
2PF5

 

11.

Reaction with Hydrocarbons

 

CH4 + 2F2 C + 4HF
C + 2F2
CF4

 

12.

Reaction with Other Halogens

 

Fluorine reacts with halogens to form inter-halogen compounds or halogen halides.
Depending on the quantity of fluorine and the temperature, different halogen fluorides are produced:

 

473 K

 

Cl2 + F2 (equal volumes)

2ClF

 

 

573 K

 

Cl2 + 3F2 (excess F2)

3ClF3

 

 

573 K

 

Br2 + 3F2 (diluted with N2)

2BrF3


Br2 + 5F2 (excess F2) 2BrF5
I2 + 5F2 (excess F2) 2IF5

 

573 K

 

IF5 + F2 (much excess of F2)

IF7

 

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