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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Fluorine and Hydrogen Fluoride Page 2

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13.

Oxidising Property of Fluorine

 

It is the most powerful oxidising agent. The oxidising property can be shown by the following equations:

KClO3 + F2 + H2O KClO4 +

2HF

 

potassium perchlorate

 

 

2K2SO4 + F2

K2S2O8

+2KF

 

potassium persulphate

 

 

2K2CO3 + F2

K2C2O6 + 2KF

 

 

potassium percarbonate

 


Its oxidising properties are due to:
(i) small atomic size
(ii) high ionisation potential
(iii) low heat of dissociation
(iv) high heat of hydration
(v) high electronegativity
(vi) low electron affinity
 

 

14.

Reaction with Silica

 

SiO2 + 2F2 SiF4 + O2

 

15.

Uses of Fluorine

 

Fluorine is used for the preparation of many useful fluorides:

Dichlorodifluoromethane(CCl2F2), known as Freon gas, is used in refrigerators and air-conditioners.

Polytetrafluoroethylene, known as teflon, is used for coating reaction vessels and heating pans.

Difluorodiphenyl trichloroethane (DDFT), similar to DDT, is used as a fungicide.

Uranium hexafluoride (UF6) is used for separation of isotopes of uranium.

Sulphur hexafluoride (SF6) is used for vulcanisation of rubber.

Hydrogen fluoride torches are used in weldings.

Liquid fluorine, with hydrazine (H2N - NH2), is used as a rocket fuel.

Metal fluorides, e.g tin (II) fluoride, are used as antiseptics.

 

16.

Hydrogen Fluoride and Hydrofluoric Acid

 

Fluorine gas, on reaction with hydrogen gas, produces hydrogen fluoride gas, which when condensed, forms liquid hydrogen fluoride. The solution of hydrogen fluoride in water is known as hydrofluoric acid.

 

17.

Preparation of Hydrofluoric Acid

 

From Fluorspar (CaF2):

 

D

 

CaF2 + H2SO4

CaSO4 + 2HF

 

(96%)

473 - 573 K

 

 

18.

Preparation of Anhydrous Hydrogen Fluoride (Liquid HF)

 

1. From H2 and F2:
F2 + H2
2HF
2. From Fremy's Salt (KHF2):

 

D

 

2KHF2

2KF + HF

 

573 K

 


Hydrogen fluoride gas, anhydrous HF and HF acid are purified using thionyl chloride as the dehydrating agent and then by fractional distillation method.
H2O + SOCl2
SO2+ 2HCl
Hydrofluoric acid cannot be stored in glass bottles. It is stored in guttapercha or polythene bottles. Anhydrous hydrogen fluoride is stored in mild steel cylinders.

 

19.

Physical Properties

 

Anhydrous HF Acid:
1. It is a colourless, poisonous liquid and shows corrosive action on the skin.
2. It is a non-conductor of electricity
3. When mixed with water, it forms a constant boiling mixture with a boiling point of 393 K.
Aqueous HF Acid:
1. It is a colourless liquid, with a B.P of 292.5 K or 19.5C.
2. It is weaker than other halogen acids.

 

20.

Chemical Properties

 

Action on Silica (quartz):
SiO2 + 4HF SiF4 + 2H2O
SiF4 + 2HF
H2SiF6 (hydrofluoro silicic acid)
Action on Glass (Metallic Silicates):
HF acid reacts with glass and dissolves it. Hence, it cannot be stored in a glass bottle.
(i) NaSiO3 + 6HF
2NaF + SiF4 + 3H2

 

2NaF + SiF4

Na2SiF6

 

 

sodium silicofluoride


(ii) CaSiO3 + 6HF
CaF2 + SiF4 + 3H2O

 

CaF2 + SiF4

CaSiF6

 

 

calcium silicofluoride

 

21.

Hydrogen Fluoride Shows the Following Characteristics

 

H-F bond has a short bond length, is highly polar and shows hydrogen bonding. Hence, HF is a weak acid.

The H-F bond is stronger in HF than in other hydrogen halides. Moreover, HF molecules are associated through hydrogen bonding also. Hence, its boiling point is higher than other hydrogen halides and it exists in a liquid state.

 

22.

Uses of Hydrogen Fluoride

 

HF is used for etching of glass.

It is used in the preparation of metal fluorides used in toothpastes.

It is used in the analysis of silica and silicates.

It is used in the removal of silica from graphite.

It is used to prepare fluorocarbons like ethylene tetrafluoride F2C = CF2, which is used in Teflon

 

23.

Role of Fluoride Ions in the Prevention of Tooth Decay

 

Toothpastes containing fluoride ions help in preventing tooth decay.

The calcium hydorxy apatite [3Ca3 (PO4)2 . Ca(OH)2] present in the enamel reacts with the fluoride ions and get converted to fluorapatite
[3Ca3(PO4)2.CaF2], which is very hard and resistant to acids.

Tooth cavities and cracks are filled by fluorapatite to prevent dental decay.

Stannous fluoride is mixed with toothpastes, which provides fluoride ions.

 

24.

Removal of Silica from Graphite

 

Powdered coke (anthracite) and silica are heated to obtain artificial graphite.

Unreacted silica is removed from graphite by treating it with HF.

HF dissolves silica by forming volatile silicon tetrafluoride.
SiO2 + 4HF
SiF4 + 2H2O

 

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