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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Group 17 - The Halogen Family

Group 17 - The Halogen Family
Group 17 of the periodic table contains the elements fluorine, chlorine, bromine, iodine and astatine. These are collectively known as HALOGENS. It is derived from two Greek words Halo and Gens meaning "Salt producer". Because most of them exist in Sea water.


 

GROUP 17 - THE HALOGEN FAMILY


Group 17 of the periodic table contains the elements fluorine, chlorine, bromine, iodine and astatine. These are collectively known as HALOGENS. It is derived from two Greek words Halo and Gens meaning "Salt producer". Because most of them exist in Sea water.

General Trends

Electronic Configuration: All these elements possess ns2np5 configuration.

Table  Electronic Configuration of group 17 elements

Fluorine  - 

Atomic Number :  9   Electronic Configuration :    Group Number : 17  Periodic Number : 2

 

Chlorine  -

Atomic Number :   17   Electronic Configuration :    Group Number : 17  Periodic Number : 3

 

Bromine  - 

Atomic Number :  35   Electronic Configuration :    Group Number : 17  Periodic Number : 4

 

Iodine  - 

Atomic Number :  53   Electronic Configuration :    Group Number : 17  Periodic Number : 5

 

Astatine  - 

Atomic Number :  85    Electronic Configuration :    Group Number : 17  Periodic Number : 6

 

Chemical Properties

1. Oxidising power: An important feature of the halogen is their oxidising

property which is due to high electron affinity of halogen atoms. The oxidising power decreases from fluorine to iodine. Fluorine is the strongest oxidising agent. It oxidises other halide ions to halogens in solution or when dry.

F2 + 2 X- 2F - + X2 (X- = Cl -, Br -, I-)

Halogen of low atomic number oxidises the halide ion of higher atomic number.

2. Solubility: Halogens, being non-polar molecules, do not dissolve to a

considerable extent in a polar solvent like water. However, fluorine reacts with water readily forming a mixture of O2 and O3.

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

Chlorine, bromine and Iodine are more soluble in organic solvents such as CCl4, CHCl3 and produce yellow, brown and violet colour.

3. Hydrides of the Halogens (Hydrogen halides):

i) All halogens react with hydrogen to form volatile covalent hydrides of formula HX.

ii) These hydrides are called hydracids.

iii) The activity of halogens towards hydrogen decreases from fluorine to iodine. Hydrogen combines explosively with fluorine even in dark. It combines with chlorine in the presence of sunlight and with bromine on heating. Hydrogen combines with iodine on heating and in presence of a catalyst.

iv) Hydracids are the reducing agents.

v)    Except HF, all hydrogen halides are gases. HF is a liquid because of inter

molecular hydrogen bonding.

H - F ....... H -F ....... H-F ....... H-F

vi) The acidic character of HX are in the following order.

HF < HCl < HBr < HI.

Anamalous Nature of Fluorine

Fluorine is the most reactive element among halogen. This is due to the minimum value of F-F bond dissociation energy.

Fluorine decomposes cold dilute alkalies liberating OF2 and with conc. alkali,

O2 is liberated. Under similar conditions, the other halogens will give rise to

the hypohalites and halates respectively.

It has the greatest affinity for hydrogen, forming HF which is associated

due to the hydrogen bonding. Hydrofluoric acid is a weak acid whereas the other hydrohalic acids are strong acids.

...... H- F...... H- F..... H-F.

It differs markedly from the other halogens in that it can form two types of

salts with metals. NaF and NaHF2.

The salts of HF differ from the corresponding salts of other hydracids. AgF

is soluble in water while the other AgX are insoluble.

Being strongly electronegative it can have only a negative oxidation state

while the other halogens can have negative as well as positive oxidation state.

HF attacks glass while others do not.

Fluorine, because of the absence of d-orbitals in its valence shell does not

form any polyhalides. Thus we have I3 -, Br3-, Cl3- ions but no F3- ion.

 

Etching glass

Industrially, hydrogen fluoride is obtained by heating fluorspar (CaF2) with

concentrated H2SO4 in a lead vessel.

CaF2 + H2SO4 CaSO4 + 2HF.

HF distils over and the vapours are condensed in water in a lead receiver.

Aqueous HF thus obtained is stored in wax bottles. It cannot be stored in glass or silica bottles as it attacks silicates and silica.

 

Na2 SiO3 + 6HF Na2SiF6 + 3H2O

SiO2 + 4HF SiF4 + 2H2O

The action of hydrofluoric acid on silica and silicates is used for etching

glass. The glass article is first covered with a film on wax. The design to be etched is now drawn on the waxed surface and is then exposed to the action of hydrofluoric acid. Now the glass can be very soon etched. The wax is finally washed off with turpentine.

 

 

ISOLATION OF FLUORINE

Symbol - F Atomic number -9                  Period Number   :2

Valency -1 Atomic mass-19    Group Number      : 17

Fluorine does not occur free in nature. It occurs in the combined form. Dennis' Method: This was devised by Dennis, Veeder and Rochow in 1931.

In this fluorine is prepared by the electrolysis of fused sodium or potassium hydrogen fluoride (perfectly dry) Electrolysis is carried out between graphite electrodes in a V-shaped electrically heated copper tube. The ends of the tube are covered with copper caps into which the graphite electrodes are fixed with bakelite cement. The copper tube is thickly lagged to prevent loss of heat.

KHF2 KF + HF

HF H+ + F

2H+ + 2e- H(At cathode)

2F - - 2e- F2 (At anode)

Fluorine liberated at the anode is passed through the U-tube containing sodium fluoride. Thisremovesthe hydrogen fluoride vapourscomingwithfluorine.

NaF +HF NaHF2


Physical Properties

1.    Fluorine is a gas and has pale greenish yellow colour.

2.    It has extremely pungent and penetrating odour. 3. It is heavier than air.

Chemical Properties

Fluorine is the most active member of halogen family.

1. Action with Hydrogen: Hydrogen explodes violently in fluorine even in

the dark.

 

H2 + F2  2HF

2. Action with non-metals: Non-metals like carbon, silicon and phosphorus

burn in fluorine forming fluorides.

 

C + 2F2  CF4

Tetra fluoromethane

 

Si + 2F2  SiF4

Silicon tetrafluoride

 

2P + 5F2  2PF5

Phosphorus pentafluoride

 

3.    Action with metals: It reacts with metals forming corresponding fluorides.

2Ag + F2  2AgF

2Al + 3F2  2AlF3

4. Formation of Interhalogen compounds: It forms a variety of inter halogen

compounds with other halogens.

 

Br2 + 3F2  2Br F3

I2 + 5F2  2 IF5

Uses

1. Fluorine is used in the manufacture of a series of compounds known as

freons. These non-toxic, non-combustible and volatile liquids are used as refrigerants in refrigerators, deep freezers and air conditioners. The most

common, freon is known as dichlorodifluoro methane CF2 Cl2.

2. CaF2 is used as flux in metallurgy.

3. NaF is used as a preservative to prevent fermentation and also for preventing

dental cavities.

4. SF6 is used as an insulating material in high voltage equipment.

5. Teflon is used as container to store hydrofluoric acid.

6. UF6 is used in the separation of U235 from U238.

 

 

INTERHALOGEN COMPOUNDS OR INTERHALOGENS

Each halogen combines with another halogen to form several compounds

known as interhalogen compounds. The less electronegative element is written first. In naming also, the less electronegative element is mentioned first.

They are divided into four types.

AX  : CIF , BrF ,BrCl, ICl, IBr.

AX3: ClF3 BrF3 ICl3

AX5  : BrFIF5

AX7 : IF7

 

They can all be prepared by direct combination or by the action of a halogen on a lower interhalogen, the product formed depends on the conditions.

 

473K

Cl2+ F2 (equal volume) ----- --- >      2ClF (AX type)

I2 + Cl2 liquid (equi molar) 2ICl (AX type)

 

573K

Cl2 + 3F2 (excess) ---- --- >        2 ClF3 (AX3 type)

Br2 + 3F2 (diluted with nitrogen) 2Br F3 Br2 + 5F2 (excess) 2Br F5 (AX5  Type)

I2 + 5F2 (Excess) 2IF5 (AX5 Type)

 

IF5 + F2 (Excess) ------- 573K  ----- --- > IF7 (AX7 Type)

The bonds are essentially covalent because of the small electronegativity

difference, and the melting and boiling points increase as the difference in electronegativity increases.

The interhalogens are generally more reactive than the halogens (except F)

because the A-X bond is weaker than the X-X bond in the halogens. The reactions are similar to those of the halogens. Hydrolysis gives halide and oxyhalide ions, the oxyhalide ion being formed from the larger halogen present.

 

BrF5 + 3 OH-  --- --- >          5F + BrO3 + 3 H+

Bromate

 

ICl + OH-   --- - ----------- ---         >         Cl  + OI  + H+

hypoiodite

Structures of interhalogen compounds

Interhalogen compounds are generally covalent compounds in which the larger halogen forms the central atom.

1. Type AX. As excepted, the compounds of the type AX are linear. Thus CIF, BrF, BrCl, IBr and ICI are all linear in structure.

Electronic structure of Chlorine atom, in the ground state and hybridised state is represented as in Fig.


Although the spatial arrangement of the four electron pairs (bp = 1 and lps = 3) round the central chlorine atom is tetrahedral, due to the presence of three lone pairs of electrons in three hybrid orbitals, the shape of AX molecule gets distorted and become linear.

2. Type AX3 Compounds of the type AX3 have trigonal bipyramidal structure,

Fig.  for the ClF3 molecule.


Bipyramidal structure arises out of sp3hybridisation involved in the formation of this compound, as illustrated in the Fig.. The three dotted arrows indicate electrons contributed by the three fluorine atoms (without lone pair it is T-shaped).

3. Type AX5 (IF5, BrF5, etc.) These compounds are formed by sp3dhybridisation and hence have an octahedral structure, as shown in Fig. for  the formation of IF5 molecule (without lone pair it is square pyramidal).

4. Type AX7 (IF7). This compound has a pentagonal bipyramidal structure since this is formed by sp3d3hybridisation.

Problem

An element A occupies group number 17 and period number 2, shows

anomalous behaviour. A reacts with water forms a mixture of B, C and acid D. B and C are allotropes. A also reacts with hydrogen violently even in dark togive an acid D. Identify A,B,C and D. write the reactions.

Solution

i) The element A that occupies group number 17 and period number 2 is

fluorine.

ii) Fluorine reacts with water and forms a mixture of B and C

2F2 + 2H2O 4HF + O2

3F2 + 3H2O 6HF + O3

Therefore, B is Oxygen and C is Ozone.

iii)    Fluorine reacts with hydrogen to give D.

 

F2 + H2  2HF

D is Hydrofluoric acid.

 



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