Chemistry

Session objectives

General trends of group 17 elementsChemical properties of group 17 elementsOxoacids of group 17 elementsGeneral properties of group 18 elementsCompounds of xenon

Occurrence

2 3 4 3 62

2 2

2

Halogen Main sources

Fluorine Fluorite (CaF ), Fluorapatite Ca PO F , cryolite Na AlF

Chlorine Sea water, salt wells, salt beds (NaCl, KCl, MgCl , CaCl )

Bromine Sea water, sea lakes (NaBr, KBr, MgBr )

I

3odine Brine wells, sea weeds (I ) Chile salt petre NaIO

The halogens are very reactive, and donot occur in free state.All except At are found in combined form in the earth’s crust.Astatine is radioactive and has a short half life.

Flourine

Flourine is found naturally in many minerals, such as flourspar.

There is no element that can be used to oxidize F- to F2

Flourine is produced by electrolysing a molten mixture of KF and HF at a carbon anode.

electrolyse2 2 2HF K[HF ] H F

2 2 4 4CaF H SO CaSO 2HF

2KF HF K[HF ]

2

2

Cathode : 2H 2e H (g)

Anode : 2F F (g) + 2e

Difficulties in obtaining flourine

HF is corrosive, and etches glass forming SiF6- ions and also causes very

painful skin wounds. Wounds are slow to heal because F- ions remove Ca2+ ions from the tissues.

Gaseous HF is very toxic (3 ppm) compared with HCN (10 ppm).

HF is poor conductor of electricity. Thus a mixture of KF and HF is electrolysed to increase the conductivity.

Flourine oxidises water to dioxygen, so water should be excluded.

It catches fire with the traces of greases or with crystalline silicon.

Anode should be graphite free carbon otherwise flourine invade into the sheet of the graphite causes an increase in the distance between sheets. It reduces the conductivity of the graphite and loss of electricity as thermal effect. Eventually a explosion may occur.

Properties and uses of flourine

Fluorine is a pale yellow, extremely reactive, corrosive gas. It has a characteristic pungent odour.

It is used to produce the volatile UF6, used in processing nuclear fuel.

To make SF6; which is very inert gas ,used as dielectric for high voltage

equipment.

In making various of freons, which are chemically inert, not oxidised by air, hot nitric acid or by concentrated sulfuric acid.

Traces of flouride ions in drinking water (~1 ppm) greatly reduce the incidence of dental caries making an enamel on teeth.

F3 4 2 2 3 4 2 2[3Ca (PO ) .Ca(OH) ] [3Ca (PO ) .CaF ]

Chlorine

Prepared by electrolysis of molten salt or brine.

Reacts directly with nearly all elements (exceptions: C, N, O and the noble gases)

Oxidizes metals to high oxidation state. For example, iron

reacts vigorously in Cl2 to form FeCl3, not FeCl2.

Uses : Manufacturing of plastics, solvents, pesticides.

As bleach in paper and textile industries.

As disinfectant in water treatment

Bromine

Br2 is obtained from sea water and brine lakes. First H2SO4 is added to adjust the pH to about 3.5. Then Cl2 gas is passed through the solution to oxidize the Br– to Br2.

2 2Cl 2Br 2Cl Br

Br2 is removed by a stream of air, because Br2 is quite volatile. The gas is passed through a solution of Na2CO3, when Br2 is absorbed, forming a mixture of NaBr and NaBrO3. Finally, the solution is acidified and distilled to give pure bromine.

2 2 3 3 23Br 3Na CO 5NaBr NaBrO 3CO

3 2 4 3 2 45NaBr NaBrO 3H SO 5HBr HBrO 3Na SO

3 2 25HBr HBrO 3Br 3H O

Bromine is corrosive, red brown fuming liquid .

Bromine compounds are used as a fire retardant in textiles and as pesticides. AgBr is used in photographic emulsions.

ZnBr2(aq) has a very high density and is used to control the

escape of oil from wells.

Iodine

Can be obtained from seaweed

It is blue black lustrous solid; sublimes easily to purple vapour.

Uses (1) Added to iodised salt.

(2) dissolves in alcohol to form tincture of iodine, an antiseptic.

(3) serves as indicator in presence of starch

3 3 4NaIO (aq) + 3NaHSO (aq) NaI (aq) + 3NaHSO (aq)

3 2 4 2 2 2 4NaIO (aq) + 5NaI (aq) + 3H SO (aq) 3I (s) + 3H O ( ) + 3Na SO (aq)

From left, Iodine dissolved in CCl4 Iodine dissolved in water.Iodine dissolved in KI.With a little starch added.

Illustrative Problem

Explain why fluorine does not undergo disproportionation reaction but other halogens do.

Fluorine being the most electronegative element undergoes only reduction but not oxidation. As a result, it shows only –1 oxidation state while other halogens show both negative (–1) and positive (+1, +3, +5, +7) oxidation states. Thus, F does not show disproportionation reactions while other halogens do.

Solution :

Electron affinity

Electron affinity values are high in case of halogens. As we move down the group, the electron affinities decrease as the size of the halogen increases.

Electron affinity of fluorine is lower than that of chlorine. This is due to small size of fluorine atom. As a result of which strong electron-electron repulsions are present in the relatively compact 2p-orbitals of fluorine.Therefore, the electron affinities decrease in the order:

Cl > F > Br > I

Stability of X-X bond

The bond energy in the X2 molecules would decrease as the atoms become larger, since increased size results in less effective overlap of orbitals.

The F — F bond energy is smaller than that of Cl — Cl. Due to small size of fluorine atoms, there are high interelectronic repulsions between non-bonding electrons in 2p-orbitals of fluorine. As a result, F — F bond is weaker in comparison to Cl — Cl and Br — Br bonds.

Illustrative Problem

Explain why F2 is a stronger oxidizing agent than Cl2 while electron affinity of fluorine is less than that of chlorine.

Solution

F2 is a stronger oxidizing agent than Cl2 because F2 has higher oxidation potential than Cl2. Electron affinity is the energy released when a gaseous atom accepts an electron to form a gaseous anion while oxidation potential is the sum of energy changes taking place in the following steps:

21 1

X (g) X (g), H D [Dissociation energy]2 2

X (g) e X (g), H E A [Electron affinity]

X (g) aq. X (aq), H HE [Hydration Energy]

Some Important Inorganic Compounds of Fluorine

Hydrides : HX

HF is liquid

HCl, HBr and HI are gases.

Thermal stability : HF > HCl > HBr > HI

Reducing character: HF < HCl < HBr < HI

Acid strength : HF < HCl < HBr < HI

H — X bond strength : HF > HCl > HBr > HI

Chemical properties

Illustrative Problem

A greenish yellow gas reacts with an alkali metal hydroxide to form a halate which can be used in fire works & safety matches.what will be the gas & halate?

Solution:

The greenish yellow gas is Cl2 & halate used in fire works & safety matches is KClO3

2 3 23Cl 6KOH KClO 5KCl 3H O

Hypohalous acid, HOX

Halous acid, HXO2

Halic acid, HXO3

Acid strength: HClO3 < HBrO3 < HIO3

HClO4 > HClO3 > HClO2 > HOCl

Oxo acids

Oxoanions of Chlorine

Illustrative Example

When NaBr is heated with Conc. H2SO4, Br2 is produced but when NaCl is heated with Conc. H2SO4, HCl is produced. Why?

When NaBr is heated with Conc. H2SO4, HBr is first produced which being a reducing agent reduces H2SO4 to SO2 while itself gets oxidised to Br2.

Solution :

2 4 4NaBr H SO NaHSO HBr

2 4 2 2 22HBr H SO 2H O SO Br

As a result, only Br2 is produced. Similarly, NaCl reacts with Conc. H2SO4 to form HCl but since HCl does not act as a reducing agent it does not get oxidised to Cl2.

2 4 4NaCl H SO NaHSO HCl

2 4HCl H SO No action

As a result, only HCl is evolved.

Pseudo halogens

Pseudohalides Pseudohalogens

CN–   (CN)2 cyanogen

SCN–   (SCN)2 thiocynogen

OCN–   (OCN)2 oxy cyanogens

Illustrative example

Iodine is liberated when KI is added to solution of Cu2+ ions but Cl2 is not liberated when KCl is added to a solution of Cu2+ ions. Why?

The I– ion, being a strong reducing agent, reduces Cu2+ to Cu+ and itself gets oxidised to I2.

Solution :

The Cl– ion is a week reducing agent.Thus, it does not reduce Cu2+ ion.

22 2 22Cu 4KI Cu I I 4K

Group 18 elements

Ionization energies are high but decrease down the group.

The research leading to the first noble-gas compound was prompted by the observation that xenon lost an electron as easily as oxygen.

Illustrative example

Liquids flow from a higher to a lower level .Give the name of the liquid that can climb up the walls of the glass vessel in which it is placed.

HeII is liquid form of helium which is obtained on cooling gaseous He to2.2K at 1 atm .It is able to flowuphill like a gas.

Solution:

Xenon and Fluorine

All three binary Xe-F compounds are crystalline solidsAll are molecular in gas phase.Solid XeF6 is ionic, being composed of XeF5

+ cations and F- anions.The binary Xe-F compound are powerful flourinating agents.

Xe

F

F

(XeF )Linear

2

XeF F

(XeF )Square planar

4

F FXeF

(XeF )Distorted O ctahedra l

6

FF

F

FF

(XeO )Pyramidal

3

Xe

O O

O

Xe

F

F

(XeO F )Trigonal bipyram idal

22

O

(XeOF )Square pyram idal

4

XeF F

F F

O

(XeOF )T-shaped

2

Xe

F

F

O

Compounds of xenon

Illustrative example

Explain the following:

(i) Why do helium and neon not form compounds with fluorine?

(ii) Neon is generally used for warning signal illumination.

Solution

(a) He and Ne do not contain d-orbitals in their valence shell and hence their electrons cannot be promoted to higher energy levels like that in Xe to form bonds.

He and Ne do not form compounds with fluorine.

(b) Neon lights are visible from long distances. The lights are even visible during fog and mist conditions.

Illustrative example

Which mixture of gases is used by sea divers to go deep in the sea water ?

Solution:

Mixture of O2 &He (1:4) is used by deep sea divers for repiration as He does not dissolve in blood at high deep sea pressure

Illustrative example

Which gas is filled in electric bulbs/tubes?

Argon is filled in electric bulbs/tubes.

Solution :

Thank you