The Halogens

Chapter 40

F

Cl

Br

I

At

Gp VII

Position in Periodic Table

General informationHalogen Atomic

radius (nm)Ionic radius (nm)

Melting Point (oC)

Boiling Point (oC)

Density (g cm-3)

Abundance on Earth (%)

Fluorine 0.072 0.133 -220 -188 1.11 0.062

Chlorine 0.099 0.181 -101 -34.7 1.56 0.013

Bromine 0.114 0.195 -7.2 58.8 3.12 0.0003

Iodine 0.133 0.216 114 184 4.93 0.00005

Astatine - - 302 380 - Trace

Characteristic properties

• Very reactive non-metals– High electronegativity– High electron affinity

• Bonding and Oxidation states

• Colour

Electronegativity

A measure of the attracting ability of bonding electrons.

Due to their large effective nuclear charge, halogens are the most electronegative element in the periodic table.

Element EN value

F 4.0

Cl 3.0

Br 2.8

I 2.5

Electron Affinity

Generally high electronaffinity. All are exothermic.

X(g) + e- X-(aq)

Element EA kJ/mol

F -348

Cl -364

Br -342

I -285

Bonding and Oxidation State

• ns2np5

• Ionic or covalent bond with oxidation state –1 or +1 (F shows –1 only)

• Except F, all other halogens can expand their octet by using the low-lying, vacant d-orbitals to form bonding. Their oxidation states range from –1 to +7.

Bonding and Oxidation State

ns

np

nd

(0, +1, -1 states)

Cl2, Cl-, OCl2

ns

np

nd

(+3 states)

HClO2, ClO2-

Bonding and Oxidation State

ns

np

nd

(+5 state)

HClO3, HBrO3, I2O5

ns

np

nd

(+7 state)

Cl2O7, HIO4

ColourElement Colour

In Standard State In Water In 1,1,1-trichloroethane

F Pale yellow Pale yellow Pale yellow

Cl Greenish yellow Pale yellow Yellow

Br Reddish brown Yellow Orange

I Violet black Brown Violet

Colour

White light

Iodine

Check point 40-1

E1

Iodine

E2

Fluorine

Variation in properties of Halogens

F Cl Br I At0

400

-300

(Temp. oC)

b.p.

m.p.

m.p./b.p. increase as Mr increasebecause Van der Waals’ Force increasewith Mr

Electronegativity

F Cl Br I

4

3

2

1

4.0

3.02.8

2.5

F form ionic compoundwith –1 oxidation state.

I shows highly positive oxidation states withmore electronegativeelement.e.g. KIO4

Electron affinity

F Cl Br I

280

320

340

EA decrease fromCl to I due to increasingatomic size which lowersthe nuclear attraction to the added electron.

F has exceptional lowerEA because its 2nd shellis already crowded with7 e-, the additional e-

will experienced a greatere-e repulsion than otherhalogens.

Variation in Chemical Properties of elements

All are good oxidizing agents. (F2>Cl2>Br2>I2)

½ X2(std.state) X-(aq)

X(g)

X-(g)

Hatom

H

HEA

Hhyd

H Hatom HEA Hhyd= + +

Oxidizing power

Element

F 79.1 -348 -506 -774.9

Cl 121 -364 -364 -607

Br 112 -342 -335 -565

I 107 -314 -293 -500

HHatom HEA Hhyd

Oxidizingstrength

Oxidizing properties

1. Oxidation of metalsa. F2 oxidizes all metals including Au and Ag.b. Cl2 oxidizes common metals such as Na and Cu.

2. Reactions with Fe2+(aq)2Fe2+ + Cl22Fe3+ + 2Cl-

2Fe2+ + Br22Fe3+ + 2Br-

2Fe3+ + 2I-2Fe2+ + I2

Standard electrode potential (V)

Cl2+e-

Cl-

1.36

Br2+e-

Br-

1.07

I2+e-I- 0.54

Fe3++e-

Fe2+

0.77

Oxidizing properties

3. Reaction with phosphorus 2P + 3X2 2PX3

2P + 5X2 2PX5

F2 and Cl2 form PX5 as they have high oxidizingStrength. They combine with P to exhibit its maximumoxidation state.

Br2 and I2 form PX3 only.

Reaction with water

Fluorine oxidizes water to form HF and O2

2F2 + 2H2O 2HF + O2

Chlorine undergoes disproportionation(self oxidationand reduction) to form HCl and HOCl.

Cl2 + H2O HCl + HOCl

Reaction with water

A mixture of Cl2(aq), HCl(aq) and HOCl(aq)is called chlorine water.

OCl- , chloric(I) ion

1. Strong oxidizing agent with bleaching property OCl- + dye (coloured) Cl- + dye (colourless)2. Unstable to heat and light 2OCl- O2 + 2Cl-

Reaction with water

Br2 is only slightly soluble in water.Br2 (aq) + H2O (l) HBr(aq) + HOBr(aq)

OBr-(aq) is unstable and with bleaching property:1. 2OBr- O2 + 2Br-

2. OBr- + dye(coloured) Br- + dye(colourless)

Reaction with water

I2 does not react with water and only veryslightly soluble in water.

I2 is more soluble in ethanol or in KI(aq). I2(aq) + I-(aq) I3

-(aq)

Reaction with alkalis

All halogens react with aqueous alkalis anddisproportionate in it. The products depend on temperature and concentration of the alkalis.

Reaction with alkalis

Cold dil.NaOH

2F2 + 2NaOH 2NaF + OF2 + H2O

Cl2 + 2NaOH NaCl + NaOCl + H2O

Br2 + 2NaOH NaBr + NaOBr + H2O3NaOBr 2NaBr + NaBrO3

(3Br2 + 6NaOH 5NaBr + NaBrO3 + 3H2O

I2 + 2NaOH NaI + NaOI + H2O3NaOI 2NaI+ NaIO3

(3I2 + 6NaOH 5NaI + NaIO3 + 3H2O

Reaction with alkalis

More concentrated NaOH

2F2 + 4NaOH 4NaF + O2 + 2H2O

Hot concentrated NaOH

Cl2 + 2NaOH NaCl + NaCl + H2O3NaOCl 2NaCl + NaClO3

(3Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O

Reaction with alkalis

The reverse reaction of I2 and KOH is usedto prepare standard iodine solution for iodometrictitration. Know amount of iodine is generated bydissolving known quantity of KIO3 in excess KIand H+.

5KI + KIO3 + 3H+ 3I2 + 6K+ + 3H2O

Halogen Displacement reactions

Oxidizing strength : F2 >Cl2 >Br2 >I2

Cl2 + 2Br- 2Cl- + Br2

Cl2 + 2I- 2Cl- + I2

Br2 + 2I- 2Br- + I2

Variation in properties of halides

Halide + c. H2SO4

NaCl + H2SO4 NaHSO4 + HCl2NaCl + H2SO4 Na2SO4 + HCl (500oC)

2NaBr + 2H2SO4 2HBr + Na2SO4

2HI + H2SO4 SO2 + Br2 + 2H2O

2NaI + 2H2SO4 2HI + Na2SO4

8HI + H2SO4 H2S + 4I2 + 4H2O

Relative reducing power: HCl < HBr < HI

Halide + c. H3PO4

3NaCl + H3PO4 Na3PO4 + 3HCl3NaBr + H3PO4 Na3PO4 + 3HBr3NaI + H3PO4 Na3PO4 + 3HI

H3PO4 is NOT a strong oxidizing agent.It reacts with halides to form HX.

Halides + Ag+(aq)

Ag+(aq) + Cl-(aq) AgCl(s) , white precipitate

Ag+(aq) + Br-(aq) AgBr(s) , pale yellow precipitate

Ag+(aq) + I-(aq) AgI(s) , yellow precipitate

Halides + Ag+(aq)

Ion Action of AgNO3=

Effect of Standing in

sunlight

Effect of adding excess aq. N

H3

Cl- White ppt. Turns grey White ppt. dissolves

Br- Pale yellow ppt. Turns yellowish grey

Pale yellow ppt. slightly dissol

ves

I- Yellow ppt. Remains yellow

Yellow ppt. does not dissolve

Relative acidity of HX

Hydrogen Halide Dissociation constant Degree of dissociation in 0.1M solution

HF 7x10-4 0.085

HCl 1x107 0.920

HBr 1x109 0.930

HI 1x1011 0.950

Relative Acidity of HX

HX(aq) H+(aq) + X-(aq)

HX(g)

H+(g) + X-(g)

H (g) + X (g)

H

H1

H5 H6

H3 H4H2

Relative Acidity of HX

HX

HF 48 566 1311 -333 -1091 -515 -14 -29 15

HCl 18 431 1311 -348 -1091 -381 -60 -13 -47

HBr 21 366 1311 -324 -1091 -347 -64 -4 -60

HI 23 299 1311 -295 -1091 -305 -58 4 -62

H1 H2 H3 H4 H5 H6 H TS G

Explanation of weak acid strength of HF:• HF has the greatest bond dissociation energy and exceptionally small electron affinity. It has the least exothermic H.2. Due to formation of strong hydrogen bond and greatest degree of hydration, HF has the smallest decrease of TS.

Anomalous behaviour of HF

According to La Chatelier’s Principle, the degreeof dissociation of a weak acid increases with dilution. HA H+ + X-

However, the acidity of conc. HF is greater thandilute HF. Why?

In conc. HF, HF molecules form dimers H2F2 by hydrogen bond. The acid strength of H2F2 isstronger than HF. H2F2 H+ + HF2

-

Uses of Halogens and Halides

• Teflon

• Na2SiF6 or NaF is used to fluoridate drinking water.

• Chlorine bleach, sterilization of water, PVC.• Anti-knocking agent (leaded petrol)• AgBr coating in film• Iodine tincture• AgI in coating film