1 Characteristic Properties of the Halogens 2 Group VIIA elements include fluorine chlorine ...

Characteristic Characteristic Properties of Properties of the Halogensthe Halogens

• Group VIIA elements include

fluorine

chlorine

bromine

iodine

astatine

IntroductionIntroduction

halogens

(Salt producers)

• Astatine

chemistry not much known

radioactive

the total amount present in the Earth's crust is probably

less than 30 g at any one time.

• Halogens are p-block elements

outermost shell electronic configuration of ns2np5

one electron short of the octet structure

• Halogens are p-block elements

• In the free elemental state

they complete their octets by sharing their single unpaired p-electrons

they either

gain an additional electron to form halide ions

When halogens react with other elements

share their single unpaired p-electrons to form

single covalent bonds

highest among the elements in the same period

have a high tendency to attract electrons

strong oxidizing agents

High Electronegativity / Electron AffinityHigh Electronegativity / Electron Affinity

-1 is the most common oxidation state of halogens in their compounds

Ionic : NaF, NaCl, NaBr, NaI

Covalent : HF, HCl, HBr, HI

High Electronegativity / Electron AffinityHigh Electronegativity / Electron Affinity

Variable Oxidation StateVariable Oxidation State

All halogens (except fluorine) can expand their octet of electrons by utilizing the

vacant,

energetically low-lying

d-orbitals.

“ Electrons-in-boxes” diagrams of the electronic configuration of a halogen atom of the ground state and various

excited states

The half-filled orbital(s) overlap(s) with those of more electronegative atoms (e.g. O)

positive oxidation state (+1, +3, +5, +7)

Oxidation state

of halogenIon / Compound

F– Cl– Br– I–

HF HCl HBr HI

0 F2 Cl2 Br2 I2

Cl2O Br2O

HOCl HOBr

OCl– OBr–

+3HClO2

ClO2–

Various oxidation states of halogens in their ions or compounds

Oxidation state

of halogenIon / Compound

+4 ClO2 BrO2

+5HClO3 HBrO3 I2O5

ClO3– BrO3

– HIO3

IO3–

+6 Cl2O6 BrO3

Cl2O7 H5IO6

HClO4 HIO4

ClO4– IO4

Various oxidation states of halogens in their ions or compounds

• Fluorine (1)

the most electronegative

element

only one unpaired p electron

available for bonding

oxidation state is limited to –1

• Fluorine (1)

cannot expand its octet

no low-lying empty d orbitals

available

the energy required to promote

electrons into the third

quantum shell is very

Absence of HFO, HFO2,

HFO3, HFO4

Variation in Physical PropertiesVariation in Physical Properties

1. Melting point / boiling point down the group

HalogenMelting point

Boiling point

Fluorine

Chlorine

Bromine

Iodine

Astatine

–220

–101

–7.2

–188

–34.7

Variations in melting point and boiling point of the

halogens

Variation in Physical PropertiesVariation in Physical Properties

1. Melting point / boiling point down the group

The molecular size down the group

The electron cloud is more easily polarized

Induced dipoles are formed more easily

Stronger London dispersion forces

2. Colour becomes darker down the group

Haloge

nF2 Cl2 Br2 I2

ColourPale

yellow

Greenis

yellow

Reddish

Violet

Appearances of halogens at room temperature and pressure: chlorine

chlorine

Appearances of halogens at room temperature and pressure: bromine

bromine

Appearances of halogens at room temperature and pressure: iodine

iodine

Colour Colour

• All halogens

coloured

the absorption of radiation in the visible light region of the electromagnetic spectrum

The colour is due to the unabsorbed radiation in the visible light region

Colour Colour

• Fluorine atom

has the smallest size

absorbs the radiation of relatively high frequency (i.e. blue light)

appears yellow (the unabsorbed radiation)

Colour Colour

• Atoms of other halogens

larger sizes

absorb radiation of lower frequency

Colour Colour

• Iodine

absorbs the radiation of relatively low frequency (i.e. yellow light)

appears violet

Q.1The colour of astatine is black.

Colour Colour

• Halogens

different colours when dissolved in different solvents

HalogenColour

in pure form in water in 1,1,1-trichloroethane

F2 Pale yellow Pale yellow Pale yellow

Cl2 Greenish

yellowPale yellow Yellow

Br2 Reddish

brown Yellow Orange

I2 Violet black

Yellow (only

slightly soluble)

Brown in

KI(aq)

Violet

Colours of halogens in pure form and in solutions

Colour Colour

• Halogens

non-polar molecules

not very soluble in polar solvents (such as water)

but very soluble in organic solvents (such as 1,1,1-trichloroethane)

Colours of halogens in water:(a) chlorine; (b) bromine; (c) iodine

(a) (b) (c)

Colours of halogens in 1,1,1-trichloroethane:(a) chlorine; (b) bromine; (c) iodine

(a) (b) (c)

3. Electron Affinity

down the group

Halogen F Cl Br I At

kJ/mol1-322 -349 -335 -295 -270

The number of electron shells and size of atoms down the group

The nuclear attraction for the additional electron down the group

Electron affinity from Cl to I

Atoms of fluorine have the smallest size among the halogens

The addition of an extra electron to the small quantum shell(n=2) results in great repulsion among the electrons.

Fluorine has a lower electron affinity than Cl and Br.

Halogen F Cl Br I At

Electronegativit

y4.0 3.0 2.8 2.5 2.2

4. Electronegativity

down the group

The number of electron shells and size of atoms down the group

The nuclear attraction for the bonding electrons down the group

Electronegativity down the group

Fluorine has the highest electronegativity because it is the most reactive elements.

The electronegativity of fluorine is arbitrarily assigned as 4.0.

Variation in Chemical Variation in Chemical PropertiesProperties

Reactivity : F2 > Cl2 > Br2 > I2

React by gaining electrons

Oxidizing power : F2 > Cl2 > Br2 > I2

1. Reactions with Sodium

• All halogens

combine directly with sodium to form sodium halides

the reactivity decreases down the group from fluorine to iodine

• Fluorine

react explosively to form sodium fluoride

2Na(s) + F2(g) 2NaF(s)

• Chlorine

reacts violently to form sodium chloride

2Na(s) + Cl2(g) 2NaCl(s)

• Bromine

burns steadily in bromine vapour to form sodium bromide

2Na(s) + Br2(g) 2NaBr(s)

• Iodine

burns steadily in iodine vapour to form sodium iodide

2Na(s) + I2(g) 2NaI(s)

Na(s) + X221

NaX(s)ofH

Na+(g) + X2(g)21

oatmHovapH

Na+(g) + X(g)

B.E. Na+(g) + X(g)E.A.

olatticeH

Vigor of reaction depends on

1.The activation energy (endothermic)

2.The lattice energy (exothermic) Activation energy

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

F has an exceptionally low B.E. & zero o

F is the most reactive

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

The lattice enthalpy of NaF is most negative

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

Cl has zero ovapH

Cl is more reactive than Br & I

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

Lattice enthalpy :

NaCl > NaBr > NaI

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

(s)/(l)

Br is more reactive than I

ovapH : Br2(l) < I2(s)

Na(s) + X221

NaX(s)ofH

Na+(g) + X(g)

Na+(g) + X2(g)21

oatmHovapH

olatticeH

Lattice enthalpy :

NaBr > NaI

Q.2(a)

Variation: bond enthalpy decreases from Cl2 to I2Reason : The size of atoms and thus the bond

length between atoms increases down the group.

The shared electron pair is getting further away from the bonding

nuclei. weaker bond and lower B.E.

F2 has an exceptionally small B.E. because the F atoms are so small that the repulsive forces between lone pairs on adjacent bonding atoms become significant.

Q.2(b)

The lattice enthalpy becomes less negative down the group.

It is because the anionic radius, r- , increases down the group.

rrHolattice

F2 reacts explosively even in the dark at 200C

Cl2 reacts explosively in sunlight

Br2 reacts moderately on heating with a catalyst

I2 reacts slowly and reversibly even on heating

2.1 Reactions with hydrogen

X2 + H2(g) 2HX(g)

Q.3 Explain the extreme reactivity of fluorine in terms of the bond enthalpies of F–F and H–F bonds.

Fluorine has an exceptionally small F-F bond enthalpy.

Thus, the activation energy of its reaction with hydrogen is also exceptionally small.

Hydrogen fluoride has the highest bond enthalpy among the hydrogen halides.

Thus, the formation of HF from H2 and F2 is the most exothermic.

The energy released from the reaction further speeds up the reaction.

F2 + H2(g) 2HF(g)

Chlorine removes hydrogen completely from turpentine(C10H16)

C10H16(l) + 8Cl2(g) 10C(s) + 16HCl(g)

Q.4The cotton wool bursts into flames and the gas jar is filled with dark smoke (of carbon) and white fumes (of HCl)

HCl gives dense white fumes with ammonia.

2.2 Reactions with phosphorus

F2 + P PF5 Cl2 + P PCl3 + PCl5Br2 + P PBr3

I2 + P PI3

F2 is the strongest oxidizing agent, it always oxidizes other elements to their highest possible oxidation states.

2.2 Reactions with phosphorus

F2 + P PF5 Cl2 + P PCl3 + PCl5Br2 + P PBr3

I2 + P PI3

Br2 and I2 are NOT strong enough to oxidize P to its highest possible oxidation state.

2.3 Reactions with xenon

Fluorine reacts directly with all non-metals except nitrogen, helium, neon and argon. It will even react with diamond and xenon on heating.

C(diamond) + 2F2 CF4

Xe + F2 XeF2

Xe + 2F2 XeF4

Xe + 3F2 XeF6

It is because(a) Xenon can expand its octet by utilizing vacant, low-lying d-orbitals.

By VB Theory,

To form two Xe-F bonds in XeF2, a 5p electron in Xe has to be promoted to a 5d orbital.

By VB Theory,

To form four Xe-F bonds in XeF4, two 5p electrons in Xe have to be promoted to two 5d orbitals.

By VB Theory,

To form six Xe-F bonds in XeF6, three 5p electrons in Xe have to be promoted to three 5d orbitals.

The gap between np and nd sub-shells down the group, thus,

Tendency to form bonds down the group : -

Xe > Kr > Ar > Ne > He

the promotion of electrons from np sub-shell to nd sub-shell becomes easier down the group.

Also, the energy released by forming more single bonds outweighs the energy required for promoting 5p electrons to 5d orbitals.

3 Reactions with other reducing agents

I2 is the weakest oxidizing agents among the halogens.

3.1 All halogens(except I2) oxidize Fe2+ to Fe3+

Half reactionStandard electrode

potential (V)

Cl2(aq) + 2e– 2Cl–(aq)

Br2(aq) + 2e – 2Br–

Fe3+(aq) + e– Fe2+(aq)

I2(aq) + 2e– 2I–(aq)

X2(aq) + 2Fe2+(aq) 2X(aq) + 2Fe3+

(aq)( X = F, Cl, Br) 0Eo

3.1 All halogens(except I2) oxidize Fe2+ to Fe3+

Half reactionStandard electrode

potential (V)

Cl2(aq) + 2e– 2Cl–(aq)

Br2(aq) + 2e – 2Br–

Fe3+(aq) + e– Fe2+(aq)

I2(aq) + 2e– 2I–(aq)

0Eocell I2(aq) + 2Fe2+(aq) No reaction

3.2 All halogens(except I2) oxidize S2O32

to SO42

4X2(aq) + S2O32(aq) + 5H2O(l) 8X(aq) + 10H+(aq) +

2SO42(aq)

(X = F, Cl, Br)

I2(aq) + 2S2O32(aq) 2I(aq) + S4O6

+5 +60

Used in iodometric titration

(i) 2I(aq) + 2Fe3+(aq) I2(aq) + 2Fe2+(aq) (excess) (unknown)

Determination of [Fe3+(aq)] by iodometric titration

Using starch as indicator

(ii) I2(aq) + 2S2O32(aq) 2I(aq) + S4O6

2(aq) (standard solution)

1:1n:n 232

3 OSFe

4 Displacement reactionsCl2(aq) + 2Br(aq) 2Cl(aq) +

Br2(aq)Cl2(aq) + 2I(aq) 2Cl(aq) + I2(aq)

Br2(aq) + 2I(aq) 2Br(aq) + I2(aq)

I2(aq) + I(aq) I3(aq)

(yellow) (brown)

More reactive

Less reactive

4 Displacement reactions

Cl2(aq) + 2I(aq) 2Cl(aq) + I2(aq)

Br2(aq) + 2I(aq) 2Br(aq) + I2(aq)

I2(aq) + I(aq) I3(aq)

(yellow) (brown)

What would be observed if an excess of Cl2(aq) or Br2(aq) is added to I(aq)? The solution turns cloudy and a black solid settles at the bottom

Aqueous

solution

Halogen added

F2 Cl2 Br2 I2

F– No reaction No reactionNo

reaction

A pale

yellow

solution is

formed (Cl2

is formed)

No reaction

reaction

Reactions of halide ions with halogens

Aqueous

solution

Halogen added

F2 Cl2 Br2 I2

A yellow

solution

is formed

(Br2 is

formed)

A yellow

solution

is formed

(Br2 is formed)

No reactionNo

reaction

A yellowish

solution is

formed

(I3 is formed)

A yellowish

solution is

formed

(I3 is formed)

A yellowish

solution is

formed (I3

is formed)

reaction

Reactions of halide ions with halogens

Q.5Shake hexane or 1,1,1-trichloroethane with the two solutions respectively.

The one that turns the organic layer violet is I3

The one that turns the organic layer orange or brown is Br2(aq).

1,1,1-trichloroethane

Br2 I2

Br2(aq) I3(aq)

If hexane is used, the upper layer will be the organic layer

Disproportionation is a chemical change in which oxidation and reduction of the same species (which may be a molecule, atom or ion) take place at the same time.

5. Disproportionation

A. Reactions with Water

HOCl : chloric(I) acid or hypochlorous acid

Chlorine water

a mixture of hydrochloric acid and chloric(I) acid

• Chlorate(I) ion, OCl is also known as hypochlorite ion

unstable

decomposes when exposed to sunlight or high

temperatures to give chloride ions and oxygen

2OCl–(aq) 2Cl–(aq) + O2(g)

• Chlorate(I) ion

bleaches by oxidation

Cl2(aq) + H2O(l)2H+(aq) + Cl–(aq) +

OCl–(aq)OCl–(aq) + dye Cl–(aq) + (dye + O)

coloured colourless

• Bromine

only slightly soluble in water

mainly exists as molecules in saturated bromine

• When the solution is diluted

hydrolysis takes place

hydrobromic acid and bromic(I) acid (hydrobromous acid) are formed

Br2(l) + H2O(l) HBr(aq) +

HOBr(aq)

• Bromate(I) ion, OBr

also unstable

bleaches dyes by oxidation

OBr–(aq) + dye coloured

Br–(aq) + (dye + O)

colourless

• Iodine

does not react with water

only slightly soluble in water

• Fluorine

reacts vigorously with water to form hydrogen fluoride and oxygen

Being the strongest oxidizing agent, F2 undergoes reduction rather than disproportionation with water.

2F2(g) + 2H2O(l) 4HF(aq) + O2(g)0 1

Chlorine reacts similarly at high temperature or when exposed to light

2Cl2(aq) + 2H2O(l) 2HCl(aq) + 2HOCl(aq)2HOCl(aq) 2HCl(aq) + O2(g)

Heat or light

Overall :2Cl2(aq) + 2H2O(l) 4HCl(aq) + O2(g)

Heat or light

• All halogens react with aqueous alkalis

• All halogens (except F2) undergoes disproportionation with alkalis• In general,

Reactivity decreases down the group

B. Reactions with Alkalis

The products formed depend on

1. Temperature

2. The type of halogen reacted

3. The concentration of alkali used

Effect of temperature

(a)At lower temperatures,

X2 Cl2 Br2 I2

T1 / C 20 0 <0

X2(aq) + 2OH(aq) XO(aq) + X(aq) + H2O(l)

T10 +1 1

Effect of temperature

(a)At higher temperatures,

3XO(aq) XO3(aq) + 2X(aq)

XO ClO BrO IO

T2 / C 70 20 0

+1 +5 1

(2) 3XO(aq) XO3(aq) + 2X(aq)

(1) X2(aq) + 2OH(aq) XO(aq) + X(aq) + H2O(l)

Overall reaction : 3(1) + (2)3X2(aq) + 6OH(aq) XO3

(aq) + 5X(aq) + 3H2O(l)

3XO(aq) XO3(aq) + 2X(aq)

XO ClO BrO IO

T2 / C 70 20 <0

On moving down the group,

1. stability of XO decreases ClO > BrO > IO

2. stability of XO3 increases ClO3

< BrO3 <

3X2(aq) + 6OH(aq) XO3(aq) + 5X(aq)

+ 3H2O(l)At lower pH (when acid is added),

the equilibrium position shifts to the left and the reversed process predominates.

XO3(aq) + 5X(aq) + 6H+(aq) 3X2(aq) +

3H2O(l)This reaction (when X=I) is often used to prepare standard iodine solution for iodometric titrations

• Dissolving a known quantity of KIO3(s) in excess KI(aq) and dilute H2SO4 generates a known amount of I2(aq)

KIO3(aq) + 5KI(aq) + 6H+(aq) 3I2(aq) + 3H2O(l) +

6K+(aq)

• The iodine produced can be used to standardize thiosulphate solution

3I2(aq) + 6S2O32(aq)

6I(aq) + 3S4O62(aq)

• This known amount of iodine generated can also be used to oxidize reducing agents (of unknown concentrations) such as SO3

2(aq) and ascorbic acid (vitamin C)• The excess iodine can be determined by back titration with sodium thiosulphate solution

I2(aq) + 2S2O32–(aq)

2I–(aq) + S4O62–

Effect of concentration of alkali

(a) At higher concentrations, XO3(aq)

is the major product.

(b) At lower concentrations, XO(aq) is the major product.

In general,

Halogens react with cold, dilute alkali to give halate(I) ions, halide ions and water

X2(aq) + 2OH XO(aq) + X(aq) + H2O(l)

Halogens react with hot, concentrated alkali to give halate(V) ions, halide ions and water.

3X2(aq) + 6OH XO3(aq) + 5X(aq) +

3H2O(l)

2F2 + 2OH(aq) OF2(aq) + 2F(aq) + H2O(l)very

dilute

2F2 + 4OH(aq) O2(aq) + 4F(aq) + 2H2O(l)concentrat

Being the strongest oxidizing agent, F2 undergoes reduction rather than disproportionation with alkalis.

Variation in chemical properties of Variation in chemical properties of halideshalides

A Comparative studyA Comparative study

1. Reactions with conc. sulphuric acid

2. Reactions with conc. phosphoric acid

3. Reactions with silver ion

• Concentrated sulphuric acid

non-volatile (b.p. ~330C)

oxidizing

Reactions with ConcentratedReactions with ConcentratedSulphuric(VI) AcidSulphuric(VI) Acid

KF(s) + H2SO4(l) KHSO4(s) + HF(g)

KCl(s) + H2SO4(l) KHSO4(s) + HCl(g)

warmwarm

non-volatile

volatile

Warming is required to speed up the reaction and to drive out the volatile acids

Fluoride and chloride : -

warmwarm

acid salt

Acid salt rather than normal salt is formed because HSO4

is a relatively weak acid

A convenient way to prepare HCl in the laboratory

warmwarm

Observation : -

White fumes are produced

Confirmatory test : -

Dense white fumes appear with NH3(aq)

Bromide: -

oxidation

reduction

KBr(s) + H2SO4(l) KHSO4(s) + HBr(g)warm

2HBr(g) + H2SO4(l) SO2(g) + Br2(g) + 2H2O(l)

(1) KBr(s) + H2SO4(l) KHSO4(s) + HBr(g)

(2) 2HBr(g) + H2SO4(l) SO2(g) + Br2(g) + 2H2O(l)

Bromide: -

Overall reaction : 2(1) + (2)

Not suitable for preparing HBr

2KBr(s) + 3H2SO4(l)

2KHSO4(s) + SO2(g) + Br2(g) + 2H2O(l)warm

• A brown gas

is evolved on

warming

• A pungent

smell is

detected

• A brown colour is

observed when

adding hexane

• It turns orange

dichromate solution

Confirmatory Test

• Dense white fumes

are formed with

aqueous ammonia

HBr• White fumes

are formed

ctObservation

2KBr(s) + 3H2SO4(l) 2KHSO4(s) + SO2(g) + Br2(g) + 2H2O(l)

iodide: -

KI(s) + H2SO4(l) KHSO4(s) + HI(g)warm

2HI(g) + H2SO4(l) SO2(g) + I2(g) + 2H2O(l)-1 0+6 +4

8HI(g) + H2SO4(l) H2S(g) + 4I2(g) + 2H2O(l)-1 0+6 -2

HI is strong enough to reduce sulphur to its lowest possible oxidation state

KI(s) + H2SO4(l) KHSO4(s) + HI(g) (1)warm

2HI(g) + H2SO4(l) SO2(g) + I2(s) + 2H2O(l) (2)

8HI(g) + H2SO4(l) H2S(g) + 4I2(s) + 2H2O(l) (3)

Overall reaction = 10(1) + (2) + (3)10KI(s) + 12H2SO4(l)

10KHSO4(s) + SO2(g) + H2S(g) + 5I2(s) + 4H2O(l) No suitable for preparing HI

10KI(s) + 12H2SO4(l)

10KHSO4(s) + SO2(g) + H2S(s) + 5I2(s) + 4H2O(l) Observation : -

A bad egg smell is detected

It turns lead(II) ethanoate paper black

(CH3COO)2Pb + H2S PbS(s) + 2CH3COOH

10KI(s) + 12H2SO4(l)

10KHSO4(s) + SO2(g) + H2S(s) + 5I2(s) + 4H2O(l) Observation : -

A violet colour is observed when added to hexane

Violet fumes are formed and

condense when cooled to give a black solid

Conclusion : -

Reducing power : HI > HBr > HCl > HF

Increases down the group

Interpretation:-

Consider the reaction,

2H–X + H2SO4 X–X + SO2 + 2H2O

The feasibility of the reaction depends on

1. the strength of H–X bond to be broken

the stronger the bond, the less feasible is the rx

2. the strength of X–X bond to be formed

the stronger the bond, the more feasible is the rx

2H–X + H2SO4 X–X + SO2 + 2H2O

The feasibility of the reaction depends on

1. the strength of H–X bond

the stronger the bond, the less feasible is the rx

2. the strength of X–X bond

the stronger the bond, the more feasible is the rxThe reaction with HF is least feasible because

1. H-F bond is the strongest

2. F-F bond is exceptionally weak due to repulsion between lone pairs of bonding atoms.

H-X B.E.(kJ mol1) X-X B.E. (kJ mol1

H-Cl 432 Cl-Cl 244

H-Br 366 Br-Br 192

H-I 298 I-I 152

both H–X bonds and X–X bonds become weaker

2H–X + H2SO4 X–X + SO2 + 2H2O

The strength of H-X bond is more important

Since two H-X bonds have to be broken for each X-X bond formed.

Reactivity : H-Cl < H-Br < H-I

Reactions with Phosphoric Acid Reactions with Phosphoric Acid

non-volatile

volatile

H3PO4(l) + HX(g) no reactionless

oxidizingSuitable for preparing HX from solid halids

NaCl(s) + H3PO4(l) NaH2PO4(s) + HCl(g)

NaBr(s) + H3PO4(l) NaH2PO4(s) + HBr(g)

NaI(s) + H3PO4(l) NaH2PO4(s) + HI(g)

e ionObservation

Confirmatory test

the product

Cl– White fumes are

formed on

warming

HCl Dense white fumes

are formed with

aqueous ammoniaBr– HBr

I– HI

NaCl(s) + H3PO4(l) NaH2PO4(s) + HCl(g)

NaBr(s) + H3PO4(l) NaH2PO4(s) + HBr(g)

NaI(s) + H3PO4(l) NaH2PO4(s) + HI(g)

Reactions with Silver Ions Reactions with Silver Ions

• Aqueous solutions of chlorides, bromides and iodides

give precipitates when reactingwith acidified silver nitrate

solution

Ag+(aq) + Cl–(aq) AgCl(s) white ppt

Ag+(aq) + Br–(aq) AgBr(s) pale yellow ppt

Ag+(aq) + I–(aq) AgI(s) yellow ppt

AgI(s)AgBr(s)AgCl(s)

Colour intensity down the group

Silver nitrate solution should be acidified with nitric acid

(a) to remove interfering ions like

SO32 or CO3

They may form white ppt with Ag+

2H+(aq) + SO32–(aq) SO2(g) + H2O(l)

2H+(aq) + CO32–(aq) CO2(g) +

H2O(l)

Silver nitrate solution should be acidified with nitric acid

(b)to avoid the formation of black ppt of Ag2O in alkaline solution.

2Ag+(aq) + 2OH(aq) Ag2O(s) + H2O(l)

The solubility(in water) of AgX down the group

AgF >> AgCl > AgBr > AgI

Ksp/mol2 dm6 1.61010 7.71013 1.51016

soluble insoluble

the size of the halide anions The electron cloud of the anions becomes more easily polarized by Ag+ The halides become more covalent and

less ionic

The halides become less soluble in polar solvents like water

The reaction can be used as a test to show the presence of halide ions.

Different halides give ppt with different colours.

Sometimes ambiguous.

Confirmatory tests are needed.

Two confirmatory tests for halides

1.Adding NH3(aq) to the AgX ppt

2.Exposing AgX ppt to sunlight

AgX(s) dissolve in NH3(aq) due to the formation of soluble complex ions.

AgCl(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Cl(aq)

AgBr(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Br(aq)

AgI(s) + 2NH3(aq) No reactionSolubility in NH3(aq) down the group

• When exposed to sunlight

light2AgCl(s) 2Ag(s) + Cl2(g)

2AgBr(s) 2Ag(s) + Br2(l)light

2AgI(s) No reactionlight

silver bromide turns yellowish grey

silver iodide remains yellow

silver chloride turns grey

Action of

acidified

solution on

halides

Confirmatory test of the

product

Effect of

adding

aqueous

ammonia

Effect of

exposure

to sunlight

Cl–A white ppt is

formed

The white ppt

dissolves

The solution

turns grey

A pale yellow

ppt is formed

The pale yellow

ppt slightly

dissolves

The solution

yellowish grey

I–A yellow ppt is

formed

The yellow ppt

does not

dissolve

The solution

remains yellow

Action of acidified silver nitrate solution on halides

Anomalous Behaviour of Anomalous Behaviour of Hydrogen FluorideHydrogen Fluoride

1. Hydrogen fluoride has abnormally high boiling point and melting point amongthe hydrogen halides HX HF HCl HBr HI

b.p./C 19.5 85 66.4 35

Formation of the extensive intermolecular hydrogen bonds among hydrogen fluoride

molecules

• Molecules of all other hydrogen halides

held together by weak van der Waal’s forces only

• The acid strength of hydrogen halides decreases in the order:

HI > HBr > HCl >> HF

2.2. Acidic Properties of Hydrogen Acidic Properties of Hydrogen HalidesHalides

Hydrogen

halide

dissociation

constant,

Ka (mol dm–3)

Degree of

dissociation in 0.1

M solution (%)

strength

5.6 × 10–4

1 × 107

1 × 109

1 × 1011

Strong

Very strong

Acid dissociation constants of hydrogen halides and their degrees of dissociation in 0.1 M solutions

In dilute (e.g. 0.1M) solution,

HF is the weakest acid among all the hydrohalic acids

HF(l) + H2O(l) H3O+(aq) + F–(aq)

Ka = 5.6 × 10–4 mol dm–3

H + F O H

H-bond

or H3O + F H3OF

Very stable ion pair

Freedom of F & H3O+ greatly (a drop in entropy of the system) due to H-bond formation

Effective concentration of F & H3O+ greatly

Thus, Ka & pH

In concentrated solution,

HF is the strongest acid among all the hydrohalic acids

H FF O H

Strength of H-bond:-

2. HF is in excess in concentrated solution F ions combine with excess HF rather than with H3O+

free H3O+ & pH

excess

H3OF(aq) + HF(aq) H3O+(aq) + HF2

For other HX acids,

acidity as concentration

It is due to the significant interaction between X and H3O+ at high concentrations

the effective concentration of H3O+

For HF, interaction between F and H3O+ is significant even at low concentrations due to the smaller size of F.

3. Pure, anhydrous liquid HF is ionic due to the formation of HF2

and H2F+ ions

Self ionization : -

2HF(l) H2F+(l) + F(l)

HF(l) + F(l) HF2(l)

Overall : -

3HF(l) [H2F]+[HF2](l)

Stabilized by resonance

Two identical H – F bonds

F H F FHF

• Heating the solid potassium hydrogen difluoride

reverses the reaction

a convenient way to obtain anhydrous hydrogen

fluoride

KF(s) + HF(l) KHF2(s)heat

Uses of fluorine and its compoundsSodium hexafluorosilicate, Na2SiF6, is used in water fluoridation.

F, being isoelectronic to OH, can replace the OH in the tooth enamel, making it less soluble in acidic solutions.

Uses of fluorine and its compoundsMolten cryolite, Na3AlF6

Lowers the temperature (2517C 1000 C) needed for extracting Al from Al2O3 by electrolysis.

Uses of fluorine and its compoundsConvert U to UF6

Separate 235UF6 from 238UF6 by diffusion for use in nuclear reactors.

The heavier 238UF6 diffuses a bit slower, making the separation possible.

Uses of fluorine and its compoundsConc. HF(aq) is used in etching glass

(e.g. making scales/graduation marks on glassware)

CaSiO3(s) + 6HF(aq)

CaF2(aq) + SiF4(aq) + 3H2O(l)(Glass)

Uses of fluorine and its compounds• The glass object to be etched

coated with wax or a similar acid-proof material

cutting through the wax layer to expose the glass

apply hydrofluoric acid

Uses of fluorine and its compounds

A glass is etched by hydrofluoric acid

Uses of fluorine and its compoundsMaking fluorocarbon compounds

Used as refrigerants, aerosol propellants, anaesthetics and

fire-fighting agents(BTM, BCF)

PTFE (teflon) used in electrical insulation, coating on surface of non-stick saucepans, etc.

Uses of fluorine and its compoundsHydrazine/fluorine mixtures are excellent rocket fuels

N2H4(g) + 2F2(g) N2(g) + 4HF(g)

H = -1166 kJ mol1 (extremely exothermic)Due to the strong NN and H-F bonds

Uses of fluorine and its compoundsExtraction of fluorine

Electrolyte : KF(s) dissolved in pure HF(l)

Anode : graphite

Cathode : steel

Q.8(a)

Anode : 2HF2 2HF + F2 + 2e

Cathode : 2H2F+ + 2e 2HF + H2

Overall :

2HF2 + 2H2F+ 4HF + F2 + H2

8.(b) Overall :

2HF2 + 2H2F+ 4HF + F2 + H2

6HF 4HF + F2 + H2

2HF F2 + H2

KF is added to increase the conductivity of the electrolyte.

KHF2 > HF or [H2F][HF2]

Q.8(c)

OH- (from H2O) rather than HF2 is

oxidized at the anode

2F2(g) + 2H2O(l) 4HF(aq) + O2(g)

vigorous reaction

Also, F2 reacts vigorously with water.

At high temperatures, fluorine produced can react vigorously with the electrodes, air, etc.

Uses of Chlorine and its compounds

Polyvinyl chloride, PVC

making electrical insulation, bottles, floor tiles, table cloth, shower curtain, etc.

CH2=CH2 + Cl2 CH2Cl – CH2Cl

CH2Cl – CH2Cl CH2=CHCl + HClheat

n(CH2=CHCl)

Making chlorine bleach

Cl2(g) + 2NaOH(aq) NaCl(aq) + NaOCl + H2O(l)

Disinfectant in sterilizing water and sewage treatment.

Extraction of bromine from sea water

Cl2(g) + 2Br(aq) 2Cl(aq) + Br2(aq)

Uses of Bromine and its compounds

Manufacture of 1,2-dibromoethane to remove Pb from petrol engine

Pb(C2H5)4, TEL : anti-knock agent added to petrol engine to prevent premature ignition.

TEL decomposes to give Pb that may cause damage to the engine

CH2Br-CH2Br + Pb(C2H5)4 PbBr2volatile and emitted to air easily

Air pollutant

AgBr is used in black-and-white photography

exposure to light

2AgBr(s) 2Ag(s) + Br2(l)coated on film

The excess AgBr(s) is removed as soluble complex ion.

AgBr(s) + 2S2O32(aq) [Ag(S2O3)2]3(aq) +

Br(aq) hypo

Uses of Iodine and its compounds

Making iodine tincture (antiseptic)

I2 in alcohol or KI(aq)

Radioactive iodine-131 as tracer in medical diagnosis

Iodide is used to make iodized table salt for preventing development of goitre.

Laboratory preparation of halogens(except F2)

conc. H2SO4

MnO2 +

2NaCl + MnO2 + 2H2SO4 Na2SO4 + MnSO4 +

2H2O + Cl2

conc. H2SO4

MnO2 +

Free from HCl and H2O

NaCl + H2SO4 HCl + NaHSO4

To remove HCl

To dry Cl2

conc. HCl

The END

1 Characteristic Properties of the Halogens 2 Group VIIA elements include fluorine chlorine ...

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