The Halogens

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The Halogens

• Chapter 40

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Position in Periodic Table
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General information
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Characteristic properties

• Very reactive non-metals
• High electronegativity
• High electron affinity
• Bonding and Oxidation states
• Colour

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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.
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Electron Affinity
Generally high electron affinity. All are
exothermic. X(g) e- ? X-(aq)
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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.

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Bonding and Oxidation State
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Bonding and Oxidation State
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Colour
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Colour
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Check point 40-1
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Variation in properties of Halogens
m.p./b.p. increase as Mr increase because Van der
Waals Force increase with Mr
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Electronegativity
F form ionic compound with 1 oxidation state. I
shows highly positive oxidation states with more
electronegative element. e.g. KIO4
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Electron affinity
EA decrease from Cl to I due to increasing atomic
size which lowers the nuclear attraction to the
added electron. F has exceptional lower EA
because its 2nd shell is already crowded with 7
e-, the additional e- will experienced a
greater e-e repulsion than other halogens.
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Variation in Chemical Properties of elements
All are good oxidizing agents. (F2gtCl2gtBr2gtI2)
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Oxidizing power
?H
?Hatom
?HEA
?Hhyd
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Oxidizing properties

• Oxidation of metals
• a. F2 oxidizes all metals including Au and
  Ag.
• b. Cl2 oxidizes common metals such as Na and Cu.
• Reactions with Fe2(aq)
• 2Fe2 Cl2?2Fe3 2Cl-
• 2Fe2 Br2?2Fe3 2Br-
• 2Fe3 2I-?2Fe2 I2

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Oxidizing properties
3. Reaction with phosphorus 2P 3X2 ?
2PX3 2P 5X2 ? 2PX5
F2 and Cl2 form PX5 as they have high
oxidizing Strength. They combine with P to
exhibit its maximum oxidation state. Br2 and I2
form PX3 only.
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Reaction with water
Fluorine oxidizes water to form HF and O2 2F2
2H2O ? 2HF O2
Chlorine undergoes disproportionation(self
oxidation and reduction) to form HCl and
HOCl. Cl2 H2O ? HCl HOCl
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Reaction with water
A mixture of Cl2(aq), HCl(aq) and HOCl(aq) is
called chlorine water.

• OCl- , chloric(I) ion
• Strong oxidizing agent with bleaching property
• OCl- dye (coloured) ? Cl- dye
  (colourless)
• Unstable to heat and light
• 2OCl- ? O2 2Cl-

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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
• 2OBr- ? O2 2Br-
• 2. OBr- dye(coloured) ? Br- dye(colourless)

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Reaction with water
I2 does not react with water and only
very slightly soluble in water. I2 is more
soluble in ethanol or in KI(aq). I2(aq) I-(aq)
? I3-(aq)
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Reaction with alkalis
All halogens react with aqueous alkalis
and disproportionate in it. The products depend
on temperature
and concentration of
the alkalis.
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Reaction with alkalis
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Reaction with alkalis
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Reaction with alkalis
The reverse reaction of I2 and KOH is used to
prepare standard iodine solution for
iodometric titration. Know amount of iodine is
generated by dissolving known quantity of KIO3 in
excess KI and H. 5KI KIO3 3H ? 3I2 6K
3H2O
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Variation in properties of halides
Halogen Displacement reactions Oxidizing
strength F2 gtCl2 gtBr2 gtI2 Cl2 2Br- ? 2Cl-
Br2 Cl2 2I- ? 2Cl- I2 Br2 2I- ? 2Br- I2
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Halide c. H2SO4
NaCl H2SO4 ? NaHSO4 HCl 2NaCl 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 lt HBr lt HI
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Halide c. H3PO4
3NaCl H3PO4 ? Na3PO4 3HCl 3NaBr H3PO4 ?
Na3PO4 3HBr 3NaI H3PO4 ? Na3PO4 3HI
H3PO4 is NOT a strong oxidizing agent. It reacts
with halides to form HX.
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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
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Halides Ag(aq)
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Relative acidity of HX
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Relative Acidity of HX
?H
HX(aq)
H(aq) X-(aq)
?H5
?H6
?H1
H(g) X-(g)
?H3
?H4
?H2
HX(g)
H (g) X (g)
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Relative Acidity of HX
?H1
?H2
?H3
?H4
?H5
?H6
?H
T?S
?G
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Anomalous behaviour of HF
According to La Chateliers Principle, the
degree of dissociation of a weak acid increases
with dilution. HA? H X- However,
the acidity of conc. HF is greater than dilute
HF. Why?
In conc. HF, HF molecules form dimers H2F2 by
hydrogen bond. The acid strength of H2F2
is stronger than HF. H2F2 ? H HF2-
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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