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Summary of Periodic Table – Group VII Halogens (X2)

Group VII element Cl2 Br2 I2 Bonding Covalent bonding (X-X) within the molecule and weak van der waals’ between the molecules (X2 ……… X2) Structure Simple molecular structure

Colour & physical state (at r.t.p)

Greenish-yellow gas Brown liquid Black solid

Colour in gaseous state Greenish-yellow gas Reddish brown gas Violet fumes Colour in Organic Solvent

(e.g. CCl4) Pale yellow Orange Violet

Volatility or Boiling point Volatility (or b.p ) from Cl2 to I2 Strength of VDW’s forces due to increase in size of electron cloud greater extent of distortion of the electron cloud. More energy is required to overcome these stronger van der Waals’ forces.

Oxidising strength X2 + 2e 2X- E The element X2 becomes less reactive down the group since electron affinity of the halogen atom decreases Thus, oxidising power from Cl2 to I2 as shown by decreasing E values (less likely for X2 to reduce to X-).

Displacement reaction Cl2 displaces Br2 from Br- and displaces I2 from I-. Br2 displaces I2 from I- but I2 cannot displace Cl- and Br-.

Reaction with thiosulphate (S2O3

2-) 4X2 (aq) + S2O3

2- (aq) + 5H2O(l) 2SO42-(aq) + 10H+(aq) + 8X-(aq)

Oxidation number of S from +2 to +6

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

2-

Oxidation number of S from +2 to +3 Reaction with

H2 gas Cl2 reacts vigorously in presence of sunlight or u.v. light; Cl2 (g) + H2 (g) 2 HCl (g)

Br2 reacts at 300 oC in the presence of Pt catalyst

Br2 (g) + H2 (g) 2 HBr (g)

I2 reacts reversibly at 400 oC in the presence of Pt catalyst.

I2 (g) + H2 (g) 2 HI (g) Reaction with NaOH At low temperature

Cl2 (g) + 2NaOH(aq) NaCl (aq) + NaClO(aq) + H2O(l) Cl2 undergoes disproportionation to form Cl- and ClO-. At high temperature Cl2 (g) + 6NaOH(aq) 5NaCl (aq) + NaClO3(aq) + 3H2O(l) At high T, ClO- undergoes further disproportionation to form Cl- and ClO3

-.

At low and high temperature Br2 (g) + 6NaOH(aq) 5NaBr (aq) + NaBrO3(aq) + 3H2O(l)

At low and high temperature I2 (g) + 6NaOH(aq) 5NaI (aq) + NaIO3(aq) + 3H2O(l)

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Hydrogen halides (HX)

Hydrogen halides HCl HBr HI

Structure and bonding

Simple molecular structure with permanent dipole-permanent dipole interactions between the molecules.

Melting point/ boiling point

M.p. or b.p. down the group Strength of VDW’s forces due to increase in size of electron cloud greater extent of distortion of the electron cloud. More energy is required to overcome these stronger van der Waals’ forces between HX molecules.

Thermal stability 2HX(g) H2(g) + X2(g) Thermal stability from HCl to HI since H-X bond energy (i.e. HX bonds become weaker). Down the group, Atomic radius of halogen increases Bond length of H-X increases Bond strength of H-X decreases Bond energy of H–X decreases

Acidity of Hydrogen Halides

HX(aq) H+(aq) + X-(aq) Acid strength : HI > HBr > HCl Down the group, Atomic radius of halogen increases bond length of H–X bond increases bond energy of H–X bond decreases weaker H–X bond breaks more easily H+ is more readily released

hydrogen halide becomes a stronger acid.

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Halides (X-)

Halides ions Cl- Br- I-

Reaction with AgNO3 followed

by NH3

white ppt of AgCl soluble in excess NH3

cream ppt of AgBr insoluble in excess NH3 (only soluble in conc. NH3)

yellow ppt of AgI insoluble in excess NH3

Ag+(aq) + Cl-(aq) AgCl (s) ----- (1) Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+ (aq) ----- (2) When excess aqueous NH3 is added, AgCl (s) is soluble due to the formation of a soluble complex, [Ag(NH3)2]+ (aq). [Ag+] decreases in eqm (1) and AgCl ppt dissolves as the ionic product [Ag+] [Cl-] < Ksp. For AgBr and AgI, the ppts remain insoluble in excess NH3 as both have relatively very small Ksp. Hence, their ionic products > Ksp.

Halides as reducing agent

X2 + 2e 2X- E The reducing power of the halides (X-) increases down the group as shown by decreasing E values. (more likely for X- to

oxidise to X2) Down the group Ionic radius of Cl- to I- anion increases, loss of valence electron occurs more readily halide ions (X-) more easily oxidised halide ions (X-) becomes a more powerful reducing agent

. Reaction with Concentrated

H2SO4

NaCl(s)+H2SO4(l)HCl(g)+NaHSO4 (aq) White fumes HCl(g) formed. Conc. H2SO4 is not powerful enough as

an oxidizing agent to oxidize HCl to Cl2.

NaBr(s) + H2SO4 (l) HBr (g) + NaHSO4 (aq) 2HBr(g) + H2SO4(l) Br2 (g)+ SO2 (g)+ 2H2O (l) White fumes HBr(g) formed. HBr gas can be further oxidised by

concentrated H2SO4 to form reddish brown Br2 gas.

Conc. H2SO4 is reduced to form pungent SO2.

2NaI (s) + H2SO4 (l) 2HI(g) + NaHSO4 (aq) 8HI (g) + H2SO4 (l) 4I2(s) +H2S (g) + 4 H2O (l) White fumes HI(g) formed. HI gas can be further oxidised by

concentrated H2SO4 to form violet I2 gas. Conc. H2SO4 is reduced to form pungent H2S.

Cl2 + 2e 2Cl- E = +1.36 V Br2 + 2e 2Br- E = + 1.07V I2 + 2e 2I- E = + 0.54V