Content p-block ELEMENTS 9.1 The Halogens 9.2 Group IV Elements Halogen Family Halogen vs. Halide The entire family consists of the Elements, X 2 Fluorine Chlorine Bromine CHLORINE Iodine their Compounds (oxidation state = -1) Hydrogen halide, HX Halide, X - All halogens have high electronegativity highly ve values of electron affinity high tendency to attract an additional e - to complete the octet CHLORIDE Cl 2 + 2 e - strong oxidizing l 2 + 2 e - weak oxidizing 2 Cl - weak 2 l - reducing strong reducing 1
Halogen vs. Halide all halogens act as oxidizing s whereas halides as reducing s reactivity of halogens decreases down the group F 2 >> Cl 2 > Br 2 > I 2 The Element - Halogens Preparation Physical Properties Chemical Properties Daily Uses reactivity of halides increases down the group F - < Cl - < Br - < I - Preparation of Gas Set-up consists of 3 stages 1) generation of gas Preparation of Gas 3) collection of gas upward delivery (lighter than air) downward delivery (heavier than air) displacement over water (insoluble in water) 2) removal of side product(s) & unreacted substance 2
Preparation of Cl 2 1) Oxidation of conc. HCl by KMnO 4 [05-2b(1)] Preparation of Cl 2 [97-1-7a] 16 HCl + 2 KMnO 4 2 KCl + 2 MnCl 2 + 8 H 2 O + 5 Cl 2 by MnO 2 4 HCl + MnO 2 MnCl 2 + 2 H 2 O + Cl 2 ideal for preparing dry Cl 2 gas 2) Action of acid on bleach ClO - + Cl - + 2 H + Cl 2 + H 2 O make use of the reversible action of dissolving Cl 2 in water: Cl 2 + H 2 O HCl + HOCl only good for preparing aqueous Cl 2 Physical Properties of Halogens b.p. [95-4a(i)] relatively low b.p. increases down the group solubility [95-4a(iii)] all X 2 readily dissolved in non-polar solvent (e.g. CH 3 CCl 3 ) Cl 2 & Br 2 slightly dissolves in water: X 2 + H 2 O HX + HOX I 2 insoluble in water, BUT dissolves in KI: I 2 + I - I 3 - brown 1) As oxidizing [01-4c] relative oxidizing power decreases down the group: F 2 >> Cl 2 > Br 2 > I 2 less electronegative down the group less tendency to gain e - oxidizing power down the group 3
1) As oxidizing rxns with: sodium metal 2 Na(s) + F 2 (g) 2 NaF(s) explosive 2 Na(s) + Cl 2 (g) 2 NaCl(s) violent 2 Na(s) + Br 2 (l)/i 2 (s) 2 NaBr/NaI(s) continue to burn iron(ii) ions 2 Fe 2+ + Cl 2 /Br 2 2 Fe 3+ + 2 Cl - /Br - Fe 2+ + I 2 no rxn 1) As oxidizing rxns with: thiosulphate 4Br 2 + S 2 O 3 + 5H 2 O 8Br - + 2SO 4 + 10H + I 2 + 2 S 2 O 3 2 I - + S 4 O 6 halogens (displacement rxn) more reactive halogens displace less reactive ones from their halides Cl 2 + 2 Br - 2 Cl - + Br 2 Br 2 + 2 I - 2 Br - + I 2 2) Disproportionation Definition [00-3a, 94-1-2a] a redox rxn in which an element in one O.S. reacts to form products in 2 different O.S., one higher than the original and the other one lower a chemical species oxidized and reduced simultaneously in the same reaction Examples [04-5a, 94-1-2a] Cl 2 + 2 NaOH NaCl + NaOCl + H 2 O production of bleach 2) Disproportionation Examples [03-1-3b, 98-8d] Cl 2 + 2 OH - Cl - + ClO - + H 2 O chlorate(i) at 25 C 3 ClO - 2 Cl - + ClO - 3 chlorate(v) Overall: Cl 2 + 6 OH - 5 Cl - + ClO 3- + 3 H 2 O at 70 C 4
Titrimetric Application of I 2 2) Disproportionation Examples [98-8d, 93-6a] l 2 + 2 OH - l - + lo - + H 2 O iodate(i) 3 lo - 2 l - + lo 3 - iodate(v) Overall: l 2 + 6 OH - 5 l - + lo 3- + 3 H 2 O lo - less stable than ClO - (lo 3- more stable than ClO 3- ) at 25 C at 70 C the eqm : l 2 + 6 OH - 5 l - + lo 3- + 3 H 2 O can be reversed in acidic medium: 5 l - + lo 3- + 6 H + l 2 + 3 H 2 O I 2 is not a good primary standard in titrations volatile / easily sublime (difficult to weigh accurately) insoluble in water (difficult to prepare aqueous solution) react with reducing s in the surroundings (unstable) indirectly prepared by KIO 3 04-3d(ii), 01-1c, 99-1-3d(i) Hydrogen Halides Preparation of HCl [01-1-7a, 96-5c(ii)] 5
Chemical Properties of HX as Bronsted acid HX + H 2 O H 3 O + + X - 03-3d(ii), 01-4d, 95-4c order of acidity of HX HF << HCl < HBr < HI H3O + & F - form tight ion-pair owing to H-bond longer H-X bond length down the group Decomposition of HI 2 HI(g) H 2 (g) + I 2 (g) 95-4c(ii) H-I bond is among the longest in H-X Ionic Halides Displacement of volatile HX from non-volatile acids NaCl(s) + H 2 SO 4 (l) NaHSO 4 (s) + HCl(g) non-volatile volatile [06-1b(iii), 00-1-3c(i), 99-1-3d(ii), 96-5c] conc. acids are normally monobasic not enough water to stabilize 2 H + formation of acid salt with only one H being replaced observation: white fumes (HX) Only HCl forms denser white fumes when in contact with NH 3 vapour NH 3 (g) + HCl(g) NH 4 Cl(s) sublime Displacement of volatile HX from non-volatile acids e.g. non-volatile acids: H 3 PO 4 (non-oxidizing) all halides produce HX without further rxn KX(s) + H 3 PO 4 (l) KH 2 PO 4 (s) + HX(g) H 2 SO 4 (oxidizing) KX(s) + H 2 SO 4 (l) KHSO 4 (s) + HX(g) HCl so formed is not reducing enough to have further rxn both HBr & HI are strong reducing s to further react with conc. H 2 SO 4 (l) 6
Displacement of volatile HX from non-volatile acids reducing power of HX: HCl << HBr < HI 2HBr(g) + H 2 SO 4 (l) Br 2 (l) + SO 2 (g) + 2H 2 O(l) observation: brown/orange fumes, choking smell 8HI(g) + H 2 SO 4 (l) 4I 2 (s) + H 2 S(g) + 4H 2 O(l) observation: black solid / purple fumes, bad egg smell aware of the state symbols volatile HX in gas state ionic cpd remain in solid state (no water in conc. acid) Br 2 in liquid state; I 2 in solid state reactivity increases down the group: F - < Cl - << Br - < I - Br - & I - can reduce KMnO 4 whereas Cl - cannot less electronegative down the group higher tendency to donate e - reducing power down the group KI reacts with persulphate, K 2 S 2 O 8 S 2 O 8 + 2I - 2 SO 4 + I 2 Fe 3+ as homogeneous catalyst (refer to Kinetics) precipitation with AgNO 3 Ag + (aq) + X - (aq) AgX(s) color of ppt: AgCl (white); AgBr (pale yellow); AgI (yellow) further tests required to distinguish % AgBr & AgI stand in sunlight AgCl & AgBr turns grey (decompose to silver) AgI remains unchanged solubility in excess NH 3 (aq) AgCl redissolve completely AgBr redissolve partially AgI remains unchanged [05-1c, 04-1-7a(ii), 95-1-4a] Daily Uses AgBr photographic films NaClO bleach Cl 2 germicide 97-1-3a 7