Presentation on theme: "1 Characteristic Properties of the Halogens 2 Group VIIA elements include fluorine chlorine bromine iodine astatine Introduction halogens (Salt."— Presentation transcript:
2 Group VIIA elements include fluorine chlorine bromine iodine astatine Introduction halogens (Salt producers)
3 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. Introduction
4 Halogens are p-block elements outermost shell electronic configuration of ns 2 np 5
5 one electron short of the octet structure Halogens are p-block elements
6 In the free elemental state Introduction they complete their octets by sharing their single unpaired p-electrons
7 they either gain an additional electron to form halide ions or When halogens react with other elements share their single unpaired p- electrons to form single covalent bonds
8 highest among the elements in the same period have a high tendency to attract electrons strong oxidizing agents High Electronegativity / Electron Affinity
9 -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 Affinity
10 Variable Oxidation State All halogens (except fluorine ) can expand their octet of electrons by utilizing the vacant, energetically low-lying d-orbitals.
11 “Electrons-in-boxes” diagrams of the electronic configuration of a halogen atom of the ground state and various excited states
12 The half-filled orbital(s) overlap(s) with those of more electronegative atoms (e.g. O ) positive oxidation state (+1, +3, +5, +7) +1 +3 +5 +7
13 Oxidation state of halogen Ion / Compound –1 F – Cl – Br – I – HFHClHBrHI OF 2 0F 2 Cl 2 Br 2 I 2 +1 Cl 2 OBr 2 O HOClHOBr OCl – OBr – +3 HClO 2 ClO 2 – Various oxidation states of halogens in their ions or compounds
14 Oxidation state of halogen Ion / Compound +4ClO 2 BrO 2 +5 HClO 3 HBrO 3 I 2 O 5 ClO 3 – BrO 3 – HIO 3 IO 3 – +6Cl 2 O 6 BrO 3 +7 Cl 2 O 7 H 5 IO 6 HClO 4 HIO 4 ClO 4 – IO 4 – Various oxidation states of halogens in their ions or compounds
15 Fluorine ( 1) the most electronegative element only one unpaired p electron available for bonding oxidation state is limited to –1
16 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 high Absence of HFO, HFO 2, HFO 3, HFO 4
17 Variation in Physical Properties 1. Melting point / boiling point down the group Halogen Melting point ( C) Boiling point ( C) Fluorine Chlorine Bromine Iodine Astatine –220 –101 –7.2 114 302 –188 –34.7 58.8 184 380
18 Variations in melting point and boiling point of the halogens
19 Variation 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
20 2. Colour becomes darker down the group HalogenF2F2 Cl 2 Br 2 I2I2 Colour Pale yellow Greenish yellow Reddish brown Violet black
21 Appearances of halogens at room temperature and pressure: chlorine chlorine
22 Appearances of halogens at room temperature and pressure: bromine bromine
23 Appearances of halogens at room temperature and pressure: iodine iodine
24 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
25 Colour Fluorine atom has the smallest size absorbs the radiation of relatively high frequency (i.e. blue light ) appears yellow (the unabsorbed radiation)
26 Colour Atoms of other halogens larger sizes absorb radiation of lower frequency
27 Colour Iodine absorbs the radiation of relatively low frequency (i.e. yellow light ) appears violet
29 Colour Halogens different colours when dissolved in different solvents
30 Halogen Colour in pure formin waterin 1,1,1-trichloroethane F2F2 Pale yellow Cl 2 Greenish yellowPale yellowYellow Br 2 Reddish brownYellowOrange I2I2 Violet black Yellow (only slightly soluble) Brown in KI(aq) Violet Colours of halogens in pure form and in solutions
31 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 )
32 Colours of halogens in water: (a) chlorine; (b) bromine; (c) iodine (a)(b)(c)
33 Colours of halogens in 1,1,1-trichloroethane: (a) chlorine; (b) bromine; (c) iodine (a)(b)(c)
34 3. Electron Affinity down the group HalogenFClBrIAt E.A. kJ/mol 1 -322-349-335-295-270
35 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
36 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.
37 HalogenFClBrIAt Electronegativity4.03.02.82.52.2 4. Electronegativity down the group
38 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
39 Fluorine has the highest electronegativity because it is the most reactive elements. The electronegativity of fluorine is arbitrarily assigned as 4.0.
40 Variation in Chemical Properties Reactivity : F 2 > Cl 2 > Br 2 > I 2 React by gaining electrons Oxidizing power : F 2 > Cl 2 > Br 2 > I 2
41 1. Reactions with Sodium All halogens combine directly with sodium to form sodium halides the reactivity decreases down the group from fluorine to iodine
42 Fluorine react explosively to form sodium fluoride 2Na(s) + F 2 (g) 2NaF(s) 1. Reactions with Sodium
43 Chlorine reacts violently to form sodium chloride 2Na(s) + Cl 2 (g) 2NaCl(s) 1. Reactions with Sodium
44 Bromine burns steadily in bromine vapour to form sodium bromide 2Na(s) + Br 2 (g) 2NaBr(s) 1. Reactions with Sodium
45 Iodine burns steadily in iodine vapour to form sodium iodide 2Na(s) + I 2 (g) 2NaI(s) 1. Reactions with Sodium
46 Na(s) + X 2 NaX(s) Na + (g) + X 2 (g) I.E. Na + (g) + X(g) B.E. Na + (g) + X (g) E.A. Vigor of reaction depends on 1.The activation energy (endothermic) 2.The lattice energy (exothermic) Activation energy
47 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. F has an exceptionally low B.E. & zero F is the most reactive (g)
48 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. The lattice enthalpy of NaF is most negative
49 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. Cl has zero Cl is more reactive than Br & I (g)
50 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. Lattice enthalpy : NaCl > NaBr > NaI
51 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. (s)/(l) Br is more reactive than I : Br 2 (l) < I 2 (s)
52 Na(s) + X 2 NaX(s) Na + (g) + X(g) Na + (g) + X 2 (g) I.E. B.E. Na + (g) + X (g) E.A. Lattice enthalpy : NaBr > NaI
53 Q.2(a) Variation:bond enthalpy decreases from Cl 2 to I 2 Reason : 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. F 2 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.
54 Q.2(b) The lattice enthalpy becomes less negative down the group. It is because the anionic radius, r -, increases down the group.
55 F 2 reacts explosively even in the dark at 200 C Cl 2 reacts explosively in sunlight Br 2 reacts moderately on heating with a catalyst I 2 reacts slowly and reversibly even on heating 2.1Reactions with hydrogen X 2 + H 2 (g) 2HX(g)
56 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 H 2 and F 2 is the most exothermic. The energy released from the reaction further speeds up the reaction. F 2 + H 2 (g) 2HF(g)
57 Chlorine removes hydrogen completely from turpentine(C 10 H 16 ) C 10 H 16 (l) + 8Cl 2 (g) 10C(s) + 16HCl(g)
58 Q.4 The 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.
59 2.2Reactions with phosphorus F 2 + P PF 5 Cl 2 + P PCl 3 + PCl 5 Br 2 + P PBr 3 I 2 + P PI 3 F 2 is the strongest oxidizing agent, it always oxidizes other elements to their highest possible oxidation states.
60 2.2Reactions with phosphorus F 2 + P PF 5 Cl 2 + P PCl 3 + PCl 5 Br 2 + P PBr 3 I 2 + P PI 3 Br 2 and I 2 are NOT strong enough to oxidize P to its highest possible oxidation state.
61 2.3Reactions 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) + 2F 2 CF 4 Xe + F 2 XeF 2 Xe + 2F 2 XeF 4 Xe + 3F 2 XeF 6
62 2.3Reactions with xenon It is because (a)Xenon can expand its octet by utilizing vacant, low-lying d-orbitals.
63 By VB Theory, To form two Xe-F bonds in XeF 2, a 5p electron in Xe has to be promoted to a 5d orbital. Xe 5s5p F 2s2p Xe* 5s5p 5d
64 By VB Theory, To form four Xe-F bonds in XeF 4, two 5p electrons in Xe have to be promoted to two 5d orbitals. Xe 5s5p 5d Xe** 5s5p 5d
65 By VB Theory, To form six Xe-F bonds in XeF 6, three 5p electrons in Xe have to be promoted to three 5d orbitals. Xe 5s5p 5d Xe*** 5s5p 5d
66 2.3Reactions with xenon 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.
67 Xe 5s5p 5d Xe*** 5s5p 5d Also, the energy released by forming more single bonds outweighs the energy required for promoting 5p electrons to 5d orbitals.
68 3Reactions with other reducing agents I 2 is the weakest oxidizing agents among the halogens.
69 3.1All halogens(except I 2 ) oxidize Fe 2+ to Fe 3+ Half reaction Standard electrode potential (V) Cl 2 (aq) + 2e – 2Cl – (aq) Br 2 (aq) + 2e – 2Br – (aq) Fe 3+ (aq) + e – Fe 2+ (aq) I 2 (aq) + 2e – 2I – (aq) +1.36 +1.07 +0.77 +0.54 X 2 (aq) + 2Fe 2+ (aq) 2X (aq) + 2Fe 3+ (aq) ( X = F, Cl, Br)
70 3.1All halogens(except I 2 ) oxidize Fe 2+ to Fe 3+ Half reaction Standard electrode potential (V) Cl 2 (aq) + 2e – 2Cl – (aq) Br 2 (aq) + 2e – 2Br – (aq) Fe 3+ (aq) + e – Fe 2+ (aq) I 2 (aq) + 2e – 2I – (aq) +1.36 +1.07 +0.77 +0.54 I 2 (aq) + 2Fe 2+ (aq) No reaction
71 3.2All halogens(except I 2 ) oxidize S 2 O 3 2 to SO 4 2 4X 2 (aq) + S 2 O 3 2 (aq) + 5H 2 O(l) 8X (aq) + 10H + (aq) + 2SO 4 2 (aq) (X = F, Cl, Br) I 2 (aq) + 2S 2 O 3 2 (aq) 2I (aq) + S 4 O 6 2 (aq) +5 +6 0 0 +4 0 Used in iodometric titration
72 (i)2I (aq) + 2Fe 3+ (aq) I 2 (aq) + 2Fe 2+ (aq) (excess) (unknown) Determination of [Fe 3+ (aq)] by iodometric titration Using starch as indicator (ii) I 2 (aq) + 2S 2 O 3 2 (aq) 2I (aq) + S 4 O 6 2 (aq) (standard solution)
74 4Displacement reactions Cl 2 (aq) + 2I (aq) 2Cl (aq) + I 2 (aq) Br 2 (aq) + 2I (aq) 2Br (aq) + I 2 (aq) I 2 (aq) + I (aq) I 3 (aq) (yellow) (brown) What would be observed if an excess of Cl 2 (aq) or Br 2 (aq) is added to I (aq)? The solution turns cloudy and a black solid settles at the bottom
75 Aqueous solution Halogen added F2F2 Cl 2 Br 2 I2I2 F–F– No reaction Cl – A pale yellow solution is formed (Cl 2 is formed) No reaction Reactions of halide ions with halogens
76 Aqueous solution Halogen added F2F2 Cl 2 Br 2 I2I2 Br – A yellow solution is formed (Br 2 is formed) No reaction I–I– A yellowish brown solution is formed (I 3 is formed) No reaction Reactions of halide ions with halogens
77 Q.5 Shake hexane or 1,1,1-trichloroethane with the two solutions respectively. The one that turns the organic layer violet is I 3 (aq). The one that turns the organic layer orange or brown is Br 2 (aq).
78 1,1,1-trichloroethane Br 2 I2I2 Br 2 (aq) I 3 (aq) If hexane is used, the upper layer will be the organic layer
79 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
80 A.Reactions with Water HOCl : chloric(I) acid or hypochlorous acid Chlorine water a mixture of hydrochloric acid and chloric(I) acid
81 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) + O 2 (g) A.Reactions with Water
82 Chlorate(I) ion bleaches by oxidation A.Reactions with Water Cl 2 (aq) + H 2 O(l) 2H + (aq) + Cl – (aq) + OCl – (aq) OCl – (aq) + dye Cl – (aq) + (dye + O) coloured colourless
83 Bromine only slightly soluble in water mainly exists as molecules in saturated bromine water A.Reactions with Water
84 When the solution is diluted hydrolysis takes place hydrobromic acid and bromic(I) acid (hydrobromous acid) are formed Br 2 (l) + H 2 O(l) HBr(aq) + HOBr(aq) A.Reactions with Water
85 Bromate(I) ion, OBr also unstable bleaches dyes by oxidation OBr – (aq) + dye coloured Br – (aq) + (dye + O) colourless A.Reactions with Water
86 Iodine does not react with water only slightly soluble in water A.Reactions with Water
87 Fluorine reacts vigorously with water to form hydrogen fluoride and oxygen A.Reactions with Water Being the strongest oxidizing agent, F 2 undergoes reduction rather than disproportionation with water. 2F 2 (g) + 2H 2 O(l) 4HF(aq) + O 2 (g) 0 11
88 Chlorine reacts similarly at high temperature or when exposed to light 2Cl 2 (aq) + 2H 2 O(l) 2HCl(aq) + 2HOCl(aq) 2HOCl(aq) 2HCl(aq) + O 2 (g) Heat or light Overall : 2Cl 2 (aq) + 2H 2 O(l) 4HCl(aq) + O 2 (g) Heat or light A.Reactions with Water
89 All halogens react with aqueous alkalis All halogens ( except F 2 ) undergoes disproportionation with alkalis In general, Reactivity decreases down the group B.Reactions with Alkalis
90 The products formed depend on 1.Temperature 2.The type of halogen reacted 3.The concentration of alkali used B.Reactions with Alkalis
91 Effect of temperature (a)At lower temperatures, X2X2 Cl 2 Br 2 I2I2 T 1 / C 20 00<0<0 X 2 (aq) + 2OH (aq) XO (aq) + X (aq) + H 2 O(l) T1T1 0+1 11 B.Reactions with Alkalis
92 Effect of temperature (a)At higher temperatures, 3XO (aq) XO 3 (aq) + 2X (aq) T2T2 XO ClO BrO IO T 2 / C 70 20 00 +1+5 11 B.Reactions with Alkalis
93 (2) 3XO (aq) XO 3 (aq) + 2X (aq) T2T2 (1) X 2 (aq) + 2OH (aq) XO (aq) + X (aq) + H 2 O(l) T1T1 Overall reaction : 3 (1) + (2) 3X 2 (aq) + 6OH (aq) XO 3 (aq) + 5X (aq) + 3H 2 O(l) T2T2 B.Reactions with Alkalis
94 3XO (aq) XO 3 (aq) + 2X (aq) T2T2 XO ClO BrO IO T 2 / C7020<0 On moving down the group, 1.stability of XO decreasesClO > BrO > IO 2.stability of XO 3 increasesClO 3 < BrO 3 < IO 3 B.Reactions with Alkalis
95 3X 2 (aq) + 6OH (aq) XO 3 (aq) + 5X (aq) + 3H 2 O(l) At lower pH (when acid is added), the equilibrium position shifts to the left and the reversed process predominates. XO 3 (aq) + 5X (aq) + 6H + (aq) 3X 2 (aq) + 3H 2 O(l) This reaction (when X=I) is often used to prepare standard iodine solution for iodometric titrations B.Reactions with Alkalis
96 Dissolving a known quantity of KIO 3 (s) in excess KI(aq) and dilute H 2 SO 4 generates a known amount of I 2 (aq) KIO 3 (aq) + 5KI(aq) + 6H + (aq) 3I 2 (aq) + 3H 2 O(l) + 6K + (aq) The iodine produced can be used to standardize thiosulphate solution 3I 2 (aq) + 6S 2 O 3 2 (aq) 6I (aq) + 3S 4 O 6 2 (aq) B.Reactions with Alkalis
97 This known amount of iodine generated can also be used to oxidize reducing agents (of unknown concentrations ) such as SO 3 2 (aq) and ascorbic acid (vitamin C) The excess iodine can be determined by back titration with sodium thiosulphate solution I 2 (aq) + 2S 2 O 3 2– (aq) 2I – (aq) + S 4 O 6 2– (aq) B.Reactions with Alkalis
98 Effect of concentration of alkali (a)At higher concentrations, XO 3 (aq) is the major product. (b)At lower concentrations, XO (aq) is the major product. B.Reactions with Alkalis
99 B.Reactions with Alkalis In general, Halogens react with cold, dilute alkali to give halate(I) ions, halide ions and water X 2 (aq) + 2OH XO (aq) + X (aq) + H 2 O(l) Halogens react with hot, concentrated alkali to give halate(V) ions, halide ions and water. 3X 2 (aq) + 6OH XO 3 (aq) + 5X (aq) + 3H 2 O(l)
100 2F 2 + 2OH (aq) OF 2 (aq) + 2F (aq) + H 2 O(l) very dilute 20 C 2F 2 + 4OH (aq) O 2 (aq) + 4F (aq) + 2H 2 O(l) concentrated 70 C 0 0 11 11 11 22 22 +2 0 Being the strongest oxidizing agent, F 2 undergoes reduction rather than disproportionation with alkalis. B.Reactions with Alkalis
101 Variation in chemical properties of halides A Comparative study 1.Reactions with conc. sulphuric acid 2.Reactions with conc. phosphoric acid 3.Reactions with silver ion
102 Concentrated sulphuric acid non-volatile (b.p. ~330 C) oxidizing Reactions with Concentrated Sulphuric(VI) Acid
103 KF(s) + H 2 SO 4 (l) KHSO 4 (s) + HF(g) KCl(s) + H 2 SO 4 (l) KHSO 4 (s) + HCl(g) warm non-volatile volatile Warming is required to speed up the reaction and to drive out the volatile acids Fluoride and chloride : -
104 KF(s) + H 2 SO 4 (l) KHSO 4 (s) + HF(g) KCl(s) + H 2 SO 4 (l) KHSO 4 (s) + HCl(g) warm acid salt Acid salt rather than normal salt is formed because HSO 4 is a relatively weak acid A convenient way to prepare HCl in the laboratory
105 KF(s) + H 2 SO 4 (l) KHSO 4 (s) + HF(g) KCl(s) + H 2 SO 4 (l) KHSO 4 (s) + HCl(g) warm Observation : - White fumes are produced Confirmatory test : - Dense white fumes appear with NH 3 (aq)
106 Bromide: - 0 oxidation +6+4 reduction KBr(s) + H 2 SO 4 (l) KHSO 4 (s) + HBr(g) warm 2HBr(g) + H 2 SO 4 (l) SO 2 (g) + Br 2 (g) + 2H 2 O(l) warm
107 (1)KBr(s) + H 2 SO 4 (l) KHSO 4 (s) + HBr(g) (2) 2HBr(g) + H 2 SO 4 (l) SO 2 (g) + Br 2 (g) + 2H 2 O(l) Bromide: - Overall reaction : 2 (1) + (2) Not suitable for preparing HBr warm 2KBr(s) + 3H 2 SO 4 (l) 2KHSO 4 (s) + SO 2 (g) + Br 2 (g) + 2H 2 O(l) warm
108 A brown gas is evolved on warming A pungent smell is detected A brown colour is observed when adding hexane Br 2 It turns orange dichromate solution green SO 2 Confirmatory Test Dense white fumes are formed with aqueous ammonia HBrWhite fumes are formed Br – ProductObservationHalide 2KBr(s) + 3H 2 SO 4 (l) 2KHSO 4 (s) + SO 2 (g) + Br 2 (g) + 2H 2 O(l)
109 iodide: - KI(s) + H 2 SO 4 (l) KHSO 4 (s) + HI(g) warm 2HI(g) + H 2 SO 4 (l) SO 2 (g) + I 2 (g) + 2H 2 O(l) 0+6+4 warm 8HI(g) + H 2 SO 4 (l) H 2 S(g) + 4I 2 (g) + 2H 2 O(l) 0+6-2 warm HI is strong enough to reduce sulphur to its lowest possible oxidation state
110 KI(s) + H 2 SO 4 (l) KHSO 4 (s) + HI(g) (1) warm 2HI(g) + H 2 SO 4 (l) SO 2 (g) + I 2 (s) + 2H 2 O(l) (2) warm 8HI(g) + H 2 SO 4 (l) H 2 S(g) + 4I 2 (s) + 2H 2 O(l) (3) warm Overall reaction = 10 (1) + (2) + (3) 10KI(s) + 12H 2 SO 4 (l) 10KHSO 4 (s) + SO 2 (g) + H 2 S(g) + 5I 2 (s) + 4H 2 O(l) No suitable for preparing HI
111 10KI(s) + 12H 2 SO 4 (l) 10KHSO 4 (s) + SO 2 (g) + H 2 S(s) + 5I 2 (s) + 4H 2 O(l) Observation : - A bad egg smell is detected Confirmatory test : - It turns lead(II) ethanoate paper black (CH 3 COO) 2 Pb + H 2 S PbS(s) + 2CH 3 COOH
112 10KI(s) + 12H 2 SO 4 (l) 10KHSO 4 (s) + SO 2 (g) + H 2 S(s) + 5I 2 (s) + 4H 2 O(l) Observation : - Confirmatory test : - A violet colour is observed when added to hexane Violet fumes are formed and condense when cooled to give a black solid
113 Conclusion : - Reducing power : HI > HBr > HCl > HF Increases down the group
114 Interpretation:- Consider the reaction, 2H–X + H 2 SO 4 X–X + SO 2 + 2H 2 O 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
115 2H–X + H 2 SO 4 X–X + SO 2 + 2H 2 O 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 rx The 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.
116 H-XB.E.(kJ mol 1 )X-XB.E. (kJ mol 1 H-Cl432Cl-Cl244 H-Br366Br-Br192 H-I298I-I152 On moving down the group, both H–X bonds and X–X bonds become weaker
117 2H–X + H 2 SO 4 X–X + SO 2 + 2H 2 O 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
118 Reactions with Phosphoric Acid non-volatile volatile H 3 PO 4 (l) + HX(g) no reaction less oxidizing Suitable for preparing HX from solid halids NaCl(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HCl(g) NaBr(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HBr(g) NaI(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HI(g) warm
119 Halide ion ObservationProduct Confirmatory test of the product Cl – White fumes are formed on warming HCl Dense white fumes are formed with aqueous ammonia Br – HBr I–I– HI NaCl(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HCl(g) NaBr(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HBr(g) NaI(s) + H 3 PO 4 (l) NaH 2 PO 4 (s) + HI(g) warm
120 Reactions with Silver Ions Aqueous solutions of chlorides, bromides and iodides give precipitates when reacting with acidified silver nitrate solution
121 Reactions with Silver Ions 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
122 AgI(s)AgBr(s)AgCl(s) Colour intensity down the group
123 Reactions with Silver Ions Silver nitrate solution should be acidified with nitric acid (a)to remove interfering ions like SO 3 2 or CO 3 2 They may form white ppt with Ag +
124 Reactions with Silver Ions 2H + (aq) + SO 3 2– (aq) SO 2 (g) + H 2 O(l) 2H + (aq) + CO 3 2– (aq) CO 2 (g) + H 2 O(l)
125 Silver nitrate solution should be acidified with nitric acid (b)to avoid the formation of black ppt of Ag 2 O in alkaline solution. 2Ag + (aq) + 2OH (aq) Ag 2 O(s) + H 2 O(l)
126 The solubility(in water) of AgX down the group AgF >> AgCl > AgBr > AgI Ksp/mol 2 dm 6 1.6 10 10 7.7 10 13 1.5 10 16 soluble insoluble
127 Q.7 On moving down the group, 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
128 Reactions with Silver Ions 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.
129 Two confirmatory tests for halides 1.Adding NH 3 (aq) to the AgX ppt 2.Exposing AgX ppt to sunlight
130 AgX(s) dissolve in NH 3 (aq) due to the formation of soluble complex ions. AgCl(s) + 2NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) + Cl (aq) AgBr(s) + 2NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) + Br (aq) AgI(s) + 2NH 3 (aq) No reaction Solubility in NH 3 (aq) down the group
132 Ion Action of acidified AgNO 3 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 Br – A pale yellow ppt is formed The pale yellow ppt slightly dissolves The solution turns yellowish grey I–I– A yellow ppt is formed The yellow ppt does not dissolve The solution remains yellow Action of acidified silver nitrate solution on halides
133 Anomalous Behaviour of Hydrogen Fluoride 1.Hydrogen fluoride has abnormally high boiling point and melting point among the hydrogen halides HXHFHClHBrHI b.p./ C19.5 85 66.4 35
134 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
135 The acid strength of hydrogen halides decreases in the order: HI > HBr > HCl >> HF 2.Acidic Properties of Hydrogen Halides
136 Hydrogen halide Acid dissociation constant, K a (mol dm –3 ) Degree of dissociation in 0.1 M solution (%) Acid strength HF HCl HBr HI 5.6 × 10 –4 1 × 10 7 1 × 10 9 1 × 10 11 8.5 92 93 95 Low Strong Very strong Acid dissociation constants of hydrogen halides and their degrees of dissociation in 0.1 M solutions
137 In dilute (e.g. 0.1M) solution, HF is the weakest acid among all the hydrohalic acids HF(l) + H 2 O(l) H 3 O + (aq) + F – (aq) Ka = 5.6 × 10 –4 mol dm –3
138 Very stable ion pair Freedom of F & H 3 O + greatly (a drop in entropy of the system) due to H-bond formation Effective concentration of F & H 3 O + greatly Thus, Ka & pH
139 In concentrated solution, HF is the strongest acid among all the hydrohalic acids
140 Strength of H-bond:- 2.HF is in excess in concentrated solution F ions combine with excess HF rather than with H 3 O + free H 3 O+ & pH excess H 3 OF(aq) + HF(aq) H 3 O + (aq) + HF 2 (aq)
141 For other HX acids, acidity as concentration It is due to the significant interaction between X and H 3 O + at high concentrations the effective concentration of H 3 O + For HF, interaction between F and H 3 O + is significant even at low concentrations due to the smaller size of F .
142 3.Pure, anhydrous liquid HF is ionic due to the formation of HF 2 and H 2 F + ions Self ionization : - 2HF(l) H 2 F + (l) + F (l) HF(l) + F (l) HF 2 (l) Overall : - 3HF(l) [H 2 F] + [HF 2 ] (l)
143 Stabilized by resonance Two identical H – F bonds
144 Heating the solid potassium hydrogen difluoride reverses the reaction a convenient way to obtain anhydrous hydrogen fluoride KF(s) + HF(l) KHF 2 (s) heat
145 Uses of fluorine and its compounds Sodium hexafluorosilicate, Na 2 SiF 6, 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.
146 Uses of fluorine and its compounds Molten cryolite, Na 3 AlF 6 Lowers the temperature (2517 C 1000 C) needed for extracting Al from Al 2 O 3 by electrolysis.
147 Uses of fluorine and its compounds Convert U to UF 6 Separate 235 UF 6 from 238 UF 6 by diffusion for use in nuclear reactors. The heavier 238 UF 6 diffuses a bit slower, making the separation possible.
148 Uses of fluorine and its compounds Conc. HF(aq) is used in etching glass (e.g. making scales/graduation marks on glassware) CaSiO 3 (s) + 6HF(aq) CaF 2 (aq) + SiF 4 (aq) + 3H 2 O(l) (Glass)
149 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
150 Uses of fluorine and its compounds A glass is etched by hydrofluoric acid
151 Uses of fluorine and its compounds Making 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.
152 Uses of fluorine and its compounds Hydrazine/fluorine mixtures are excellent rocket fuels N 2 H 4 (g) + 2F 2 (g) N 2 (g) + 4HF(g) H = -1166 kJ mol 1 (extremely exothermic) Due to the strong N N and H-F bonds
153 Uses of fluorine and its compounds Extraction of fluorine Electrolyte : KF(s) dissolved in pure HF(l) Anode : graphite Cathode : steel
154 Q.8(a) Anode : 2HF 2 2HF + F 2 + 2e Cathode : 2H 2 F + + 2e 2HF + H 2 Overall : 2HF 2 + 2H 2 F + 4HF + F 2 + H 2
155 8.(b)Overall : 2HF 2 + 2H 2 F + 4HF + F 2 + H 2 6HF 4HF + F 2 + H 2 2HF F 2 + H 2 KF is added to increase the conductivity of the electrolyte. KHF 2 > HF or [H 2 F][HF 2 ]
156 Q.8(c) OH - (from H 2 O) rather than HF 2 is oxidized at the anode 2F 2 (g) + 2H 2 O(l) 4HF(aq) + O 2 (g) vigorous reaction Also, F 2 reacts vigorously with water.
157 8.(d) At high temperatures, fluorine produced can react vigorously with the electrodes, air, etc.
158 Uses of Chlorine and its compounds Polyvinyl chloride, PVC making electrical insulation, bottles, floor tiles, table cloth, shower curtain, etc.
160 Making chlorine bleach Cl 2 (g) + 2NaOH(aq) NaCl(aq) + NaOCl + H 2 O(l) Disinfectant in sterilizing water and sewage treatment. Extraction of bromine from sea water Cl 2 (g) + 2Br (aq) 2Cl (aq) + Br 2 (aq)
161 Uses of Bromine and its compounds Manufacture of 1,2-dibromoethane to remove Pb from petrol engine Pb(C 2 H 5 ) 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 CH 2 Br-CH 2 Br + Pb(C 2 H 5 ) 4 PbBr 2 volatile and emitted to air easily Air pollutant
162 AgBr is used in black-and-white photography exposure to light 2AgBr(s) 2Ag(s) + Br 2 (l) coated on filmblack The excess AgBr(s) is removed as soluble complex ion. AgBr(s) + 2S 2 O 3 2 (aq) [Ag(S 2 O 3 ) 2 ] 3 (aq) + Br (aq) hypo
163 Uses of Iodine and its compounds Making iodine tincture (antiseptic) I 2 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.
164 Laboratory preparation of halogens(except F 2 ) conc. H 2 SO 4 MnO 2 + NaCl
165 2NaCl + MnO 2 + 2H 2 SO 4 Na 2 SO 4 + MnSO 4 + 2H 2 O + Cl 2 conc. H 2 SO 4 MnO 2 + NaCl 0 +4 +2 Free from HCl and H 2 O NaCl + H 2 SO 4 HCl + NaHSO 4 To remove HClTo dry Cl 2
166 Laboratory preparation of halogens(except F 2 ) conc. HCl MnO 2
167 Laboratory preparation of halogens(except F 2 ) conc. HCl MnO 4