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The Halogens

Chapter 42. The Halogens. 42.1 Characteristic Properties of the Halogens 42.2 Variation in Properties of the Halogens 42.3 Comparative Study of the Reactions of Halide Ions 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride

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The Halogens

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  1. Chapter 42 The Halogens 42.1Characteristic Properties of the Halogens 42.2Variation in Properties of the Halogens 42.3Comparative Study of the Reactions of Halide Ions 42.4Acidic Properties of Hydrogen Halides and theAnomalous Behaviour of Hydrogen Fluoride 42.5Uses of Halogens and Halogen-ContainingCompounds

  2. 42.1 Characteristic Properties of the Halogens (SB p.82) • Halogen (Group VIIA): • Consists of fluorine, chlorine, bromine, iodine, astatine • Outermost shell electronic configuration: ns2np5 • Diatomic molecules in free elemental state by sharing the unpaired e– • React with other elements, complete the octet by gaining 1e–or sharing the unpaired e– to form 1 covalent bond

  3. Bromine is a reddish brown liquid Chlorine is a greenish-yellow gas Iodine is a violet black solid 42.1 Characteristic Properties of the Halogens (SB p.83)

  4. 42.1 Characteristic Properties of the Halogens (SB p.83) Electronegativity Electronegativity is a measure of the relative tendency of an atom to attract bond pair(s) of electrons towards itself in a covalent bond • Halogens have high tendency to attract an additional e– to complete the octet which has extra stability • Halogens have high electronegativity values

  5. 42.1 Characteristic Properties of the Halogens (SB p.83) Electron Affinity Electron affinity is the enthalpy change when one mole of electrons is added to one mole of atoms or ions in the gaseous phase. • All halogens have negative values of electron affinity • i.e. all halogens have a high tendency to attract an additional electron to form the respective halide ions

  6. 42.1 Characteristic Properties of the Halogens (SB p.84) Bonding and Oxidation State • The outermost shell electronic configuration of halogens: ns2np5 • To complete the octet by gaining 1e– •  the oxidation state is –1 • To complete the octet by sharing their unpaired e– •  the oxidation state would be –1 or +1 (depends on the electronegativity of the elements covalently bonded, except fluorine)

  7. 42.1 Characteristic Properties of the Halogens (SB p.84) • All halogens (except fluorine) can expand their octet of electrons by utilizing the vacant, low-lying d-orbitals • By promoting e– into d-orbitals, halogens can have variable no. of unpaired e– to form bonds with other atoms •  Form compounds of different oxidation states

  8. 42.1 Characteristic Properties of the Halogens (SB p.84) • Example:Chlorine • 7 outermost shell electrons •  Expand the octet to form a maximum 7 covalent bonds •  Oxidation state = +7 as in dichlorine heptoxide (Cl2O7) •  If fewer than 7 electrons are used in bonding, the halogen atom can have 1, 2 and 3 lone pairs of electrons •  The oxidation state would be +5, +3 and +1

  9. 42.1 Characteristic Properties of the Halogens (SB p.84) • Fluorine • • The most electronegative atom • 1 unpaired e– • cannot expand its octet • ∵ no vacant low-lying d-orbitals • • cannot promote e– into 3rd quantum shell • ∵ too high in energy •  Oxidation state = –1

  10. 42.1 Characteristic Properties of the Halogens (SB p.84) “Electrons-in-boxes” representation of the electronic configuration of a halogen atom showing different oxidation states

  11. 42.1 Characteristic Properties of the Halogens (SB p.85) Various oxidation states of halogens in their compounds

  12. 42.1 Characteristic Properties of the Halogens (SB p.85) Colour • All halogens are coloured • ∵ absorption of radiation in visible region of spectrum •  absorbed radiation causes the excitation of electrons • Small fluorine atoms absorb relatively high frequency (i.e. blue light) •  appears yellow • Larger atoms like iodine absorb relatively low frequency (i.e. yellow light) •  appears violet

  13. 42.1 Characteristic Properties of the Halogens (SB p.85) • Halogens show different colours when dissolved in different solvents • Halogens are non-polar molecules Not very soluble in water but very soluble in organic solvents

  14. (a) (c) (b) (a) - (c): Cl2, Br2, I2 in H2O (from left to right) (d) (f) (e) (d) - (e): Cl2, Br2, I2 in CH3CCl3 42.1 Characteristic Properties of the Halogens (SB p.86)

  15. Halogen Electronic configuration F 1s22s22p5 Cl 1s22s22p63s23p5 Br 1s22s22p63s23p63d104s24p5 I 1s22s22p63s23p63d104s24p64d105s25p5 At 1s22s22p63s23p63d104s24p64d104f145s25p65d106s26p5 They have an outermost shell electronic configuration of ns2np5 42.1 Characteristic Properties of the Halogens (SB p.86) Check Point 42-1 (a) Write the electronic configuration of each of the halogens. What is in common about these configuration? Answer

  16. (b) A halogen atom tends to gain an electron when forming a compound. 42.1 Characteristic Properties of the Halogens (SB p.86) Check Point 42-1 (cont’d) (b) Does a halogen atom gain or lose an electron more readily when forming a compound? Answer

  17. (c) Going down the halogen group, as the sizes of the halogen atoms increase, radiation of lower frequency is absorbed. For example, for the first member of the halogen group (i.e. fluorine), radiation of high frequency (i.e. blue light) is absorbed, hence fluorine appears yellow. For iodine, radiation of low frequency (i.e. yellow light) is absorbed, thus it appears violet. 42.1 Characteristic Properties of the Halogens (SB p.86) Check Point 42-1 (cont’d) (c) The colour of halogens darkens down the group. Explain why. Answer

  18. 42.2 Variation in Properties of the Halogens (SB p.87) Physical properties of the halogens

  19. 42.2 Variation in Properties of the Halogens (SB p.87) Variation in Physical Properties Melting point and boiling point • Increases down the group • Depends on the strength of van der Waals’ forces (i.e. instantaneous dipole-induced dipole interaction) •  molecular size increases, electron clouds get larger and more easily polarized • instantaneous dipole-induced dipole interaction becomes stronger •  more energy is needed to separate the molecules

  20. 42.2 Variation in Properties of the Halogens (SB p.88) Electronegativity • Decreases down the group • ∵ atomic size increases down the group •  Increasing no. of electron shells creates greater screening effect •  The tendency to attract bonding electrons to itself decreases

  21. 42.2 Variation in Properties of the Halogens (SB p.88) Electron Affinity • Reach maximum at chlorine • ∵ atomic size increases down the group •  Decrease in effective nuclear charge •  The tendency to attract additional electrons decreases • Fluorine’s E.A. is unexpectedly low • ∵smallest in size •  Addition of electrons introduces significant electron-electron repulsion

  22. 42.2 Variation in Properties of the Halogens (SB p.88) Bond Enthalpy Bond enthalpy is the enthalpy change when one mole of covalent bond in gaseous species is broken X2(g)  2X(g)

  23. 42.2 Variation in Properties of the Halogens (SB p.88) • Decrease down the group except F2 • ∵ increase in atomic size, increase in bond length •  Decrease in attractive force between atoms •  Decrease in bond enthalpy • The bond enthalpy of F2 is unexpectedly low • ∵ smallest in size results in very short F–F bond •  Significant electron repulsion between non-bonding electrons •  F – F bond is weakened, less energy is required to break the bond

  24. (a) Electron affinity is the enthalpy change when one mole of electrons is added to one mole of atoms or ions in the gaseous phase. The electron affinity reaches the maximum at chlorine. It is because the increase in atomic size and number of electron shells further down the group leads to a decrease in effective nuclear charge. The tendency of halogens to attract additional electrons therefore decreases as the group is descended. Besides, fluorine has an unexpectedly low electron affinity because fluorine is the smallest member of halogens. The addition of an electron introduces a significant electron-electron repulsion. Its electron affinity is therefore lower than expected. 42.2 Variation in Properties of the Halogens (SB p.89) Check Point 42-2 (a) Explain the term “electron affinity”. How does it vary among the halogens? Answer

  25. (b) (i) Increase (ii) Increase (iii) Increase (iv) Decrease (v) Increase 42.2 Variation in Properties of the Halogens (SB p.89) Check Point 42-2 (b) What is the trend down the group for each of the following physical properties of the halogens? (i) atomic radius (ii) ionic radius (iii) melting point (iv) electronegativity (v) colour intensity Answer

  26. 42.2 Variation in Properties of the Halogens (SB p.89) Variation in Chemical Properties • Halogens : • The most reactive non-metallic elements • Tends to attract 1 e– to complete the octet • Highly electronegative and have high electron affinity values • Strong oxidizing agents • Fluorine is the most reactive halogen • ∵ abnormal low bond enthalpy

  27. 42.2 Variation in Properties of the Halogens (SB p.89) Relative Oxidizing Power of Halogens Reaction with Sodium • All halogens combine with Na directly to form sodium halides • 2Na(s) + F2(g)  2NaF(s) • 2Na(s) + Cl2(g)  2NaCl(s) H = –411 kJ • 2Na(s) + Br2(g)  2NaBr(s) H = –360 kJ • 2Na(s) + I2(g)  2NaI(s) H = –288 kJ • Na & F2 react explosively to form NaF2 • Na & Cl2 react violently to form NaCl • Na burns in Br2 & I2 vapours to form NaBr & NaI respectively

  28. 42.2 Variation in Properties of the Halogens (SB p.90) Reaction with Iron(II) Ions • If the overall standard electrode potential is positive value, the reaction is spontaneous • Aqueous Cl2 & Br2 oxidize green Fe2+(aq) to yellowish brown Fe3+(aq) • 2Fe2+(aq) + Cl2(aq)  2Fe3+(aq) + 2Cl–(aq) Ecell = +0.59 V • 2Fe2+(aq) + Br2(aq)  2Fe3+(aq) + 2Br–(aq) Ecell = +0.30 V • 2Fe2+(aq) + I2(aq)  2Fe3+(aq) + 2I–(aq) Ecell = –0.23 V

  29. 42.2 Variation in Properties of the Halogens (SB p.90) Reaction with Phosphorus • All halogens react with red phosphorus • As P has low-lying vacant 3d orbitals, the compound formed can have more than 8 electrons in the outermost shell • P can form PX3 and PX5 depending on the oxidation power of X2

  30. Due to very strong oxidizing powerof F2, only PF5isformed • 2P(s) + 5F2(g)  2PF5(s) • Cl2 has strong oxidizing power, the major product is PCl5 • 2P(s) + 5Cl2(g)  2PCl5(g) • PCl3 can be formed under certain conditions • Br2 & I2 are relatively mild oxidizing agents, the major product would be PBr3 & PI3 respectively • 2P(s) + 3Br2(l)  2PBr3(l) • 2P(s) + 3I2(s)  2PI3(s)  42.2 Variation in Properties of the Halogens (SB p.90)

  31. 42.2 Variation in Properties of the Halogens (SB p.91)

  32. 42.2 Variation in Properties of the Halogens (SB p.91) • The above reactions show that fluorine is the most electronegative and most reactive elementamong halogens •  React readily with all substances and bring out the highest oxidation state of other elements in the product • The relative oxidizing power: F2 > Cl2 > Br2 > I2

  33. 42.2 Variation in Properties of the Halogens (SB p.91) • Fluorine is one of the few elements which can combine with noble gas directly∵ extremely strong oxidizing power • Depending on condition and amount of reagent, xenon can form XeF2, XeF4 or XeF6 • Xe(g) + F2(g)  XeF2(s)Xe(g) + 2F2(g)  XeF4(s)Xe(g) + 3F2(g)  XeF6(s) • All of these fluorides are powerful oxidizing agents

  34. 42.2 Variation in Properties of the Halogens (SB p.92) Disproportionation of the Halogens in Alkalis Reaction with Water • F2 oxidizes H2O to form HF and O2 vigorously • 2F2(g) + 2H2O(l)  4HF(aq) + O2(g) • Cl2 reacts with H2O to form HCl and HOCl (chloric(I) acid)

  35. 42.2 Variation in Properties of the Halogens (SB p.92) • The O.N. of Cl changes from 0 in Cl2 to –1 in HCl • Also, the O.N. of Cl changes from 0 in Cl2 to +1in HOCl • ∴ Cl2 is oxidized and reduced simultaneously • This is call disproportionation 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

  36. Chlorine water possesses bleaching property • ∵ OCl–oxidizes dyes to form colourless compounds • Cl2(aq) + H2O(l) 2H+(aq) + Cl–(aq) + OCl–(aq) • OCl–(aq) + dye  Cl–(aq) + (dye + O) colourless coloured 42.2 Variation in Properties of the Halogens (SB p.92) • Mixture of HCl and HOCl is called chlorine water • Chlorate(I) ion is not stable and will decompose on exposure to sunlight or high temperature • 2OCl–(aq)  2Cl–(aq) + O2(g)

  37. Br2 is slightly soluble in water • On diluting saturated bromine water, hydrolysis occurs • Br2(l) + 2H2O(l) HBr(aq) + HOBr(aq) • OBr– ion is also unstable and will form colourless compoundwhen reacting with dyes which is similar to OCl– ion • OBr–(aq) + dye  Br–(aq) + (dye + O) coloured colourless 42.2 Variation in Properties of the Halogens (SB p.92)

  38. 42.2 Variation in Properties of the Halogens (SB p.92) • Iodine does not react with water and is only slightly soluble in water • But I2 is very soluble in KI because it exists as I3– (triiodide ion) • I2(s) + KI(aq)  KI3(aq)

  39. All halogens (except F2) react with aqueous alkalis and disproportionate in alkalis • Their reactivities decrease down the group • When F2 is passed through cold and very dilute (2%) NaOH, OF2 is formed • 2F2(g) + 2NaOH(aq)  2NaF(aq) + OF2(g) + H2O(l) • When F2 is passed through hot and conc. NaOH, O2 is formed • 2F2(g) + 4NaOH(aq)  4NaF(aq) + O2(g) + 2H2O(l) –1 –1 0 2%, cold –1 0 0 –2 hot, conc. 42.2 Variation in Properties of the Halogens (SB p.93) Reaction with Alkalis

  40. Cl2 react with cold and dilute NaOH, NaCl and NaOCl are formed • Cl2(g) + 2NaOH(aq)  NaCl(aq) + NaOCl(aq) + H2O(l) • Cl2 reacts with hot and conc. NaOH, NaCl and NaClO3 are formed • 3Cl2(g) + 6NaOH(aq)  • 5NaCl(aq) + NaClO3(aq) + 3H2O(l) 0 –1 +1 cold, dilute 0 hot, conc. –1 +5 42.2 Variation in Properties of the Halogens (SB p.93)

  41. Br2 undergoes similar reactions with alkalis as Cl2 • The OBr– ion formed is not stable and disproportionates readily at room temperature • Br2(g) + 2NaOH(aq)  NaBr(aq) + NaOBr(aq) + H2O(l) • 3NaOBr(aq) 2NaBr(aq) + NaBrO3(aq) • The overall reaction: • 3Br2(g) + 6NaOH(aq)  • 5NaBr(aq) + NaBrO3(aq) + 3H2O(l) cold, dilute 0 +5 –1 cold, dilute 42.2 Variation in Properties of the Halogens (SB p.93)

  42. I2 behaves similarly as Br2 • 3I2(g) + 6NaOH(aq) 5NaI(aq) + NaIO3(aq) + 3H2O(l) • The reverse reaction is used to prepare standard iodine solution for iodometric titration by dissolving a known quantity of KIO3 in excess KI and dilute H2SO4 • KIO3(aq) + 5KI(aq) + 6H+(aq) •  3I2(aq) + 3H2O(l) + 6K+(aq) 0 –1 +5 cold, dilute 42.2 Variation in Properties of the Halogens (SB p.94)

  43. 42.2 Variation in Properties of the Halogens (SB p.94) • The iodine generated is used to oxidize reducing agents, e.g. sulphate(IV) ions and ascorbic acid (vitamin C) • Excess I2 is determined by back titration with sodium thiosulphate solution • I2(aq) + 2S2O32–(aq)  2I–(aq) + S4O62–(aq)

  44. (a) Because all of the halogens have one electron short of the octet electronic configuration, they are highly electronegative and have high electron affinity values. Therefore, they tend to attract an additional electron to complete the octet. Halogens are thus strong oxidizing agents. 42.2 Variation in Properties of the Halogens (SB p.94) Check Point 42-3 (a) Explain why halogens are strong oxidizing agents. Answer

  45. (b) (i) Cl2(aq), Cl–(aq), ClO–(aq), H+(aq), H2O(l) (ii) Br2(aq), Br–(aq), BrO–(aq), H+(aq), H2O(l) 42.2 Variation in Properties of the Halogens (SB p.94) • Check Point 42-3 (cont’d) • What chemical species are present in the following solutions? • (i) chlorine water • (ii) bromine water Answer

  46. 42.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Halogens • The reactions of halogens with halide ions follow the relative oxidizing power: F2 > Cl2 > Br2 > I2 • F2 can displace all other halogens from the halide ions • F2(g) + 2Cl–(aq)  2F–(aq) + Cl2(aq)F2(g) + 2Br–(aq)  2F–(aq) + Br2(aq)F2(g) + 2I–(aq)  2F–(aq) + I2(aq)

  47. The mixture of chlorine water with KBr(aq) (left) and KI(aq) (right) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.95) • Cl2 can displace Br2 and I2 from Br– and I– ions respectively • Cl2(g) + 2Br–(aq)  2Cl–(aq) + Br2(aq)Cl2(g) + 2I–(aq)  2Cl–(aq) + I2(aq) • Br2 can displace I2 from I– ions only • Br2(g) + 2I–(aq)  2Br–(aq) + I2(aq) • I2 displaces none of the other three

  48. 42.3 Comparative Study of the Reactions of Halide Ions (SB p.95) • The feasibility of redox reactions at standard state in aqueous solutions can be predicted by using the values of standard electrode potentials • If the Ecell of the overall reaction is a positive value, the reaction is spontaneous

  49. e.g. Br2(g) + 2I–(aq)  2Br–(aq) + I2(aq) The reaction can be considered as the combination of two oxidizing equilibria competing each other Br2(aq) + 2e– 2Br–(aq) E = +1.07V I2(aq) + 2e– 2I–(aq) E = +0.54V As Br2(aq) + 2e– 2Br–(aq) has more positive value, Br2 has a higher tendency to gain e–(stronger oxidizing power) Br2(aq) + 2e– 2Br–(aq) E = +1.07V –) I2(aq) + 2e– 2I–(aq) E = +0.54V Br2(aq) + 2I–(aq) 2Br–(aq) + I2(aq) Ecell = +0.53V Ecell of the overall reaction is +ve value, the reaction proceeds spontaneously 42.3 Comparative Study of the Reactions of Halide Ions (SB p.95)

  50. 42.3 Comparative Study of the Reactions of Halide Ions (SB p.96)

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