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New Way Chemistry for Hong Kong A-Level Book 41 1 The Halogens 42.1Characteristic Properties of the Halogens 42.2Variation in Properties of the Halogens.

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Presentation on theme: "New Way Chemistry for Hong Kong A-Level Book 41 1 The Halogens 42.1Characteristic Properties of the Halogens 42.2Variation in Properties of the Halogens."— Presentation transcript:

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2 New Way Chemistry for Hong Kong A-Level Book 41 1 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 the Anomalous Behaviour of Hydrogen Fluoride 42.5Uses of Halogens and Halogen-Containing Compounds Chapter 42

3 New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the Halogens (SB p.82) Halogen (Group VIIA): Consists of fluorine, chlorine, bromine, iodine, astatine Outermost shell electronic configuration: ns 2 np 5 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

4 New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the Halogens (SB p.83) Chlorine is a greenish- yellow gas Bromine is a reddish brown liquid Iodine is a violet black solid

5 New Way Chemistry for Hong Kong A-Level Book 44 4 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 42.1 Characteristic Properties of the Halogens (SB p.83) ElementElectronegativity F Cl Br I At Halogens have high tendency to attract an additional e – to complete the octet which has extra stability Halogens have high electronegativity values

6 New Way Chemistry for Hong Kong A-Level Book 45 5 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 Characteristic Properties of the Halogens (SB p.83) Element Electron affinity (kJ mol –1 ) F Cl Br I At –348 –364 –342 –314 –285 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

7 New Way Chemistry for Hong Kong A-Level Book 46 6 Bonding and Oxidation State The outermost shell electronic configuration of halogens: ns 2 np 5 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) 42.1 Characteristic Properties of the Halogens (SB p.84)

8 New Way Chemistry for Hong Kong A-Level Book 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

9 New Way Chemistry for Hong Kong A-Level Book 48 8 Example: Chlorine 7 outermost shell electrons  Expand the octet to form a maximum 7 covalent bonds  Oxidation state = +7 as in dichlorine heptoxide (Cl 2 O 7 )  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 Characteristic Properties of the Halogens (SB p.84)

10 New Way Chemistry for Hong Kong A-Level Book 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

11 New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the Halogens (SB p.84) “Electrons-in-boxes” representation of the electronic configuration of a halogen atom showing different oxidation states

12 New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the Halogens (SB p.85) Oxidation state of halogenIon / Compound –1 F – Cl – Br – I – HF HCl HBr HI OF 2 0F 2 Cl 2 Br 2 I 2 +1 Cl 2 O Br 2 O HOCl HOBr OCl – OBr – +3 HClO 2 ClO 2 – +4 ClO 2 BrO 2 +5 HClO 3 HBrO 3 I 2 O 5 ClO 3 – BrO 3 – HIO 3 IO 3 – +6 Cl 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 compounds

13 New Way Chemistry for Hong Kong A-Level Book 412 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 42.1 Characteristic Properties of the Halogens (SB p.85)

14 New Way Chemistry for Hong Kong A-Level Book 413 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 42.1 Characteristic Properties of the Halogens (SB p.85) Element Colour in standard state in H 2 Oin CH 3 CCl 3 F Cl Br I Pale yellow Greenish yellow Reddish brown Violet black Pale yellow Yellow Yellow (only slightly soluble) Pale yellow Yellow Orange Violet

15 New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the Halogens (SB p.86) (b) (c) (a) (a) - (c): Cl 2, Br 2, I 2 in H 2 O (from left to right) (d) (e) (f) (d) - (e): Cl 2, Br 2, I 2 in CH 3 CCl 3

16 New Way Chemistry for Hong Kong A-Level Book 415 Check Point 42-1 (a)Write the electronic configuration of each of the halogens. What is in common about these configuration? Answer 42.1 Characteristic Properties of the Halogens (SB p.86) They have an outermost shell electronic configuration of ns 2 np 5 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2 6p 5 At 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 5p 5 I 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 5 Br 1s22s22p63s23p51s22s22p63s23p5 Cl 1s22s22p51s22s22p5 F Electronic configurationHalogen

17 New Way Chemistry for Hong Kong A-Level Book 416 Check Point 42-1 (cont’d) (b)Does a halogen atom gain or lose an electron more readily when forming a compound? Answer (b) A halogen atom tends to gain an electron when forming a compound Characteristic Properties of the Halogens (SB p.86)

18 New Way Chemistry for Hong Kong A-Level Book 417 Check Point 42-1 (cont’d) (c)The colour of halogens darkens down the group. Explain why. Answer (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 Characteristic Properties of the Halogens (SB p.86)

19 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.87) Halogen Colour and state at room temperature and pressure Atomic radius (nm) Ionic radius (nm) Melting point (°C) Boiling point (°C) Density (g cm –3 ) Fluorine Chlorine Bromine Iodine Astatine Pale yellow gas Greenish yellow gas Reddish brown liquid Violet black solid — — — –220 –101 – –188 – — Physical properties of the halogens

20 New Way Chemistry for Hong Kong A-Level Book 419 Variation in Physical Properties 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 42.2 Variation in Properties of the Halogens (SB p.87) Melting point and boiling point

21 New Way Chemistry for Hong Kong A-Level Book 420 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 42.2 Variation in Properties of the Halogens (SB p.88) Electronegativity

22 New Way Chemistry for Hong Kong A-Level Book 421 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 42.2 Variation in Properties of the Halogens (SB p.88) Electron Affinity

23 New Way Chemistry for Hong Kong A-Level Book 422 Bond enthalpy is the enthalpy change when one mole of covalent bond in gaseous species is broken X 2 (g)  2X(g) 42.2 Variation in Properties of the Halogens (SB p.88) Bond Enthalpy

24 New Way Chemistry for Hong Kong A-Level Book 423 Decrease down the group except F 2 ∵ increase in atomic size, increase in bond length  Decrease in attractive force between atoms  Decrease in bond enthalpy The bond enthalpy of F 2 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 42.2 Variation in Properties of the Halogens (SB p.88)

25 New Way Chemistry for Hong Kong A-Level Book 424 Check Point 42-2 (a)Explain the term “electron affinity”. How does it vary among the halogens? Answer (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 Variation in Properties of the Halogens (SB p.89)

26 New Way Chemistry for Hong Kong A-Level Book 425 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 (b)(i)Increase (ii)Increase (iii)Increase (iv)Decrease (v)Increase 42.2 Variation in Properties of the Halogens (SB p.89)

27 New Way Chemistry for Hong Kong A-Level Book 426 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 42.2 Variation in Properties of the Halogens (SB p.89)

28 New Way Chemistry for Hong Kong A-Level Book 427 All halogens combine with Na directly to form sodium halides 2Na(s) + F 2 (g)  2NaF(s) 2Na(s) + Cl 2 (g)  2NaCl(s)  H = –411 kJ 2Na(s) + Br 2 (g)  2NaBr(s)  H = –360 kJ 2Na(s) + I 2 (g)  2NaI(s)  H = –288 kJ Na & F 2 react explosively to form NaF 2 Na & Cl 2 react violently to form NaCl Na burns in Br 2 & I 2 vapours to form NaBr & NaI respectively 42.2 Variation in Properties of the Halogens (SB p.89) Relative Oxidizing Power of Halogens Reaction with Sodium

29 New Way Chemistry for Hong Kong A-Level Book 428 If the overall standard electrode potential is positive value, the reaction is spontaneous  Aqueous Cl 2 & Br 2 oxidize green Fe 2+ (aq) to yellowish brown Fe 3+ (aq) 2Fe 2+ (aq) + Cl 2 (aq)  2Fe 3+ (aq) + 2Cl – (aq)E cell = V 2Fe 2+ (aq) + Br 2 (aq)  2Fe 3+ (aq) + 2Br – (aq)E cell = V 2Fe 2+ (aq) + I 2 (aq)  2Fe 3+ (aq) + 2I – (aq)E cell = –0.23 V 42.2 Variation in Properties of the Halogens (SB p.90) Reaction with Iron(II) Ions Half equationStandard electrode potential (V) Cl 2 (aq) + 2e –  2Cl – (aq) Br 2 (aq) + 2e –  2Br – (aq) I 2 (aq) + 2e –  2I – (aq) Fe 2+ (aq) + e –  Fe 3+ (aq)

30 New Way Chemistry for Hong Kong A-Level Book 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 PX 3 and PX 5 depending on the oxidation power of X 2

31 New Way Chemistry for Hong Kong A-Level Book 430 Due to very strong oxidizing power of F 2, only PF 5 is formed 2P(s) + 5F 2 (g)  2PF 5 (s) Cl 2 has strong oxidizing power, the major product is PCl 5 2P(s) + 5Cl 2 (g)  2PCl 5 (g) PCl 3 can be formed under certain conditions Br 2 & I 2 are relatively mild oxidizing agents, the major product would be PBr 3 & PI 3 respectively 2P(s) + 3Br 2 (l)  2PBr 3 (l) 2P(s) + 3I 2 (s)  2PI 3 (s)  42.2 Variation in Properties of the Halogens (SB p.90)

32 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.91) ReactantF2F2 Cl 2 Br 2 I2I2 Sodium Reacts explosively to form NaF Reacts violently to form NaCl Na burns continuously in Br 2 vapour to form NaBr Na burns continuously in I 2 vapour to form NaI Iron(II) solution Reaction unsuitable to be carried out as water is oxidized to H 2 O 2 and some OF 2 Green Fe 2+ (aq) solution oxidized to yellowish brown Fe 3+ (aq) solution Solution remains green as Fe 2+ (aq) is not oxidized by I 2 Phosphorus Reacts spontaneously to form PF 5 PCl 5 and PCl 3 are formed on heating Reacts spontaneously to form PBr 3 Reacts spontaneously to form PI 3

33 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.91) The above reactions show that fluorine is the most electronegative and most reactive element among halogens  React readily with all substances and bring out the highest oxidation state of other elements in the product The relative oxidizing power: F 2 > Cl 2 > Br 2 > I 2

34 New Way Chemistry for Hong Kong A-Level Book 433 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 XeF 2, XeF 4 or XeF 6 Xe(g) + F 2 (g)  XeF 2 (s) Xe(g) + 2F 2 (g)  XeF 4 (s) Xe(g) + 3F 2 (g)  XeF 6 (s) All of these fluorides are powerful oxidizing agents 42.2 Variation in Properties of the Halogens (SB p.91)

35 New Way Chemistry for Hong Kong A-Level Book 434 F 2 oxidizes H 2 O to form HF and O 2 vigorously 2F 2 (g) + 2H 2 O(l)  4HF(aq) + O 2 (g) 42.2 Variation in Properties of the Halogens (SB p.92) Disproportionation of the Halogens in Alkalis Reaction with Water Cl 2 reacts with H 2 O to form HCl and HOCl (chloric(I) acid)

36 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.92) The O.N. of Cl changes from 0 in Cl 2 to –1 in HCl Also, the O.N. of Cl changes from 0 in Cl 2 to +1in HOCl ∴ Cl 2 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

37 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.92) Chlorine water possesses bleaching property ∵ OCl – oxidizes dyes to form colourless compounds Cl 2 (aq) + H 2 O(l) 2H + (aq) + Cl – (aq) + OCl – (aq) OCl – (aq) + dye  Cl – (aq) + (dye + O) coloured colourless 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) + O 2 (g)

38 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.92) Br 2 is slightly soluble in water On diluting saturated bromine water, hydrolysis occurs Br 2 (l) + 2H 2 O(l)HBr(aq) + HOBr(aq) coloured colourless OBr – ion is also unstable and will form colourless compound when reacting with dyes which is similar to OCl – ion OBr – (aq) + dye  Br – (aq) + (dye + O)

39 New Way Chemistry for Hong Kong A-Level Book 438 Iodine does not react with water and is only slightly soluble in water But I 2 is very soluble in KI because it exists as I 3 – (triiodide ion) I 2 (s) + KI(aq)  KI 3 (aq) 42.2 Variation in Properties of the Halogens (SB p.92)

40 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.93) Reaction with Alkalis All halogens (except F 2 ) react with aqueous alkalis and disproportionate in alkalis Their reactivities decrease down the group When F 2 is passed through cold and very dilute (2%) NaOH, OF 2 is formed 2F 2 (g) + 2NaOH(aq)  2NaF(aq) + OF 2 (g) + H 2 O(l) When F 2 is passed through hot and conc. NaOH, O 2 is formed 2F 2 (g) + 4NaOH(aq)  4NaF(aq) + O 2 (g) + 2H 2 O(l) 0 –1 2%, cold 0 0 –2 hot, conc. –1

41 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.93) Cl 2 react with cold and dilute NaOH, NaCl and NaOCl are formed Cl 2 (g) + 2NaOH(aq)  NaCl(aq) + NaOCl(aq) + H 2 O(l) Cl 2 reacts with hot and conc. NaOH, NaCl and NaClO 3 are formed 3Cl 2 (g) + 6NaOH(aq)  5NaCl(aq) + NaClO 3 (aq) + 3H 2 O(l) 0 +1 –1 cold, dilute 0 +5 –1 hot, conc.

42 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.93) Br 2 undergoes similar reactions with alkalis as Cl 2 The OBr – ion formed is not stable and disproportionates readily at room temperature Br 2 (g) + 2NaOH(aq)  NaBr(aq) + NaOBr(aq) + H 2 O(l) 3NaOBr(aq)2NaBr(aq) + NaBrO 3 (aq) The overall reaction: 3Br 2 (g) + 6NaOH(aq)  5NaBr(aq) + NaBrO 3 (aq) + 3H 2 O(l) cold, dilute 0 +5 –1 cold, dilute

43 New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the Halogens (SB p.94) I 2 behaves similarly as Br 2 3I 2 (g) + 6NaOH(aq) 5NaI(aq) + NaIO 3 (aq) + 3H 2 O(l) The reverse reaction is used to prepare standard iodine solution for iodometric titration by dissolving a known quantity of KIO 3 in excess KI and dilute H 2 SO 4 KIO 3 (aq) + 5KI(aq) + 6H + (aq)  3I 2 (aq) + 3H 2 O(l) + 6K + (aq) 0 +5 –1 cold, dilute

44 New Way Chemistry for Hong Kong A-Level Book 443 The iodine generated is used to oxidize reducing agents, e.g. sulphate(IV) ions and ascorbic acid (vitamin C) Excess I 2 is 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) 42.2 Variation in Properties of the Halogens (SB p.94)

45 New Way Chemistry for Hong Kong A-Level Book 444 Check Point 42-3 (a)Explain why halogens are strong oxidizing agents. Answer (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 Variation in Properties of the Halogens (SB p.94)

46 New Way Chemistry for Hong Kong A-Level Book 445 Check Point 42-3 (cont’d) (b)What chemical species are present in the following solutions? (i)chlorine water (ii)bromine water Answer (b)(i)Cl 2 (aq), Cl – (aq), ClO – (aq), H + (aq), H 2 O(l) (ii)Br 2 (aq), Br – (aq), BrO – (aq), H + (aq), H 2 O(l) 42.2 Variation in Properties of the Halogens (SB p.94)

47 New Way Chemistry for Hong Kong A-Level Book 446 The reactions of halogens with halide ions follow the relative oxidizing power: F 2 > Cl 2 > Br 2 > I 2 F 2 can displace all other halogens from the halide ions F 2 (g) + 2Cl – (aq)  2F – (aq) + Cl 2 (aq) F 2 (g) + 2Br – (aq)  2F – (aq) + Br 2 (aq) F 2 (g) + 2I – (aq)  2F – (aq) + I 2 (aq) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Halogens

48 New Way Chemistry for Hong Kong A-Level Book 447 Cl 2 can displace Br 2 and I 2 from Br – and I – ions respectively Cl 2 (g) + 2Br – (aq)  2Cl – (aq) + Br 2 (aq) Cl 2 (g) + 2I – (aq)  2Cl – (aq) + I 2 (aq) Br 2 can displace I 2 from I – ions only Br 2 (g) + 2I – (aq)  2Br – (aq) + I 2 (aq) I 2 displaces none of the other three 42.3 Comparative Study of the Reactions of Halide Ions (SB p.95) The mixture of chlorine water with KBr(aq) (left) and KI(aq) (right)

49 New Way Chemistry for Hong Kong A-Level Book 448 The feasibility of redox reactions at standard state in aqueous solutions can be predicted by using the values of standard electrode potentials If the E cell of the overall reaction is a positive value, the reaction is spontaneous 42.3 Comparative Study of the Reactions of Halide Ions (SB p.95)

50 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.95) e.g. Br 2 (g) + 2I – (aq)  2Br – (aq) + I 2 (aq) The reaction can be considered as the combination of two oxidizing equilibria competing each other Br 2 (aq) + 2e – 2Br – (aq)E = +1.07V I 2 (aq) + 2e – 2I – (aq)E = +0.54V As Br 2 (aq) + 2e – 2Br – (aq) has more positive value, Br 2 has a higher tendency to gain e – (stronger oxidizing power) Br 2 (aq) + 2e – 2Br – (aq)E = +1.07V –)I 2 (aq) + 2e – 2I – (aq)E = +0.54V Br 2 (aq) + 2I – (aq)2Br – (aq) + I 2 (aq)E cell = +0.53V E cell of the overall reaction is +ve value, the reaction proceeds spontaneously

51 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.96) Aqueous solution Halogen added F2F2 Cl 2 Br 2 I2I2 F–F– No reaction Cl – A pale yellow solution is formed (Cl 2 is released) No reaction Br – A yellow solution is formed (Br 2 is released) No reaction I–I– A yellowish brown solution is formed (I 2 is released) No reaction

52 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.96) It is sometimes difficult to determine whether certain reactions have been taken place by observing the colour change only e.g.a solution containing a small amount of Br 2 can appear colourless, and a very dilute solution of iodine in KI looks like a bromine solution

53 New Way Chemistry for Hong Kong A-Level Book 452 Solution: Adding a small amount of 1,1,1- trichloroethane into the reaction mixture. Any bromine or iodine present will dissolve in purely covalent organic solvents readily The organic layer becomes orange if bromine is present The organic layer becomes violet if iodine is present 42.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Addition of CH 3 CCl 3 : formation of orange bottom layer in Br 2 -containing solution (left); formation of violet bottom layer in I 2 - containing solution (right)

54 New Way Chemistry for Hong Kong A-Level Book 453 Check Point 42-4 (a)Is bromine and iodine more soluble in water or 1,1,1- trichloroethane? Why? Answer (a) Both bromine and iodine are more soluble in 1,1,1-trichloroethane than in water. It is because they consist of non-polar molecules and are less soluble in polar solvents like water Comparative Study of the Reactions of Halide Ions (SB p.97)

55 New Way Chemistry for Hong Kong A-Level Book 454 Check Point 42-4 (b)Name a simple way to increase the solubility of iodine in water. Answer 42.3 Comparative Study of the Reactions of Halide Ions (SB p.97) (d) Dissolve some potassium (or sodium) iodide in water. Iodine will be more soluble in aqueous iodide than water because of the following equilibrium: I 2 (aq) + I – (aq)I 3 – (aq)

56 New Way Chemistry for Hong Kong A-Level Book 455 Concentrated sulphuric(VI) acid : an oxidizing acid  Exhibit oxidizing and acidic properties Conc. H 2 SO 4 reacts with F – and Cl – to give HF and HCl NaF(s) + H 2 SO 4 (l)  NaHSO 4 (s) + HF(g) NaCl(s) + H 2 SO 4 (l)  NaHSO 4 (s) + HCl(g) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.97) Reactions with Concentrated Sulphuric(VI) Acid

57 New Way Chemistry for Hong Kong A-Level Book 456 Conc. H 2 SO 4 react with Br – and I – to give SO 2 and H 2 S respectively For Br – : NaBr(s) + H 2 SO 4 (l)  NaHSO 4 (s) + HBr(g) 2HBr(g) + H 2 SO 4 (l)  SO 2 (g) + Br 2 (g) + 2H 2 O(l) Overall equation: 2NaBr(s) + 3H 2 SO 4 (l)  2NaHSO 4 (s) + SO 2 (g) + Br 2 (g) + 2H 2 O(l) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.97)

58 New Way Chemistry for Hong Kong A-Level Book 457 For I – : NaI(s) + H 2 SO 4 (l)  NaHSO 4 (s) + HI(g) 8HI(g) + H 2 SO 4 (l)  H 2 S(g) + 4I 2 (g) + 4H 2 O(l) Overall equation: 8NaI(s) + 9H 2 SO 4 (l)  8NaHSO 4 (s) + H 2 S(g) + 4I 2 (g) + 4H 2 O(l) Different products are formed in these reactions ∵ HBr and HI are oxidized by H 2 SO 4 to Br 2 and I 2 respectively

59 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.98) IonAction of conc. H 2 SO 4 Product Confirmatory test of the product Cl – Steamy fumes in cold No green gas evolved even on heating HCl White fumes with ammonia Blue litmus paper turns red but not bleached Br – Steamy fumes in cold Pungent smell Brown gas evolved on warming HBrWhite fumes with ammonia SO 2 Turns orange dichromate(VI) solution green Br 2 Red colour in hexane I–I– Steamy violet fumes in cold Bad egg smell HIWhite fumes with ammonia H2SH2S Turns lead(II) ethanoate paper black I2I2 Violet colour in hexane Reducing power of halides : Cl – < Br – < I – Action of concentrated sulphuric(VI) acid on halides

60 New Way Chemistry for Hong Kong A-Level Book 459 H 3 PO 4 reacts with halides to give the corresponding hydrogen halides ∵ H 3 PO 4 is not an oxidizing agent General method to prepare hydrogen halides in the laboratory 3NaCl(s) + H 3 PO 4 (l)  Na 3 PO 4 (s) + 3HCl(g) 3NaBr(s) + H 3 PO 4 (l)  Na 3 PO 4 (s) + 3HBr(g) 3NaI(s) + H 3 PO 4 (l)  Na 3 PO 4 (s) + 3HI(g) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.98) Reactions with Phosphoric(V) Acid

61 New Way Chemistry for Hong Kong A-Level Book 460 Ion Action of conc. H 3 PO 4 Product Confirmatory test of the product Cl – Steamy fumes on warming HCl White fumes with ammonia Br – HBr I–I– HI 42.3 Comparative Study of the Reactions of Halide Ions (SB p.98) Action of concentrated phosphoric(V) acid on halides

62 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.99) Reactions with Silver Ions white precipitate pale yellow precipitate yellow precipitate Aqueous solution of Cl –, Br – and I – give precipitation when reacting with AgNO 3 Characteristic test to determine the presence of halide ions (except F – ) Ag + (aq) + Cl – (aq)  AgCl(s) Ag + (aq) + Br – (aq)  AgBr(s) Ag + (aq) + I – (aq)  AgI(s)

63 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.99) Silver chloride, AgCl Silver bromide, AgBr Silver iodide, AgI

64 New Way Chemistry for Hong Kong A-Level Book 463 All AgX are insoluble in acids Dilute HNO 3 is added to prevent the precipitation of other insoluble silver compounds  Interfering ions, e.g. SO 3 2– & CO 3 2– can be removed 2H + (aq) + SO 3 2– (aq)  SO 2 (g) + H 2 O(l) 2H + (aq) + CO 3 2– (aq)  SO 2 (g) + H 2 O(l)  Formation of Ag 2 SO 3 (s) and Ag 2 CO 3 (s), which may be mistaken as silver halides, can be prevented 42.3 Comparative Study of the Reactions of Halide Ions (SB p.99)

65 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.99) Different silver halides can be identified by 1.their colours 2.their reactiond with ammonia solution AgCl dissolves readily in NH 3 (aq) AgCl(s) + 2NH 3 (aq)  [Ag(NH 3 ) 2 ] + (aq) + Cl – (aq) AgBr is slightly soluble in NH 3 (aq) AgI is not soluble in NH 3 (aq) diamminesilver(I) ion

66 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.100) When AgX are exposed to sunlight  AgCl turns grey  AgBr turns yellowish grey  AgI remains yellow light The colour changes of AgCl and AgBr are due to photochemical decomposition of AgX into their constituent elements (i.e. silver & halogens) 2AgCl(s)  2Ag(s) + Cl 2 (g) 2AgBr(s)  2Ag(s) + Br 2 (g)

67 New Way Chemistry for Hong Kong A-Level Book Comparative Study of the Reactions of Halide Ions (SB p.100) Ion Action of acidified AgNO 3 Confirmatory test of the product Effect of exposure of sunlight Effect of adding aqueous NH 3 Cl – White precipitate of AgCl Turns grey White precipitate dissolves Br – Pale yellow precipitate of AgBr Turns yellowish grey Pale yellow precipitate slightly dissolves I–I– Yellow precipitate of AgI Remains yellow Yellow precipitate does not dissolve Action of acidified silver nitrate(V) on halides

68 New Way Chemistry for Hong Kong A-Level Book 467 Check Point 42-5 (a)What would happen when the following substances are added to sodium iodide solution? Give appropriate equations, if any. (i)iron(II) sulphate(VI) solution (ii)chlorine water (iii)sodium iodate(V) solution and dilute sulphuric(VI) acid Answer (a)(i)There is no observable change. (ii)The solution turns brown. Cl 2 (aq) + 2NaI(aq)  I 2 (aq) + 2NaCl(aq) (iii)The solution turns brown. 5I – (aq) + IO 3 – (aq) 6H + (aq)  3I 2 (aq) + 3H 2 O(l) 42.3 Comparative Study of the Reactions of Halide Ions (SB p.100)

69 New Way Chemistry for Hong Kong A-Level Book 468 Check Point 42-5 (cont’d) (b)If you are given two solutions, sodium fluoride and sodium chloride, how would you distinguish them with a simple chemical test? State all observations and write equations whenever appropriate. Answer 42.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (b) Sodium chloride and sodium fluoride solutions can be distinguished with the addition of acidified silver nitrate solution. Sodium chloride solution will give a white precipitate when reacting with acidified silver nitrate solution, whereas sodium fluoride solution will not. Ag + (aq) + Cl – (aq)  AgCl(s) white ppt.

70 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.101) Acidic Properties of Hydrogen Halides Hydrogen halide Acid dissociation constant, K a (mol dm –3 ) Degree of dissociation in 0.1 M solution (%) HF HCl HBr HI 7  10 –4 1    Larger dissociation constant, greater the acidic strength Acidic strength of HX : HI > HBr > HCl >> HF HX dissolve in aqueous solvents for form acidic solutions HX(g) + H 2 O(l)H 3 O + (aq) + X – (aq)

71 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.101) Anomalous Behaviour of Hydrogen Fluoride HF has abnormally high boiling and melting point when compared with HCl, HBr and HI ∵ the presence of extensive intermolecular hydrogen bonding  Due to high electronegativity value of F 1.

72 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) 2.HF behaves as weak acid in dilute HF solution HF(l) + H 2 O(l) H 3 O + (aq) + F – (aq) K a = 7  10 –4 mol dm –3 However, in conc. HF solution, another equilibrium is established to form the complex ion [HF 2 ] – F – (aq) + HF(l) [HF 2 ] – (aq)K = 5.1 dm 3 mol –1 The equilibrium of 2nd reaction shifts to the right as [HF] increases  The equilibrium of 1st reaction also shifts to the right due to consumption of F – in 2nd reaction  Strength of acid enhanced HF behaves as strong acid at concentration around 5 M to 15 M

73 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) Other fluorides (e.g. KF) also react with HF forming acid salts containing the stable [HF 2 ] – ion KF(s) + HF(l)KHF 2 (s) Heating KHF 2 (s) reverses the reaction to give anhydrous HF 3.

74 New Way Chemistry for Hong Kong A-Level Book HF can react with glass  Used to etch glass  By coating the glass object to be etched with wax or acid-proof substance  The required pattern is made by removing the wax layer to expose the glass below  Then apply the acid Principle of etching glass: HF reacts with the silicate of glass CaSiO 3 (s) + 6HF(aq)  CaF 2 (aq) + SiF 4 (aq) + 3H 2 O(l) 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) A glass is etched by hydrofluoric acid

75 New Way Chemistry for Hong Kong A-Level Book 474 Check Point 42-6 (a)Which one in each of the following pairs is a stronger acid? Explain why. (i)dilute hydrochloric acid and dilute hydrofluoric acid (ii)concentrated hydrochloric acid and concentrated hydrofluoric acid Answer 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102)

76 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) (a) (i) Dilute hydrochloric acid is a stronger acid than dilute hydrofluoric acid, as HCl has a much larger K a value than HF. HCl(aq) + H 2 O(l)H 3 O + (aq) + Cl – (aq) K a = 1  10 6 mol dm –3 HF(l) + H 2 O(l)H 3 O + (aq) + F – (aq) K a = 7  10 –4 mol dm –3 The small K a value of HF indicates that only a small amount of HF(l) is ionized. Most of it still exist in the undissociated form. The large K a value of HCl indicates that almost all HCl(aq) are ionized in water. This can be explained by the fact that strong intermolecular hydrogen bonds are formed among HF molecules. This makes its ionization in water more difficult.

77 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) (a) (ii) Concentrated hydrofluoric acid is a stronger acid than concentrated hydrochloric acid. It is because in a more concentrated solution of hydrogen fluoride, apart from the equilibrium stated above, a second equilibrium is established with the fluoride ion forming the complex ion [HF 2 ] –. F – (aq) + HF(l) [HF 2 ] – (aq) K = 5.1 dm 3 mol –1 This equilibrium shifts to the right as the concentration of hydrogen fluoride increases. The equilibrium of the reaction stated in (a)(i) also shifts to the right due to the consumption of fluoride ions in the second equilibrium. The strength of the acid is therefore enhanced. Hence, concentrated hydrofluoric acid is effectively a strong acid.

78 New Way Chemistry for Hong Kong A-Level Book 477 Check Point 42-6 (cont’d) (b)Explain why hydrogen fluoride (i)is a liquid at room temperature; (ii)forms acid salts such as KHF 2 ; (iii)can be used to etch glass. Answer 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102)

79 New Way Chemistry for Hong Kong A-Level Book Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.102) (b) (i)HF exists as a liquid at room temperature because there is extensive hydrogen bonding formed among HF molecules. (ii)HF is able to react with other fluorides (e.g. KF), forming acid salts containing the stable [HF 2 ] – ion. KF(s) + HF(l)KHF 2 (s) (iii)The principle of etching glass by hydrofluoric acid can be explained by its reaction with the silicate of the glass forming soluble compounds. CaSiO 3 (s) + 6HF(aq)  CaF 2 (aq) + SiF 4 (aq) + 3H 2 O(l)

80 New Way Chemistry for Hong Kong A-Level Book 479 Check Point 42-6 (c)Complete and balance the following equations. (i)Xe(g) + F 2 (g)  (ii)F 2 (g) + H 2 O(l)  (iii)F 2 (g) + KOH(aq) (cold, dilute)  Answer 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.103) (c) (i)Xe(g) + F 2 (g)  XeF 2 (s) or Xe(g) + 2F 2 (g)  XeF 4 (s) or Xe(g) + 3F 2 (g)  XeF 6 (s) (ii)2F 2 (g) + 2H 2 O(l)  4HF(aq) + O 2 (g) (iii)2F 2 (g) + 2KOH(aq)  2KF(aq) + OF 2 (aq) + H 2 O(l)

81 New Way Chemistry for Hong Kong A-Level Book 480 Halogens seldom used directly ∵ high reactivity and toxicity Halogen compounds are chemically stable  Used extensively in many aspects 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.103) Fluorine Fluorine is used to make teflon (polytetrafluoroethene)  non-toxic, used as non-stick coating on frying pans Also used to make Freon (difluorodichoromethane)  Used as refrigerant and propellant for aerosols

82 New Way Chemistry for Hong Kong A-Level Book 481 The drinking water in Hong Kong is fluoridated by sodium hexafluorosilcate (Na 2 SiF 6 ) or sodium fluoride (NaF) ∵ reduce the incidence of tooth decay 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.103) Fluoride Explanation: 1.Fluoride inhibits certain enzymes, e.g. those catalyze the fermentation of sugar to lactic acid, which may dissolve tooth enamel and result in tooth decay 2.F – can replace the OH – ion in the structure of tooth enamel, and the resulting enamel is less soluble in acid, minimizing tooth decay Excess fluoride is harmful to teeth Fluoride-containing toothpaste

83 New Way Chemistry for Hong Kong A-Level Book Uses of Halogens and Halogen-containing Compounds (SB p.104) Chlorine Cl 2 is one of the raw material for the production of CHCl = CH 2 CHCl = CH 2 can be polymerized to give poly(chloroethene) (PVC) PVC is widely used in electrical insulation, pipe making, etc CH 2 =CH 2 + Cl 2  CH 2 Cl — CH 2 Cl CH 2 Cl — CH 2 Cl  CHCl=CH 2 + HCl nCHCl=CH 2  (CHCl — CH 2 ) n where n is a large number

84 New Way Chemistry for Hong Kong A-Level Book 483 Chlorine is important in manufacture of industrial and domestic bleaches Cl reacts with NaOH to give chlorine bleach Cl 2 (g) + NaOH(aq)  NaCl(aq) + NaOCl(aq) + H 2 O(l) The OCl – is responsible for the bleaching action Chlorine is used to sterilize water for domestic and industrial use, and also for swimming pools 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.104)

85 New Way Chemistry for Hong Kong A-Level Book 484 Br 2 is used in manufacture of many important organic compounds e.g. 1,2-dibromoethane (CH 2 Br — CH 2 Br) CH 2 Br — CH 2 Br is added to petrol to prevent lead deposition in car engines Lead deposition occurs when the “anti-knocking” agent, tetraethyl lead, decomposes 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.104) Bromine

86 New Way Chemistry for Hong Kong A-Level Book 485 AgBr is coated on the film for black and white photography When exposed to light, AgBr decomposes to give Ag 2AgBr(s)  2Ag(s) + Br 2 (g) When the film is developed, the unexposed silver bromide is removed by some chemicals, and the silver remains on the film as an opaque shadow 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.105) Silver Bromide

87 New Way Chemistry for Hong Kong A-Level Book 486 Iodine tincture  By dissolving I 2 in alcohol, water or potassium iodide  Used as antiseptic for cuts and wounds  Irritating 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.105) Iodine

88 New Way Chemistry for Hong Kong A-Level Book 487 AgI is extensively used in film industry, like AgBr Iodide ions are added to table salt (NaCl) to prevent goitre Iodine-131 is used in medical diagnosis to monitor and trace the flow of thyroxine from the thyroid gland 42.5 Uses of Halogens and Halogen-containing Compounds (SB p.105) Silver Iodide

89 New Way Chemistry for Hong Kong A-Level Book 488 Check Point 42-7 List some of the uses of the halogens Answer 42.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.105) Fluorine is used in the manufacture of teflon as a non-stick coating for frying pans. Chlorine is used in the manufacture of PVC, bleach and disinfectants. Bromine is used in the manufacture of 1,2-dibromoethane which is added to petrol to prevent lead deposition in car engines. Iodine is used as an antiseptic for cuts and wounds.

90 New Way Chemistry for Hong Kong A-Level Book 489 The END


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