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Chapter 6 The Periodic Table: Group7 6.1 The Halogens 6.2 Reactions of The Halogens and Their Ions.

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Presentation on theme: "Chapter 6 The Periodic Table: Group7 6.1 The Halogens 6.2 Reactions of The Halogens and Their Ions."— Presentation transcript:

1 Chapter 6 The Periodic Table: Group7 6.1 The Halogens 6.2 Reactions of The Halogens and Their Ions

2 6.1 What is the outcome from syllabus? Candidates should be able to: ♣ describe the trends in volatility and colour of chlorine, bromine and iodine ♣ interpret the volatility of the elements in terms of van der Waals’ forces ♣ describe the relative reactivity of the elements as oxidising agents ♣ describe and explain the reactions of the elements with hydrogen ♣ describe and explain the relative thermal stabilities of the hydrides and their relative stabilities in terms of bond energies

3 All the elements in Group 7 are nonmetals except for astatine, which is a radioactive metalloid. These elements are called halogens, which means “salt-former.” All of the halogens form salts with sodium and with the other alkali metals. 6.1 What is the outcome from syllabus?

4 6.1 The Halogens greenish gas red-brown liquidblack solid

5 6.1 The Halogens Covalent radius/nm T m /KT b /K Cl Br I The melting and boiling temperatures increase as going down the group, because larger atoms makes the van der Waals forces between the molecules stronger.

6 6.1 The Halogens X + e - → Xˉ halogen halide F Cl Br I Increasing oxidising power This is because the electronegativity increases as we go up the group, which means that the elements gain electrons more easily. For example: Cl 2 (aq) + 2Br ˉ (aq) → Br 2 (aq) + 2Cl ˉ (aq) reduction

7 6.1 The Halogens

8 Formation of hydrogen hylides with hydrogen H 2 (g) + Cl 2 (g) hν or heating HCl (g) vigorously reaction H 2 (g) + F 2 (g) hν or heating HF (g) hard to control rather slowly, almost no reactions H 2 (g) + Br 2 (g) hν or heating H 2 (g) + I 2 (g) hν or heating

9 6.1 The Halogens Laboratory preparations of HX CaF 2 + H 2 SO 4 (conc.) → CaSO 4 + 2HF(g) NaCl + H 2 SO 4 (conc.) → NaHSO 4 + HCl(g) ∵ 2HBr(g) + H 2 SO 4 (conc.) → SO 2 (g) + 2H 2 O(g) + Br 2 (l) 8HI(g) + H 2 SO 4 (conc.) → H 2 S(g) + 4H 2 O(g) + 4I 2 (l) ∴ NaBr(s) + H 3 PO 4 (l) → NaH 2 PO 4 (aq) + HBr(g) NaI(s) + H 3 PO 4 (l) → NaH 2 PO 4 (aq) + HI(g)  

10 6.1 The Halogens HF Hydrogen fluoride Hydrogen chloride Hydrogen bromide Hydrogen iodide HCl HBr HI H2OH2O H2OH2O H2OH2O H2OH2O Hydrofluoric acid Hydrochloric acid Hydrobromic acid Hydroiodic acid Strong acid Weak acid Acidity of hydrogen hylides in aqueous solution The strong H—F bond must be broken to release H +

11 6.1 The Halogens Bond Tabulated bond energy/kJ mol -1 H-F568 H-Cl432 H-Br366 H-I298 Thermal Stability of HX 2HX(g) → H 2 (g) + X 2 (g,l,s) HI: easily decomposed. HBr: may or may not decompose depending on the exact temperature of the wire. HCl and HF: not decomposed. By plunging a red-hot wire into a test tube of the gas:

12 6.1 The Halogens √ √

13 6.2 What is the outcome from syllabus? ♣ describe and explain the reactions of halide ions with aqueous silver ions followed by: (i) aqueous ammonia (ii) concentrated sulphuric acid ♣ recognise the industrial importance and environmental significance of the halogens and their compounds, (e.g. for bleaches; PVC; halogenated hydrocarbons as solvents, refrigerants and in aerosols) ♣ explain the use of chlorine in water purification ♣ describe and interpret in terms of changes of oxidation number the reaction of chlorine with cold, and with hot, aqueous sodium hydroxide Candidates should be able to: ♣ outline a method for the manufacture of chlorine from brine by a diaphragm cell

14 6.2 Reactions of the Halogens and Their Ions

15 Test for halide ions The presence of Cl - (aq), Br - (aq) and I - (aq) can be confirmed by adding a few drops of silver nitrate solution (fluorides are soluble): AgNO 3 (aq) + X - (aq) AgX(s) + NO 3 - (aq) Silver halideColour ChlorideWhite BromideCream IodideYellow Silver chloride and bromide dissolve in concentrated ammonia, but the iodide does not. AgBr(s) + 2NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) + Br - (aq)

16 6.1 Reactions of Halogens and their ions Reactions of halide ions with conc. H 2 SO 4 CaF 2 + H 2 SO 4 (conc.) → CaSO 4 + 2HF(g) NaCl + H 2 SO 4 (conc.) → NaHSO 4 + HCl(g) NaBr(s) + H 2 SO 4 (conc.) → NaHSO 4 + HBr(g) 2HBr(g) + H 2 SO 4 (conc.) → SO 2 (g) + 2H 2 O(g) + Br 2 (l) NaI(s) + H 2 SO 4 (conc.) → NaHSO 4 + HI(g) 8HI(g) + H 2 SO 4 (conc.) → H 2 S(g) + 4H 2 O(g) + 4I 2 (l) With bromides and iodides, a redox reaction occurs:

17 6.2 Reactions of the Halogens and Their Ions Colours of Silver halides Ag + (aq) + Cl - (aq) AgCl(s) Ag + (aq) + Br - (aq) AgBr(s) Ag + (aq) + I - (aq) AgI(s)

18 6.2 Manufacture of Chlorine from brine An Anode (+): 2Cl - (aq) Cl 2 (g) + 2e - At Cathode (-): 2H 2 O(l) + 2e - 2OH - (aq) + H 2 (g) Overall: 2NaCl(aq) + 2H 2 O(l) Cl 2 (g) + H 2 (g) + 2NaOH(aq)

19 6.2 Reactions of Halogens and their ions Reactions of chlorine with Alkalis Cold dilute alkali: Cl 2 (g) + 2NaOH(aq) Cl - (aq) + ClO - (aq) + H 2 O(l) Hot concentrated alkali: 3Cl 2 (g) + 6NaOH(aq) 5Cl - (aq) + ClO 3 - (aq) + H 2 O(l) Both examples of disproportionation Cl - : Cl (-I) Cl 2 : Cl (0) ClO - : Cl (+I) ClO 3 - : Cl (+V) (Roman numerals: I = 1; V = 5)

20 6.2 Reactions of Halogens and their ions Reactions of chlorine with Alkalis Cold dilute alkali: Cl 2 (g) + 2NaOH(aq) Cl - (aq) + ClO - (aq) + H 2 O(l) Hot concentrated alkali: 3Cl 2 (g) + 6NaOH(aq) 5Cl - (aq) + ClO 3 - (aq) + H 2 O(l) Both examples of disproportionation Cl - : Cl (-I) Cl 2 : Cl (0) ClO - : Cl (+I) chlorate(I) ClO 3 - : Cl (+V) chlorate(V) (Roman numerals: I = 1; V = 5)

21 6.2 Reactions of Halogens and their ions Commercial uses of halogens and their compounds Chlorine is used in water purification as it destroys harmful bacteria that could accumulate in old and unrecycled drinking water. Sodium chlorate(I), NaClO(aq) is used in bleaches. Halogenated hydrocarbons are used as solvents (dichloromethane CH 2 Cl 2 dissolves many organic compounds). CFCs were used as refrigerants and aerosols; section 12.3 They do not contain hydrogen atoms! Some are used as anaesthetics: Fluothane - CF 3 CHBr.


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