2. New Way Chemistry for Hong Kong A-Level Book 41 1 Sulphur and its Compound 44.1Introduction 44.2Burning of Sulphur 44.3Sulphur Dioxide 44.4Sulphuric(VI) Acid Chapter 44

3. New Way Chemistry for Hong Kong A-Level Book 42 2 44.1 Introduction (SB p.128) Sulphur (second member of Group VIA): Electronic configuration: 1s 2 2s 2 2p 6 3s 2 3p 4 Non-metal, solid at room temp. Low melting point Poor conductor of heat and non-conductor of electricity Occurs as free S 8 moleclues Covalent radius (nm)0.104 Melting point (°C)113 Boiling point (°C)445 Bond enthalpy (kJ mol –1 )+264 First ionization enthalpy (kJ mol –1 ) 1 000 Electron affinity (kJ mol –1 ) –194 Electronegativity2.5 Some information about sulphur

4. New Way Chemistry for Hong Kong A-Level Book 43 3 44.1 Introduction (SB p.128) Sulphur Five allotropes: rhombic, monoclinic, plastic, colloidal and flowers Appearance of rhombic sulphur Crystal of rhombic sulphur Rhombic sulphur (transparent yellow crystals) is stable up to 96°C Consists of 8 S atom covalently bonded in crown-shaped S 8 ring Insoluble in water but soluble in organic solvents

5. New Way Chemistry for Hong Kong A-Level Book 44 4 44.1 Introduction (SB p.129) Monoclinic sulphur (amber yellow crystals) is stable between 95.5°C and 119°C (also composed of S 8 molecules) Plastic sulphur is formed in chemical reactions Crystal of monoclinic sulphur Plastic sulphur

6. New Way Chemistry for Hong Kong A-Level Book 45 5 44.2 Burning of Sulphur (SB p.130) When sulphur is heated, the S 8 molecules vibrate more and more vigorously. At the melting point, they can move freely. The sulphur formed contains S 8 rings which can easily flow past each other. It is therefore mobile. 1. The yellow solid melts between 113°C and 119°C, and changes to a yellowish brown liquid ExplanationObservation

7. New Way Chemistry for Hong Kong A-Level Book 46 6 44.2 Burning of Sulphur (SB p.130) The S 8 rings are broken by heat energy to form short chains of eight atoms. They link together to form long polymeric chains. These get tangled and are unable to move past each other easily. Thus the liquid becomes more and more viscous. At 190°C, the liquid consists mainly of very long polymeric chains. The liquid thus reaches the maximum viscosity. 2. On further heating, the liquid darkens and becomes more viscous. At about 190°C, it appears dark reddish brown. It reaches its maximum viscosity and will not run out even if the test tube is inverted. ExplanationObservation

8. New Way Chemistry for Hong Kong A-Level Book 47 7 44.2 Burning of Sulphur (SB p.130) When the temperature increases further, the very long polymeric chains break up into shorter chains. These are less tangled, so the liquid becomes less viscous and more mobile again. The boiling point of sulphur is 445°C, so sulphur boils at 445°C. The molten sulphur and the sulphur vapour both consists mainly of S 8 rings, together with a small proportion of short chains. 3. When heated still further, the liquid continues to darken, but becomes less and less viscous. The sulphur boils at 445°C, giving off yellow sulphur vapour. The sulphur appears as a very dark reddish brown liquid which is quite mobile. ExplanationObservation

9. New Way Chemistry for Hong Kong A-Level Book 48 8 44.2 Burning of Sulphur (SB p.131) In the presence of O 2, sulphur burns with a dull blue flame to form SO 2 S(s) + O 2 (g)  SO 2 (g) Traces of misty SO 3 are also formed  This experiment should be carried out in a fume cupboard, as SO 2 (g) is toxic with a characteristic pungent and choking smell

10. New Way Chemistry for Hong Kong A-Level Book 49 9 Check Point 44-1 Give the structures of the following compounds or ions. (a)Sulphur dioxide (b)Sulphur trioxide (c)Sulphate(VI) ion (d)Sulphuric(VI) acid (e)Sulphur hexafluoride Answer 44.2 Burning of Sulphur (SB p.131) (a)Sulphur Dioxide (b)Sulphur trioxide

11. New Way Chemistry for Hong Kong A-Level Book 410 44.2 Burning of Sulphur (SB p.131) (c)Sulphate(VI) ion (d)Sulphuric(VI) acid

12. New Way Chemistry for Hong Kong A-Level Book 411 44.2 Burning of Sulphur (SB p.131) (e) Sulphur hexafluoride

13. New Way Chemistry for Hong Kong A-Level Book 412 44.3 Sulphur Dioxide (SB p.131) Sulphur dioxide (SO 2 ) Colourless, toxic gas with pungent and choking smell Readily liquefied under pressure Very soluble in water and different ions H + (aq), HSO 3 – (aq) and SO 3 2– (aq) are formed Aqueous solution of SO 2 is acidic –H + +H + –H + +H + SO 2 (g) + H 2 O(l)H 2 SO 3 (aq)HSO 3 – (aq)SO 3 2– (aq) sulphuric(IV) acid Hydrogen- sulphate(IV) acid sulphate(IV) ion The V-shaped SO 2 molecule

14. New Way Chemistry for Hong Kong A-Level Book 413 44.3 Sulphur Dioxide (SB p.131) SO 2 is an acidic gaseous pollutant SO 2 dissolves in raindrops to form acid rain which damages plants and buildings Highly irritating and have harmful effects on the respiratory system of humans SO 2 will be oxidized to SO 3 by atmospheric O 2 2SO 2 (g) + O 2 (g)2SO 3 (g) SO 3 is highly irritating too and it dissolves in rain water to form H 2 SO 4 SO 3 (g) + H 2 O(l)  H 2 SO 4 (aq)

15. New Way Chemistry for Hong Kong A-Level Book 414 Reducing Properties of Sulphur Dioxide 44.3 Sulphur Dioxide (SB p.132) SO 2 in aqueous solution (i.e. sulphuric(IV) acid (H 2 SO 3 )) is powerful reducing agent SO 2 (g) + H 2 O(l) H 2 SO 3 (aq) 2H + (aq) + SO 3 2– (aq) +4+6 SO 3 2– ion acts as an electron donor and reacts with electron acceptors to give SO 4 2– ion SO 3 2– (aq) + H 2 O(l)  SO 4 2– (aq) + 2H + (aq) + 2e – The O.N. of sulphur increases from +4 to +6 when SO 2 changes to SO 4 2– The e – are accepted by the oxidizing agent

16. New Way Chemistry for Hong Kong A-Level Book 415 44.3 Sulphur Dioxide (SB p.132) Reaction with Manganate(VII) Ion MnO 4 – is a strong oxidizing agent, reacts with a reducing agent in an acidic medium to give Mn 2+ MnO 4 – (aq) + 8H + (aq) + 5e –  Mn 2+ (aq) + 4H 2 O(l) purple colourless Reaction between aqueous SO 2 and MnO 4 – : 2MnO 4 – (aq) + 5SO 3 2– (aq) + 6H + (aq)  2Mn 2+ (aq) + 5SO 4 2– (aq) + 3H 2 O(l) Addition of SO 3 2-

17. New Way Chemistry for Hong Kong A-Level Book 416 44.3 Sulphur Dioxide (SB p.132) Reaction with Dichromate(VI) Ion Cr 2 O 7 2– is a strong oxidizing agent, reacts with a reducing agent in an acidic medium to give Cr 3+ Cr 2 O 7 2– (aq) + 14H + (aq) + 6e –  2Cr 3+ (aq) + 7H 2 O(l) orange green Reaction between aqueous SO 2 and Cr 2 O 7 2– : Cr 2 O 7 2– (aq) + 3SO 3 2– (aq) + 8H + (aq)  2Cr 3+ (aq) +3SO 4 2– (aq) + 4H 2 O(l) Addition of SO 3 2-

18. New Way Chemistry for Hong Kong A-Level Book 417 44.3 Sulphur Dioxide (SB p.133) Reaction with Bromine Br 2 is an oxidizing agent and reacts with a reducing agent in to give Br – Br 2 (aq) + 2e –  2Br – (aq) reddish brown colourless Reaction between aqueous SO 2 and Br 2 : Br 2 (aq) + SO 3 2– (aq) + H 2 O(l)  2Br – (aq) +SO 4 2– (aq) + 2H + (aq) Addition of SO 3 2-

19. New Way Chemistry for Hong Kong A-Level Book 418 44.3 Sulphur Dioxide (SB p.133) Bleaching Action SO 2 bleaches by reduction It combines with moisture to form sulphuric(IV) acid and then reduces the dye to form a colourless product Dye(s) + SO 3 2– (aq)  (dye – O)(s) + SO 4 2– (aq) coloured colourless Bleaching action of moist SO 2

20. New Way Chemistry for Hong Kong A-Level Book 419 SO 2 is a mild reducing agent. It is used to bleach delicate materials, e.g. paper, straw, silk and wool SO 2 is also commonly used to whiten some foodstuffs, e.g. flour and cheese It is used as a food preservative for juices and jam 44.3 Sulphur Dioxide (SB p.133) SO 2 is used to whiten flour and cheese

21. New Way Chemistry for Hong Kong A-Level Book 420 Newspaper is bleached by SO 2 O 2 from the air, with the presence of sunlight, replaces the oxygen removed during bleaching and restores the original colour  Old newspaper turns brown 44.3 Sulphur Dioxide (SB p.133) Old newspaper turns brown

22. New Way Chemistry for Hong Kong A-Level Book 421 Oxidizing Properties of Sulphur Dioxide SO 2 normally behaves as a reducing agent However, if a stronger reducing agent is present in the reaction, SO 2 is forced to act as an oxidizing agent The O.N. of sulphur would change from +4 to 0 SO 2 (g) + 4e –  S(s) + 2O 2– (aq) or SO 2 (g) + 4H + (aq) + 4e –  S(s) + 2H 2 O(l) The electrons are supplied by the reducing agent 44.3 Sulphur Dioxide (SB p.134)

23. New Way Chemistry for Hong Kong A-Level Book 422 Mg is a strong reducing agent. It reacts with SO 2 to give yellow specks of S and white MgO 2Mg(s) + SO 2 (g)  2MgO(s) + S(s) 44.3 Sulphur Dioxide (SB p.134) Reaction with Magnesium Laboratory set-up for burning Mg in SO 2 gas The reaction is performed by putting a burning piece of Mg into a jar of SO 2. The Mg is hot enough to liberate free oxygen from SO 2 and therefore continues to burn in the gas

24. New Way Chemistry for Hong Kong A-Level Book 423 SO 2 (aq) oxidizes H 2 S in the presence of moisture, giving water and sulphur 2H 2 S(g) + SO 2 (aq)  2H 2 O(l) + 3S(s) Dry H 2 S(g) and SO 2 (g) do not react 44.3 Sulphur Dioxide (SB p.134) Reaction with Hydrogen Sulphide

25. New Way Chemistry for Hong Kong A-Level Book 424 Check Point 44-2 State whether following is a redox reaction. Explain your answer with reference to the oxidation number of sulphur. (a)H 2 SO 3 (aq)  2H + (aq) + SO 3 2– (aq) Answer (a)H 2 SO 3 (aq)  2H + (aq) + SO 3 2– (aq)+4 This is not a redox reaction. The oxidation number of sulphur remains unchanged. 44.2 Burning of Sulphur (SB p.135)

26. New Way Chemistry for Hong Kong A-Level Book 425 Check Point 44-2 (cont’d) State whether following is a redox reaction. Explain your answer with reference to the oxidation number of sulphur. (b)2Mg(s) + SO 2 (g)  2MgO(s) + S(s) Answer (b)2Mg(s) + SO 2 (g)  2MgO(s) + S(s) +40 This is a redox reaction. The oxidation number of sulphur changes from +4 in SO 2 (g) to 0 in S(s). 44.2 Burning of Sulphur (SB p.135)

27. New Way Chemistry for Hong Kong A-Level Book 426 Check Point 44-2 (cont’d) State whether following is a redox reaction. Explain your answer with reference to the oxidation number of sulphur. (c)2S 2 O 3 2– (aq) + I 2 (aq)  S 4 O 6 2– (aq) + 2I – (aq) Answer (c)2S 2 O 3 2– (aq) + I 2 (g)  S 4 O 6 2– (aq) + 2I – (aq) +2+2.5 This is a redox reaction. The oxidation number of sulphur changes from +2 in S 2 O 3 2– (aq) to +2.5 in S 4 O 6 2– (aq). 44.2 Burning of Sulphur (SB p.135)

28. New Way Chemistry for Hong Kong A-Level Book 427 Sulphuric(VI) acid corrosive, colourless, oily liquid strong dibasic acid pure H 2 SO 4 boils and decomposes at 340°C, giving off SO 3 and steam 44.4 Sulphuric(VI) Acid (SB p.135)  H 2 SO 4 (l)  SO 3 (g) + H 2 O(g) pure H 2 SO 4 has a high boiling point and high viscosity, because extensive hydrogen bonds are formed between H 2 SO 4 molecules

29. New Way Chemistry for Hong Kong A-Level Book 428 Sulphuric(VI) acid is one of the most important industrial chemicals Uses: mainly used in the manufacture of nitrogenous and phosphate fertilizers manufacture of detergents and paint additives 44.4 Sulphuric(VI) Acid (SB p.135)

30. New Way Chemistry for Hong Kong A-Level Book 429 Manufacture of Sulphuric(VI) Acid by the Contact Process Most of the world’s supply of sulphuric(VI) acid is produced by Contact Process Three stages: 1.Preparation and purification of SO 2 2.Catalytic oxidation of SO 2 to SO 3 3.Conversion of SO 3 to H 2 SO 4 44.4 Sulphuric(VI) Acid (SB p.135)

31. New Way Chemistry for Hong Kong A-Level Book 430 44.4 Sulphuric(VI) Acid (SB p.136) A flow diagram for the Contact process

32. New Way Chemistry for Hong Kong A-Level Book 431 S is obtained naturally in elemental form in large underground deposits Two methods for preparing SO 2 : 1.SO 2 is prepared by burning S in air S(s) + O 2 (g)  SO 2 (g) 2.SO 2 is obtained by roasting sulphide ores, e.g. iron pyrite (FeS 2 ) or black galena (PbS), in oxygen or air 4FeS 2 (s) + 11O 2 (g)  2Fe 2 O 3 (s) + 8SO 2 (g) 2PbS(s) + 3O 2 (g)  2PbO(s) + 2SO 2 (g) Preparation and Purification of Sulphur Dioxide 44.4 Sulphuric(VI) Acid (SB p.136)

33. New Way Chemistry for Hong Kong A-Level Book 432 44.4 Sulphuric(VI) Acid (SB p.136) Elemental sulphur is mined in huge quantities Iron pyrite Black galena Iron pyrite and black galena

34. New Way Chemistry for Hong Kong A-Level Book 433 SO 2 and air must be purified before entering the subsequent oxidation reaction because impurities may poison the catalyst The gases are purified with H 2 O and dried with conc. H 2 SO 4 before entering the catalytic chamber 44.4 Sulphuric(VI) Acid (SB p.136)

35. New Way Chemistry for Hong Kong A-Level Book 434 1.The purified SO 2 is mixed with air 2.The mixture is preheated to 450°C in a heat exchanger and allow to enter the catalytic chamber which contains V 2 O 5 at 450°C and atmospheric pressure 3.About 99% of SO 2 is converted to SO 3 Catalytic Oxidation of Sulphur Dioxide to Sulphur Trioxide 44.4 Sulphuric(VI) Acid (SB p.137) A simplified diagram of the catalytic chamber and the heat exchanger

36. New Way Chemistry for Hong Kong A-Level Book 435 SO 3 reacts violently with H 2 O to form a mist of H 2 SO 4 It is a highly exothermic reaction SO 3 is dissolved in conc. H 2 SO 4 firstly to form oleum SO 3 (g) + H 2 SO 4 (l)  H 2 S 2 O 7 (l) Then the oleum is treated with H 2 O to give conc. H 2 SO 4 H 2 S 2 O 7 (l) + H 2 O(l)  2H 2 SO 4 (l) Conversion of Sulphur Trioxide to Sulphuric(VI) Acid 44.4 Sulphuric(VI) Acid (SB p.138)

37. New Way Chemistry for Hong Kong A-Level Book 436 44.4 Sulphuric(VI) Acid (SB p.138) The key reaction for the production of H 2 SO 4 is the oxidation of SO 2 to SO 3 SO 2 (g) + O 2 (g)SO 3 (g)  H = –197 kJ According to Le Chatelier’s principle, (1)High pressure will increase the yield  Under the operation temperature of 450°C, the conversion rate is already high  Not necessary to use high pressure which will cost a lot (2)Low temperature will increase the yield Physico-chemical principles:

38. New Way Chemistry for Hong Kong A-Level Book 437 In addition to increase the yield, the reaction should be allow to attain equilibrium at a fast rate Operating temperature is 450°C which results in the highest conversion rate 44.4 Sulphuric(VI) Acid (SB p.138) A sulphuric acid production plant

39. New Way Chemistry for Hong Kong A-Level Book 438 Catalyst is used to increase the rate of the reaction  Pt has a higher efficiency than V 2 O 5  But Pt is easily poisoned by arsenic compounds and has a higher cost  nowadays, V 2 O 5 acts as catalyst instead of Pt although it is less efficient  V 2 O 5 is cheaper and less susceptible to poisoning 44.4 Sulphuric(VI) Acid (SB p.138)

40. New Way Chemistry for Hong Kong A-Level Book 439 Chemical Properties of Sulphuric(VI) Acid H 2 SO 4 is completely ionized in H 2 O to give SO 4 2– (aq), HSO 4 – (aq) and H 3 O + (aq) H 2 SO 4 (l) + H 2 O(l)  H 3 O + (aq) + HSO 4 – (aq) HSO 4 – (aq) + H 2 O(l)  H 3 O + (aq) + SO 4 2– (aq) Dilute H 2 SO 4 is a typical acid without oxidizing power It reacts with metals that are above the H + in the E.C.S. to give H 2 Dilute H 2 SO 4 reacts with metals, alkalis, solid bases, carbonates and hydrogencarbonates 44.4 Sulphuric(VI) Acid (SB p.139) As a Typical Acid

41. New Way Chemistry for Hong Kong A-Level Book 440 Examples: Zn(s) + H 2 SO 4 (aq)  ZnSO 4 (aq) + H 2 (g) 2NaOH(aq) + H 2 SO 4 (aq)  Na 2 SO 4 (aq) + 2H 2 O(l) 2NH 3( aq) + H 2 SO 4 (aq)  (NH 4 ) 2 SO 4 (aq) CuO(s) + H 2 SO 4 (aq)  CuSO 4 (aq) + H 2 O(l) MgCO 3 (s) + H 2 SO 4 (aq)  MgSO 4 (aq) + H 2 O(l) + CO 2 (g) 2NaHCO 3 (aq) + H 2 SO 4 (aq)  Na 2 SO 4 (aq) + 2H 2 O(l) + 2CO 2 (g) 44.4 Sulphuric(VI) Acid (SB p.139) Reaction between Zn metal and dilute H 2 SO 4

42. New Way Chemistry for Hong Kong A-Level Book 441 Conc. H 2 SO 4 is a strong oxidizing agent, especially when hot There are different changes in the oxidation no. of sulphur when sulphate(VI) ions react with different reducing agents 44.4 Sulphuric(VI) Acid (SB p.139) As an Oxidizing Agent

43. New Way Chemistry for Hong Kong A-Level Book 442 44.4 Sulphuric(VI) Acid (SB p.139) +4 +6 1.SO 2 is given out SO 4 2– (aq) + 4H + (aq) + 2e –  SO 2 (g) + 2H 2 O(l) 0 +6 2.S is deposited SO 4 2– (aq) + 8H + (aq) + 6e –  S(s) + 4H 2 O(l) +6 –2–2 3.H 2 S is given out SO 4 2– (aq) + 10H + (aq) + 8e –  H 2 S(g) + 4H 2 O(l)

44. New Way Chemistry for Hong Kong A-Level Book 443 Hot conc. H 2 SO 4 reacts with all metals (except Au & Pt) to form the corresponding sulphates(VI), SO 2 (g) and H 2 O(l) e.g. Cu(s) + 2H 2 SO 4 (l)  CuSO 4 (aq) + SO 2 (g) + 2H 2 O(l) Zn(s) + 2H 2 SO 4 (l)  ZnSO 4 (aq) + SO 2 (g) + 2H 2 O(l) 44.4 Sulphuric(VI) Acid (SB p.140) Reaction with Metals

45. New Way Chemistry for Hong Kong A-Level Book 444 Hot conc. H 2 SO 4 oxidizes some non-metals to their oxides slowly e.g. C(s) + 2H 2 SO 4 (l)  CO 2 (g) + 2SO 2 (g) + 2H 2 O(l) S(s) + 2H 2 SO 4 (l)  3SO 2 (g) + 2H 2 O(l) 44.4 Sulphuric(VI) Acid (SB p.140) Reaction with Non-metals

46. New Way Chemistry for Hong Kong A-Level Book 445 Hot conc. H 2 SO 4 oxidizes HBr and HI to Br 2 and I 2 respectively 2HBr(g) + H 2 SO 4 (l)  Br 2 (g) + SO 2 (g) + 2H 2 O(l) 8HI(g) + H 2 SO 4 (l)  4I 2 (g) + H 2 S (g) + 4H 2 O(l) But conc. H 2 SO 4 cannot oxidize HF(g) and HCl(g) 44.4 Sulphuric(VI) Acid (SB p.140) Reaction with Hydrogen Halides

47. New Way Chemistry for Hong Kong A-Level Book 446 Conc. H 2 SO 4 has a strong affinity for water Lots of heat are envolved when two liquids are mixed The water added to conc. H 2 SO 4 is likely to turn to steam and causes spurting of the acid When diluting conc. H 2 SO 4, the correct way is to add the acid slowly and carefully to water with continuous stirring 44.4 Sulphuric(VI) Acid (SB p.140) As a Dehydrating Agent

48. New Way Chemistry for Hong Kong A-Level Book 447 Conc. H 2 SO 4 can remove chemically combined water from hydrated salts Example: CuSO 4 5H 2 O crystals change from blue to white when conc. H 2 SO 4 is added The anhydrous salt is white 44.4 Sulphuric(VI) Acid (SB p.140) Dehydrating Hydrated Salts conc. H 2 SO 4 (l) hydrated form (blue crystals) anhydrous form (white powder) CuSO 4 5H 2 O(s)  CuSO 4 (s) + 5H 2 O(l) Dehydration of CuSO 4 5H 2 O by hot conc. H 2 SO 4

49. New Way Chemistry for Hong Kong A-Level Book 448 Conc. H 2 SO 4 can remove the elements of water (H : O in 2 : 1) from a number of compounds which do not possess water molecules Example: Sucrose (C 12 H 22 O 11 ) is dehydrated slowly by cold, rapidly by hot concentrated H 2 SO 4 giving a black mass of carbon C 12 H 22 O 11 (s)  12C(s) + 11H 2 O(l) 44.4 Sulphuric(VI) Acid (SB p.141) Dehydrating Organic Compounds

50. New Way Chemistry for Hong Kong A-Level Book 449 44.4 Sulphuric(VI) Acid (SB p.141) Dehydration of sucrose by conc. H 2 SO 4

51. New Way Chemistry for Hong Kong A-Level Book 450 Check Point 44-3 (a)Complete and balance the following equations. (i)CuO(s) + H 2 SO 4 (aq)  (ii)Cu(s) + H 2 SO 4 (l)  (iii)CuSO 4 5H 2 O(s)  Answer 44.4 Sulphuric(VI) Acid (SB p.142) conc. H 2 SO 4 (l) (a)(i)CuO(s) + H 2 SO 4 (aq)  CuSO 4 (aq) + H 2 O(l) (ii)Cu(s) + 2H 2 SO 4 (l)  CuSO 4 (aq) + SO 2 (g) + 2H 2 O(l) (iii)CuSO 4 5H 2 O(s)  CuSO 4 (s) + 5H 2 O(l) conc. H 2 SO 4 (l)

52. New Way Chemistry for Hong Kong A-Level Book 451 Check Point 44-3 (cont’d) (b)State the types of reactions in (a). Answer 44.4 Sulphuric(VI) Acid (SB p.142) (b)(i)Neutralization (ii)Redox reaction (iii)Dehydration

53. New Way Chemistry for Hong Kong A-Level Book 452 Check Point 44-3 (cont’d) (c)State the observations when dilute and concentrated sulphuric(VI) acid are added to the following substance respectively. (i)copper Give chemical equations whenever appropriate. Answer 44.4 Sulphuric(VI) Acid (SB p.142) (c)(i)There is no observable change when copper is added to dilute sulphuric(VI) acid. However, when copper is added to concentrated sulphuric(VI) acid, a blue solution is formed and a gas of pungent smell is evolved. It is because copper is oxidized by concentrated sulphuric(VI) acid to give a blue solution of copper(II) ion and sulphur dioxide gas which has a pungent smell. Cu(s) + 2H 2 SO 4 (l)  CuSO 4 (aq) + SO 2 (g) + 2H 2 O(l)

54. New Way Chemistry for Hong Kong A-Level Book 453 Check Point 44-3 (cont’d) (c)State the observations when dilute and concentrated sulphuric(VI) acid are added to the following substance respectively. (ii)sugar Give chemical equations whenever appropriate. Answer 44.4 Sulphuric(VI) Acid (SB p.142) (c)(ii)There is no observable change when dilute sulphuric(VI) acid is added to sucrose. However, when concentrated sulphuric(VI) acid is added to sucrose, a black mass of carbon is formed. It is because sucrose is dehydrated by concentrated sulphuric(VI) acid. conc. H 2 SO 4 (l) C 12 H 22 O 11 (s)  12C(s) + 11H 2 O(l) sucrose

55. New Way Chemistry for Hong Kong A-Level Book 454 Test for Sulphate(VI) Ions The presence of sulphate(VI) ions in a solution can be tested by using a solution of BaCl 2 (aq) acidified with dilute HNO 3 (aq) To the solution to be tested, BaCl 2 (aq) is added followed by excess HNO 3 (aq) Appearance of white ppt due to the formation of BaSO 4 (s) indicates the presence of sulphate(VI) ions Ba 2+ (aq) + SO 4 2– (aq)  BaSO 4 (s) 44.4 Sulphuric(VI) Acid (SB p.142) White precipitate of BaSO 4

56. New Way Chemistry for Hong Kong A-Level Book 455 Note: Sulphate(IV) ions (SO 3 2– ) and carbonate ions (CO 3 2– ) also give white ppt with Ba 2+ (aq) Ba 2+ (aq) + SO 3 2– (aq)  BaSO 3 (s) Ba 2+ (aq) + CO 3 2– (aq)  BaCO 3 (s) BaSO 3 (s) & BaCO 3 (s) will redissolve in excess acid to form a colourless solution ∴ it is necessary to add excess dilute HNO 3 BaSO 3 (s) + 2HNO 3 (aq)  Ba(NO 3 ) 2 (aq) + H 2 O(l) + SO 2 (g) BaCO 3 (s) + 2HNO 3 (aq)  Ba(NO 3 ) 2 (aq) + H 2 O(l) + CO 2 (g) 44.4 Sulphuric(VI) Acid (SB p.142)

57. New Way Chemistry for Hong Kong A-Level Book 456 Check Point 44-4 Describe how can you test for the presence of (a)sulphate(IV) ions; Write balanced equations for chemical reactions occurred. Answer 44.4 Sulphuric(VI) Acid (SB p.143) (a) Sulphate(IV) ions turn acidified potassium dichromate solution from orange to green. 3SO 3 2– (aq) + Cr 2 O 7 2– (aq) + 8H + (aq)  3SO 4 2– (aq) + 2Cr 3+ (aq) + 4H 2 O(l) orange green

58. New Way Chemistry for Hong Kong A-Level Book 457 Check Point 44-4 (cont’d) Describe how can you test for the presence of (b)sulphate(VI) ions. Write balanced equations for chemical reactions occurred. Answer 44.4 Sulphuric(VI) Acid (SB p.143) (b) Sulphate(VI) ions form a white precipitate with acidified barium chloride solution. BaCl 2 (aq) + SO 4 2– (aq)  BaSO 4 (aq) + 2Cl – (aq) white

59. New Way Chemistry for Hong Kong A-Level Book 458 Uses of Sulphuric(VI) Acid Sulphuric(VI) acid is an important chemical used in the manufacture of: detergents, polymers, fibres, dyestuffs, paints, pigments, fertilizers, etc. 44.4 Sulphuric(VI) Acid (SB p.142)

60. New Way Chemistry for Hong Kong A-Level Book 459 Phosphorus fertilizer (Ca(H 2 PO 4 ) 2 ) is prepared by reacting calcium phosphate(V) with conc. H 2 SO 4 (l) to form more water soluble Ca(H 2 PO 4 ) 2 Ca 3 (PO 4 ) 2 (s) + 2H 2 SO 4 (l)  Ca(H 2 PO 4 ) 2 (s) + 2CaSO 4 (s) Ammonium sulphate(VI) ((NH 4 ) 2 SO 4 ) is used as a fertilizer which is produced by the reaction between NH 3 and H 2 SO 4 2NH 3 (g) + H 2 SO 4 (l)  (NH 4 ) 2 SO 4 (aq) crystallization (NH 4 ) 2 SO 4 (aq)  (NH 4 ) 2 SO 4 (s) 44.4 Sulphuric(VI) Acid (SB p.144) Fertilizers

61. New Way Chemistry for Hong Kong A-Level Book 460 Soapless detergents (also known as synthetic detergents) can be made by treating hydrocarbons with conc. H 2 SO 4 followed by NaOH 44.4 Sulphuric(VI) Acid (SB p.144) Detergents Some soapless detergents

62. New Way Chemistry for Hong Kong A-Level Book 461 Azo dyes are made from nitrobenzene, which is prepared by the reaction between benzene and a mixture of conc. H 2 SO 4 and conc. HNO 3 44.4 Sulphuric(VI) Acid (SB p.145) Dyestuffs

63. New Way Chemistry for Hong Kong A-Level Book 462 White pigment, titanyl sulphate (TiOSO 4 ) made by dissolving titanium(IV) oxide (TiO 2 ) in hot conc. H 2 SO 4 Paints additives, e.g. barium sulphate(VI) (BaSO 4 ) & calcium sulphate(VI) (CaSO 4 ) can also be made from H 2 SO 4 Ba 2+ (aq) + SO 4 2– (aq)  BaSO 4 (s) Ca 2+ (aq) + SO 4 2– (aq)  CaSO 4 (s) 44.4 Sulphuric(VI) Acid (SB p.145) Paints and Pigments

64. New Way Chemistry for Hong Kong A-Level Book 463 The END