Sulphur and its Compound

1
Chapter 44
Sulphur and its Compound
44.1 Introduction 44.2 Burning of
Sulphur 44.3 Sulphur Dioxide 44.4 Sulphuric(VI)
Acid
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44.1 Introduction (SB p.128)

• Sulphur (second member of Group VIA)
• Electronic configuration 1s22s22p63s23p4
• Non-metal, solid at room temp.
• Low melting point
• Poor conductor of heat and non-conductor of
  electricity
• Occurs as free S8 moleclues

Some information about sulphur
Covalent radius (nm) 0.104
Melting point (C) 113
Boiling point (C) 445
Bond enthalpy (kJ mol1) 264
First ionization enthalpy (kJ mol1) 1 000
Electron affinity (kJ mol1) 194
Electronegativity 2.5
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44.1 Introduction (SB p.128)

• Sulphur
• Five allotropes rhombic, monoclinic, plastic,
  colloidal and flowers

• Rhombic sulphur (transparent yellow crystals) is
  stable up to 96C
• Consists of 8 S atom covalently bonded in
  crown-shaped S8 ring
• Insoluble in water but soluble in organic solvents

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44.1 Introduction (SB p.129)

• Monoclinic sulphur (amber yellow crystals) is
  stable between 95.5C and 119C (also composed
  of S8 molecules)
• Plastic sulphur is formed in chemical reactions

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44.2 Burning of Sulphur (SB p.130)
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44.2 Burning of Sulphur (SB p.130)
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44.2 Burning of Sulphur (SB p.130)
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44.2 Burning of Sulphur (SB p.131)

• This experiment should be carried out in a fume
  cupboard, as SO2(g) is toxic with a
  characteristic pungent and choking smell

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44.2 Burning of Sulphur (SB p.131)
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
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44.2 Burning of Sulphur (SB p.131)
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44.2 Burning of Sulphur (SB p.131)
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44.3 Sulphur Dioxide (SB p.131)

• Sulphur dioxide (SO2)
• Colourless, toxic gas with pungent and choking
  smell
• Readily liquefied under pressure
• Very soluble in water and different ions
  H(aq), HSO3(aq) and SO32(aq) are formed
• Aqueous solution of SO2 is acidic

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44.3 Sulphur Dioxide (SB p.131)
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44.3 Sulphur Dioxide (SB p.132)
Reducing Properties of Sulphur Dioxide
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44.3 Sulphur Dioxide (SB p.132)
Reaction with Manganate(VII) Ion

• MnO4 is a strong oxidizing agent, reacts with a
  reducing agent in an acidic medium to give Mn2
• MnO4(aq) 8H(aq) 5e ?? Mn2(aq) 4H2O(l)

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44.3 Sulphur Dioxide (SB p.132)
Reaction with Dichromate(VI) Ion

• Cr2O72 is a strong oxidizing agent, reacts with
  a reducing agent in an acidic medium to give Cr3
• Cr2O72(aq) 14H(aq) 6e ?? 2Cr3(aq)
  7H2O(l)

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44.3 Sulphur Dioxide (SB p.133)
Reaction with Bromine

• Br2 is an oxidizing agent and reacts with a
  reducing agent in to give Br
• Br2(aq) 2e ?? 2Br(aq)

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44.3 Sulphur Dioxide (SB p.133)
Bleaching Action
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44.3 Sulphur Dioxide (SB p.133)

• SO2 is a mild reducing agent. It is used to
  bleach delicate materials, e.g. paper, straw,
  silk and wool
• SO2 is also commonly used to whiten some
  foodstuffs, e.g. flour and cheese
• It is used as a food preservative for juices and
  jam

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44.3 Sulphur Dioxide (SB p.133)

• Newspaper is bleached by SO2
• O2 from the air, with the presence of sunlight,
  replaces the oxygen removed during bleaching and
  restores the original colour
• ? Old newspaper turns brown

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44.3 Sulphur Dioxide (SB p.134)
Oxidizing Properties of Sulphur Dioxide

• SO2 normally behaves as a reducing agent
• However, if a stronger reducing agent is present
  in the reaction, SO2 is forced to act as an
  oxidizing agent
• The O.N. of sulphur would change from 4 to 0
• SO2(g) 4e ?? S(s) 2O2 (aq)
• or SO2(g) 4H(aq) 4e ?? S(s) 2H2O(l)
• The electrons are supplied by the reducing agent

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44.3 Sulphur Dioxide (SB p.134)
Reaction with Magnesium

• Mg is a strong reducing agent. It reacts with SO2
  to give yellow specks of S and white MgO
• 2Mg(s) SO2(g) ?? 2MgO(s) S(s)

• The reaction is performed by putting a burning
  piece of Mg into a jar of SO2. The Mg is hot
  enough to liberate free oxygen from SO2 and
  therefore continues to burn in the gas

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44.3 Sulphur Dioxide (SB p.134)
Reaction with Hydrogen Sulphide

• SO2(aq) oxidizes H2S in the presence of moisture,
  giving water and sulphur
• 2H2S(g) SO2(aq) ?? 2H2O(l) 3S(s)
• Dry H2S(g) and SO2(g) do not react

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44.2 Burning of Sulphur (SB p.135)
Check Point 44-2 State whether following is a
redox reaction. Explain your answer with
reference to the oxidation number of
sulphur. (a) H2SO3(aq) ?? 2H(aq) SO32(aq)
Answer
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44.2 Burning of Sulphur (SB p.135)
Check Point 44-2 (contd) State whether following
is a redox reaction. Explain your answer with
reference to the oxidation number of
sulphur. (b) 2Mg(s) SO2(g) ?? 2MgO(s) S(s)
Answer
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44.2 Burning of Sulphur (SB p.135)
Check Point 44-2 (contd) State whether
following is a redox reaction. Explain your
answer with reference to the oxidation number of
sulphur. (c) 2S2O32(aq) I2(aq) ?? S4O62(aq)
2I(aq)
Answer
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44.4 Sulphuric(VI) Acid (SB p.135)

• Sulphuric(VI) acid
• corrosive, colourless, oily liquid
• strong dibasic acid
• pure H2SO4 boils and decomposes at 340C, giving
  off SO3 and steam

• pure H2SO4 has a high boiling point and high
  viscosity, because extensive hydrogen bonds are
  formed between H2SO4 molecules

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44.4 Sulphuric(VI) Acid (SB p.135)

• 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

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44.4 Sulphuric(VI) Acid (SB p.135)
Manufacture of Sulphuric(VI) Acid by the Contact
Process

• Most of the worlds supply of sulphuric(VI) acid
  is produced by Contact Process
• Three stages
• 1. Preparation and purification of SO2
• 2. Catalytic oxidation of SO2 to SO3
• 3. Conversion of SO3 to H2SO4

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44.4 Sulphuric(VI) Acid (SB p.136)
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44.4 Sulphuric(VI) Acid (SB p.136)
Preparation and Purification of Sulphur Dioxide

• S is obtained naturally in elemental form in
  large underground deposits
• Two methods for preparing SO2
• 1. SO2 is prepared by burning S in air
• S(s) O2(g) ?? SO2(g)
• 2. SO2 is obtained by roasting sulphide ores,
  e.g. iron pyrite (FeS2) or black galena (PbS),
  in oxygen or air
• 4FeS2(s) 11O2(g) ?? 2Fe2O3(s)
  8SO2(g) 2PbS(s) 3O2(g) ?? 2PbO(s) 2SO2(g)

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44.4 Sulphuric(VI) Acid (SB p.136)
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44.4 Sulphuric(VI) Acid (SB p.136)

• SO2 and air must be purified before entering the
  subsequent oxidation reaction because impurities
  may poison the catalyst
• The gases are purified with H2O and dried with
  conc. H2SO4 before entering the catalytic chamber

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44.4 Sulphuric(VI) Acid (SB p.137)
Catalytic Oxidation of Sulphur Dioxide to Sulphur
Trioxide
1. The purified SO2 is mixed with air 2. The
mixture is preheated to 450C in a heat exchanger
and allow to enter the catalytic chamber which
contains V2O5 at 450C and atmospheric
pressure 3. About 99 of SO2 is converted to SO3
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44.4 Sulphuric(VI) Acid (SB p.138)
Conversion of Sulphur Trioxide to Sulphuric(VI)
Acid

• SO3 reacts violently with H2O to form a mist of
  H2SO4
• It is a highly exothermic reaction
• SO3 is dissolved in conc. H2SO4 firstly to form
  oleum
• SO3(g) H2SO4(l) ?? H2S2O7(l)
• Then the oleum is treated with H2O to give conc.
  H2SO4
• H2S2O7(l) H2O(l) ?? 2H2SO4(l)

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44.4 Sulphuric(VI) Acid (SB p.138)
Physico-chemical principles

• According to Le Chateliers principle,
• (1) High pressure will increase the yield
• ? Under the operation temperature of 450C,
  the conversion rate is already high
• ? Not necessary to use high pressure which
  will cost a lot
• (2) Low temperature will increase the yield

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44.4 Sulphuric(VI) Acid (SB p.138)

• In addition to increase the yield, the reaction
  should be allow to attain equilibrium at a fast
  rate
• Operating temperature is 450C which results in
  the highest conversion rate

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44.4 Sulphuric(VI) Acid (SB p.138)

• Catalyst is used to increase the rate of the
  reaction
• ? Pt has a higher efficiency than V2O5
• ? But Pt is easily poisoned by arsenic compounds
  and has a higher cost
• ? nowadays, V2O5 acts as catalyst instead of Pt
  although it is less efficient
• ? V2O5 is cheaper and less susceptible to
  poisoning

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44.4 Sulphuric(VI) Acid (SB p.139)
Chemical Properties of Sulphuric(VI) Acid
As a Typical Acid

• H2SO4 is completely ionized in H2O to give
  SO42(aq), HSO4(aq) and H3O(aq)
• H2SO4(l) H2O(l) ?? H3O(aq)
  HSO4(aq)HSO4(aq) H2O(l) ?? H3O(aq)
  SO42(aq)
• Dilute H2SO4 is a typical acid without oxidizing
  power
• It reacts with metals that are above the H in
  the E.C.S. to give H2
• Dilute H2SO4 reacts with metals, alkalis, solid
  bases, carbonates and hydrogencarbonates

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44.4 Sulphuric(VI) Acid (SB p.139)
Examples Zn(s) H2SO4(aq) ?? ZnSO4(aq)
H2(g) 2NaOH(aq) H2SO4(aq) ?? Na2SO4(aq)
2H2O(l) 2NH3(aq) H2SO4(aq) ??
(NH4)2SO4(aq) CuO(s) H2SO4(aq) ?? CuSO4(aq)
H2O(l) MgCO3(s) H2SO4(aq) ?? MgSO4(aq)
H2O(l) CO2(g) 2NaHCO3(aq) H2SO4(aq) ??
Na2SO4(aq) 2H2O(l) 2CO2(g)
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44.4 Sulphuric(VI) Acid (SB p.139)
As an Oxidizing Agent

• Conc. H2SO4 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

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44.4 Sulphuric(VI) Acid (SB p.139)
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44.4 Sulphuric(VI) Acid (SB p.140)
Reaction with Metals

• Hot conc. H2SO4 reacts with all metals (except Au
  Pt) to form the corresponding sulphates(VI),
  SO2(g) and H2O(l)
• e.g.
• Cu(s) 2H2SO4(l) ?? CuSO4(aq) SO2(g)
  2H2O(l)
• Zn(s) 2H2SO4(l) ?? ZnSO4(aq) SO2(g) 2H2O(l)

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44.4 Sulphuric(VI) Acid (SB p.140)
Reaction with Non-metals

• Hot conc. H2SO4 oxidizes some non-metals to their
  oxides slowly
• e.g.
• C(s) 2H2SO4(l) ?? CO2(g) 2SO2(g) 2H2O(l)
• S(s) 2H2SO4(l) ?? 3SO2(g) 2H2O(l)

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44.4 Sulphuric(VI) Acid (SB p.140)
Reaction with Hydrogen Halides

• Hot conc. H2SO4 oxidizes HBr and HI to Br2 and I2
  respectively
• 2HBr(g) H2SO4(l) ?? Br2(g) SO2(g) 2H2O(l)
• 8HI(g) H2SO4(l) ?? 4I2(g) H2S (g) 4H2O(l)
• But conc. H2SO4 cannot oxidize HF(g) and HCl(g)

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44.4 Sulphuric(VI) Acid (SB p.140)
As a Dehydrating Agent

• Conc. H2SO4 has a strong affinity for water
• Lots of heat are envolved when two liquids are
  mixed
• The water added to conc. H2SO4 is likely to turn
  to steam and causes spurting of the acid
• When diluting conc. H2SO4, the correct way is to
  add the acid slowly and carefully to water with
  continuous stirring

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44.4 Sulphuric(VI) Acid (SB p.140)
Dehydrating Hydrated Salts

• Conc. H2SO4 can remove chemically combined water
  from hydrated salts
• Example
• CuSO4 5H2O crystals change from blue to white
  when conc. H2SO4 is added
• The anhydrous salt is white

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44.4 Sulphuric(VI) Acid (SB p.141)
Dehydrating Organic Compounds

• Conc. H2SO4 can remove the elements of water (H
  O in 2 1) from a number of compounds which do
  not possess water molecules
• Example
• Sucrose (C12H22O11) is dehydrated slowly by
  cold, rapidly by hot concentrated H2SO4 giving a
  black mass of carbon
• C12H22O11(s) ?? 12C(s) 11H2O(l)

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44.4 Sulphuric(VI) Acid (SB p.141)
Dehydration of sucrose by conc. H2SO4
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44.4 Sulphuric(VI) Acid (SB p.142)
Check Point 44-3 (a) Complete and balance the
following equations. (i) CuO(s) H2SO4(aq)
?? (ii) Cu(s) H2SO4(l) ?? (iii) CuSO4
5H2O(s) ?????
Answer
conc. H2SO4(l)
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44.4 Sulphuric(VI) Acid (SB p.142)
Check Point 44-3 (contd) (b) State the types of
reactions in (a).
Answer
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44.4 Sulphuric(VI) Acid (SB p.142)
Check Point 44-3 (contd) (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
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44.4 Sulphuric(VI) Acid (SB p.142)
Check Point 44-3 (contd) (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
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44.4 Sulphuric(VI) Acid (SB p.142)
Test for Sulphate(VI) Ions

• The presence of sulphate(VI) ions in a solution
  can be tested by using a solution of BaCl2(aq)
  acidified with dilute HNO3(aq)
• To the solution to be tested, BaCl2(aq) is added
  followed by excess HNO3(aq)
• Appearance of white ppt due to the formation of
  BaSO4(s) indicates the presence of sulphate(VI)
  ions
• Ba2(aq) SO42(aq) ?? BaSO4(s)

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44.4 Sulphuric(VI) Acid (SB p.142)

• Note
• Sulphate(IV) ions (SO32) and carbonate ions
  (CO32) also give white ppt with Ba2(aq)
• Ba2(aq) SO32(aq) ?? BaSO3(s)
• Ba2(aq) CO32(aq) ?? BaCO3(s)
• BaSO3(s) BaCO3(s) will redissolve in excess
  acid to form a colourless solution? it is
  necessary to add excess dilute HNO3
• BaSO3(s) 2HNO3(aq) ?? Ba(NO3)2(aq) H2O(l)
  SO2(g)
• BaCO3(s) 2HNO3(aq) ?? Ba(NO3)2(aq) H2O(l)
  CO2(g)

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44.4 Sulphuric(VI) Acid (SB p.143)

• Check Point 44-4
• Describe how can you test for the presence of
• sulphate(IV) ions
• Write balanced equations for chemical reactions
  occurred.

Answer
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44.4 Sulphuric(VI) Acid (SB p.143)
Check Point 44-4 (contd) Describe how can you
test for the presence of (b) sulphate(VI)
ions. Write balanced equations for chemical
reactions occurred.
Answer
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44.4 Sulphuric(VI) Acid (SB p.142)
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.

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44.4 Sulphuric(VI) Acid (SB p.144)
Fertilizers

• Phosphorus fertilizer (Ca(H2PO4)2) is prepared by
  reacting calcium phosphate(V) with conc. H2SO4(l)
  to form more water soluble Ca(H2PO4)2
• Ca3(PO4)2(s) 2H2SO4(l) ?? Ca(H2PO4)2(s)
  2CaSO4(s)
• Ammonium sulphate(VI) ((NH4)2SO4) is used as a
  fertilizer which is produced by the reaction
  between NH3 and H2SO4
• 2NH3(g) H2SO4(l) ?? (NH4)2SO4(aq)
• crystallization (NH4)2
  SO4(aq) ?????? (NH4)2SO4(s)

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44.4 Sulphuric(VI) Acid (SB p.144)
Detergents

• Soapless detergents (also known as synthetic
  detergents) can be made by treating hydrocarbons
  with conc. H2SO4 followed by NaOH

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44.4 Sulphuric(VI) Acid (SB p.145)
Dyestuffs

• Azo dyes are made from nitrobenzene, which is
  prepared by the reaction between benzene and a
  mixture of conc. H2SO4 and conc. HNO3

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44.4 Sulphuric(VI) Acid (SB p.145)
Paints and Pigments

• White pigment, titanyl sulphate (TiOSO4)
• made by dissolving titanium(IV) oxide (TiO2) in
  hot conc. H2SO4
• Paints additives, e.g. barium sulphate(VI)
  (BaSO4) calcium sulphate(VI) (CaSO4)
• can also be made from H2SO4
• Ba2(aq) SO42(aq) ?? BaSO4(s) Ca2(aq)
  SO42(aq) ?? CaSO4(s)

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The END