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)

1New Way Chemistry for Hong Kong A-Level Book 4

1

Sulphur and its Compound

44.144.1 IntroductionIntroduction

44.244.2 Burning of SulphurBurning of Sulphur

44.344.3 Sulphur DioxideSulphur Dioxide

44.444.4 Sulphuric(VI) AcidSulphuric(VI) Acid

Chapter 44Chapter 44


2

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

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

Electronegativity 2.5

Some information about sulphur


3


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 S8 ring

• Insoluble in water but soluble in organic solvents


4


• Monoclinic sulphur (amber yellow crystals) is stable between 95.5°C and 119°C (also composed of S8 molecules)

• Plastic sulphur is formed in chemical reactions

Crystal of monoclinic sulphur Plastic sulphur


5

44.2 Burning of Sulphur (SB p.130)

When sulphur is heated, the S8 molecules vibrate more and more vigorously. At the melting point, they can move freely. The sulphur formed contains S8 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


6


The S8 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.



7


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 S8 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.



8


• In the presence of O2, sulphur bur

ns with a dull blue flame to form

SO2

S(s) + O2(g) SO2(g)

• Traces of misty SO3 are also for

med

• This experiment should be carried out in a fume cupboard,

as SO2(g) is toxic with a characteristic pungent and

choking smell


9

Check Point 44-1 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 hexafluorideAnswer


(a) Sulphur Dioxide

(b) Sulphur trioxide


10


(c) Sulphate(VI) ion

(d) Sulphuric(VI) acid


11


(e) Sulphur hexafluoride


12

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 SO3

2–(aq) are formed

• Aqueous solution of SO2 is acidic

–H+

+H+

–H+

+H+SO2(g) + H2O(l)H2SO3(aq) HSO3

–(aq) SO32–(aq)

sulphuric(IV) acid

Hydrogen-sulphate(IV) acid

sulphate(IV) ion

The V-shaped SO2 molecule


13


• SO2 is an acidic gaseous pollutant

• SO2 dissolves in raindrops to form acid rain which damages plants and buildings

• Highly irritating and have harmful effects on the respiratory system of humans

• SO2 will be oxidized to SO3 by atmospheric O2

2SO2(g) + O2(g) 2SO3(g)

• SO3 is highly irritating too and it dissolves in rain water to form H2SO4

SO3(g) + H2O(l) H2SO4(aq)


14

Reducing Properties of Sulphur DioxideReducing Properties of Sulphur Dioxide


• SO2 in aqueous solution (i.e. sulphuric(IV) acid (H2SO3)) is powerful reducing agent

SO2(g) + H2O(l) H2SO3(aq) 2H+(aq) + SO32–(a

q)

+4 +6

• SO32– ion acts as an electron donor and reacts with electron

acceptors to give SO42– ion

SO32–(aq) + H2O(l) SO4

2–(aq) + 2H+(aq) + 2e–

• The O.N. of sulphur increases from +4 to +6 when SO2 changes to SO4

2–

• The e– are accepted by the oxidizing agent


15


Reaction with Manganate(VII) Ion

• MnO4– is a strong oxidizing agent, reacts with a reducing a

gent in an acidic medium to give Mn2+

MnO4–(aq) + 8H+(aq) + 5e– Mn2+(aq) + 4H2O(l)

purple

colourless

• Reaction between aqueous SO

2 and MnO4– :

2MnO4–(aq) + 5SO3

2–(aq) + 6H+(aq) 2Mn2+(aq) + 5SO4

2–(aq) + 3H2O(l)

Addition of SO3

2-


16


Reaction with Dichromate(VI) Ion

• Cr2O72– is a strong oxidizing agent, reacts with a reducing a

gent in an acidic medium to give Cr3+

Cr2O72–(aq) + 14H+(aq) + 6e– 2Cr3+(aq) + 7H2O(l)

orange

green

• Reaction between aqueous SO2 and Cr2O7

2– :

Cr2O72–(aq) + 3SO3

2–(aq) + 8H+(aq) 2Cr3+(aq) +3SO4

2–(aq) + 4H2

O(l)

Addition of SO3

2-


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Reaction with Bromine

• Br2 is an oxidizing agent and reacts with a reducing agent in to give Br–

Br2(aq) + 2e– 2Br–(aq)

reddish brown

colourless

• Reaction between aqueous SO2 and Br2:

Br2 (aq) + SO32–(aq) + H2O(l)

2Br–(aq) +SO42–(aq) +

2H+(aq)

Addition of SO3

2-


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Bleaching Action

• SO2 bleaches by reduction

• It combines with moisture to form sulphuric(IV) acid and t

hen reduces the dye to form a colourless product

Dye(s) + SO32–(aq) (dye – O)(s) + SO4

2–(aq)coloured colourless

Bleaching action of moist SO2


19

• 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


SO2 is used to whiten flour and cheese


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• Newspaper is bleached by SO2

• O2 from the air, with the presence of

sunlight, replaces the oxygen remov

ed during bleaching and restores the

original colour

Old newspaper turns brown


Old newspaper turns brown


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Oxidizing Properties of Sulphur DioxideOxidizing Properties of Sulphur Dioxide

• SO2 normally behaves as a reducing agent

• However, if a stronger reducing agent is present in the rea

ction, 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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• Mg is a strong reducing agent. It reacts with SO2 to give y

ellow specks of S and white MgO

2Mg(s) + SO2(g) 2MgO(s) + S(s)


Reaction with Magnesium

Laboratory set-up for burning Mg in SO2 gas

• 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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• 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


Reaction with Hydrogen Sulphide


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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

(a) H2SO3(aq) 2H+(aq) + SO32–(aq)

+4 +4

This is not a redox reaction. The oxidation number of sulphur remains unchanged.



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Check Point 44-2 (cont’d)Check Point 44-2 (cont’d)


(b) 2Mg(s) + SO2(g) 2MgO(s) + S(s)Answer

(b) 2Mg(s) + SO2(g) 2MgO(s) + S(s)+4 0

This is a redox reaction. The oxidation number of sulphur changes from +4 in SO2(g) to 0 in S(s).



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Check Point 44-2 (cont’d) Check Point 44-2 (cont’d)


(c) 2S2O32–(aq) + I2(aq) S4O6

2–(aq) + 2I–(aq)Answer

(c) 2S2O32–(aq) + I2(g) S4O6

2–(aq) + 2I–(aq)+2 +2.5

This is a redox reaction. The oxidation number of sulphur changes from +2 in S2O3

2–(aq) to +2.5 in S4O6

2–(aq).



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Sulphuric(VI) acid

• corrosive, colourless, oily liquid

• strong dibasic acid

• pure H2SO4 boils and decomposes at 340°C, giving off SO3 and steam

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

H2SO4(l) SO3(g) + H2O(g)

• pure H2SO4 has a high

boiling point and high

viscosity, because extensive

hydrogen bonds are formed

between H2SO4 molecules


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• Sulphuric(VI) acid is one of the most important industr

ial chemicals

Uses:

• mainly used in the manufacture of nitrogenous and p

hosphate fertilizers

• manufacture of detergents and paint additives



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Manufacture of Sulphuric(VI) Acid by the Contact ProcessManufacture 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 SO2

2. Catalytic oxidation of SO2 to SO3

3. Conversion of SO3 to H2SO4



30


A flow diagram for the Contact process


31

• S is obtained naturally in elemental form in large undergr

ound 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)

Preparation and Purification of Sulphur Dioxide



32


Elemental sulphur is mined in huge quantities

Iron pyrite Black galena

Iron pyrite and black galena


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• 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



34

1. The purified SO2 is m

ixed with air

2. The mixture is preheated to 450°C in a heat exchanger and allow to enter the catalytic chamber which contains V2O5 at 450°C and at

mospheric pressure

3. About 99% of SO2

is converted to SO3

Catalytic Oxidation of Sulphur Dioxide to Sulphur Trioxide


A simplified diagram of the catalytic chamber and the heat exchanger


35

• 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)

Conversion of Sulphur Trioxide to Sulphuric(VI) Acid



36


• The key reaction for the production of H2SO4 is the

oxidation of SO2 to SO3

SO2(g) + O2(g) SO3(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:


37

• 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


A sulphuric acid production plant


38

• 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



39

Chemical Properties of Sulphuric(VI) AcidChemical Properties of Sulphuric(VI) Acid

• H2SO4 is completely ionized in H2O to give SO42–(aq), H

SO4–(aq) and H3O+(aq)

H2SO4(l) + H2O(l) H3O+(aq) + HSO4–(aq)

HSO4–(aq) + H2O(l) H3O+(aq) + SO4

2–(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, ca

rbonates and hydrogencarbonates


As a Typical Acid


40

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)


Reaction between Zn metal and dilute H2SO4


41

• Conc. H2SO4 is a strong oxidizing agent, especially wh

en hot

• There are different changes in the oxidation no. of sul

phur when sulphate(VI) ions react with different redu

cing agents


As an Oxidizing Agent


42


+4+6

1. SO2 is given out

SO42–(aq) + 4H+(aq) + 2e– SO2(g) + 2H2O(l)

0+6

2. S is deposited

SO42–(aq) + 8H+(aq) + 6e– S(s) + 4H2O(l)

+6 –2

3. H2S is given out

SO42–(aq) + 10H+(aq) + 8e– H2S(g) + 4H2O(l)


43

• Hot conc. H2SO4 reacts with all metals (except Au & Pt) t

o 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)


Reaction with Metals


44

• 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)


Reaction with Non-metals


45

• 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)


Reaction with Hydrogen Halides


46

• 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 stirr

ing


As a Dehydrating Agent


47

• 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


Dehydrating Hydrated Salts

conc. H2SO4(l)

hydrated form(blue crystals)

anhydrous form(white powder)

CuSO4 • 5H2O(s) CuSO4(s) + 5H2O(l)

Dehydration of CuSO4 • 5H2O by hot conc. H2SO4


48

• 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)


Dehydrating Organic Compounds


49


Dehydration of sucrose by conc. H2SO4


50


(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)

(a) (i) CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)

(ii) Cu(s) + 2H2SO4(l) CuSO4(aq) + SO2(g) + 2H2O(l)

(iii) CuSO4 • 5H2O(s) CuSO4(s) + 5H2O(l)conc. H2SO4(l)


51


(b) State the types of reactions in (a).Answer


(b) (i) Neutralization

(ii) Redox reaction

(iii) Dehydration


52

Check Point 44-3 (cont’d)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


(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) + 2H2SO4(l) CuSO4(aq) + SO2(g) + 2H2O(l)


53


(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


(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. H2SO4(l)

C12H22O11(s) 12C(s) + 11H2O(l)sucrose


54

Test for Sulphate(VI) IonsTest 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 adde

d followed by excess HNO3(aq)

• Appearance of white ppt due to the formation of BaSO4(s) indicates the presence of sulphat

e(VI) ions

Ba2+(aq) + SO42–(aq) BaSO4(s)


White precipitate of

BaSO4


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Note:

• Sulphate(IV) ions (SO32–) and carbonate ions (CO3

2–) also gi

ve white ppt with Ba2+(aq)


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)



56


Describe how can you test for the presence of

(a) sulphate(IV) ions;

Write balanced equations for chemical reactions occurred.

Answer


(a) Sulphate(IV) ions turn acidified potassium dichromate solution from orange to green.

3SO32–(aq) + Cr2O7

2–(aq) + 8H+(aq)

3SO42–(aq) + 2Cr3+(aq) + 4H2O(l)

orange

green


57


Describe how can you test for the presence of

(b) sulphate(VI) ions.

Write balanced equations for chemical reactions occurred.

Answer


(b) Sulphate(VI) ions form a white precipitate with acidified barium chloride solution.

BaCl2(aq) + SO42–(aq) BaSO4(aq) + 2Cl–(aq)

white


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Uses of Sulphuric(VI) AcidUses of Sulphuric(VI) Acid

• Sulphuric(VI) acid is an important chemical used in the manufacture of:

detergents, polymers, fibres, dyestuffs, paints, pigments, fertilizers, etc.



59

• 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 H2

SO4

2NH3(g) + H2SO4(l) (NH4)2SO4(aq)

crystallization(NH4)2SO4(aq) (NH4)2SO4(s)


Fertilizers


60

• Soapless detergents (also kno

wn as synthetic detergents) can

be made by treating hydroca

rbons with conc. H2SO4 follo

wed by NaOH


Detergents

Some soapless detergents


61

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

and conc. HNO3


Dyestuffs


62

White pigment, titanyl sulphate (TiOSO4)

• made by dissolving titanium(IV) oxide (TiO2) in hot

conc. H2SO4

Paints additives, e.g. barium sulphate(VI) (BaSO4) & cal

cium sulphate(VI) (CaSO4)

• can also be made from H2SO4


Ca2+(aq) + SO42–(aq) CaSO4(s)


Paints and Pigments


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The END

Documents

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)