The d -Block Elements

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The The dd-Block Elements-Block Elements

2

IntroductionIntroduction

• d-block elements

locate between the s-block andp-block

known as transition elements

occur in the fourth and subsequent periods of the Periodic Table

3

period 4

period 5

period 6

period 7

d-block elements

4


Transition elements are elements that contain an incomplete d sub-shell (i.e. d1 to d9) in at least one of the oxidation states of their compounds.

3d0

3d10

5


Sc and Zn are not transition elements because

They form compounds with only one oxidation state in which the d sub-shell are NOT imcomplete.

Sc Sc3+ 3d0 Zn Zn2+ 3d10

6


Cu

Cu+ 3d10 not transitional Cu2+ 3d9 transitional

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The first transition series

the first horizontal row of the d-block elements

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Characteristics of transition elements

(d-block metals vs s-block metals)

1. Physical properties vary slightly with atomic number across the series (cf. s-block and p-block elements)

2. Higher m.p./b.p./density/hardness than s-block elements of the same periods.

3. Variable oxidation states(cf. fixed oxidation states of s-block metals)

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Characteristics of transition elements

4. Formation of coloured compounds/ions(cf. colourless ions of s-block elements)

5. Formation of complexes

6. Catalytic properties

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The building up of electronic configurations of elements follow:

Aufbau principle

Pauli exclusion principle

Hund’s rule

Electronic ConfigurationsElectronic Configurations

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• 3d and 4s sub-shells are very close to each other in energy.

• Relative energy of electrons in sub-shells depends on the effective nuclear charge they experience.

• Electrons enter 4s sub-shell first

• Electrons leave 4s sub-shell first


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Cu Cu2+

Relative energy levels of orbitals in atom and in ion

13

• Valence electrons in the inner 3d orbitals


• Examples:

The electronic configuration of scandium:

1s22s22p63s23p63d14s2

The electronic configuration of zinc: 1s22s22p63s23p63d104s2

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Element Atomic number Electronic configuration

Scandium

Titanium

Vanadium

Chromium

Manganese

Iron

Cobalt

Nickel

Copper

Zinc

21

22

23

24

25

26

27

28

29

30

[Ar] 3d 14s2

[Ar] 3d 24s2

[Ar] 3d 34s2

[Ar] 3d 54s1

[Ar] 3d 54s2

[Ar] 3d 64s2

[Ar] 3d 74s2

[Ar] 3d 84s2

[Ar] 3d 104s1

[Ar] 3d 104s2

Electronic configurations of the first series of the d-block elements

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• A half-filled or fully-filled d sub-shell

has extra stability

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dd -Block Elements as Metals-Block Elements as Metals

Physical properties of d-Block elements :

good conductors of heat and electricity

hard and strong

malleable and ductile

• d-Block elements are typical metals

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• Physical properties of d-Block elements:

• Exceptions : Mercury

low melting point

liquid at room temperature and pressure

lustrous

high melting points and boiling points

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• d-block elements

extremely useful as construction materials

strong and unreactive

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used for construction and making machinery nowadays

abundant

easy to extract

• Iron

cheap

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• Iron

corrodes easily

often combined with other elements to form steel

harder and more resistant to corrosion

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• Titanium

used to make aircraft and space shuttles

expensive

Corrosion resistant, light, strong and withstand large temperature changes

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• The similar atomic radii of the transition metals facilitate the formation of substitutional alloys

the atoms of one element to replace those of another

element

modify their solid structures and physical properties

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• Manganese

confers hardness & wearing resistance to its alloys

e.g. duralumin : alloy of Al with Mn/Mg/Cu

• Chromium

confers inertness to stainless steel

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Atomic Radii and Ionic RadiiAtomic Radii and Ionic Radii

• Two features can be observed:

1. The d-block elements have smaller atomic radii than the s-block

elements2. The atomic radii of the d-block

elements do not show much variation across the series

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Variation in atomic radius of the first 36 elements


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27

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(i) Nuclear charge

(ii) Shielding effect (repulsion between e-)

(i) > (ii)

(i) (ii)

(ii) > (i)

On moving across the Period,

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• At the beginning of the series

atomic number

effective nuclear charge

the electron clouds are pulled closer to the nucleus

atomic size


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• In the middle of the series

the effective nuclear charge experienced by 4s electrons

increases very slowly

only a slow decrease in atomic radius in this region

more electrons enter the inner3d sub-shell

The inner 3d electrons shield the outer 4s electrons

effectively

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• At the end of the series

the screening and repulsive effects of the electrons in the 3d sub- shell become even stronger

Atomic size


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• Many of the differences in physical and chemical properties between the d-block and s-block elements

explained in terms of their differences in electronic configurations and atomic radii

Comparison of Some Physical Comparison of Some Physical and Chemical Properties and Chemical Properties between the between the dd-Block and -Block and ss-Block -Block ElementsElements

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1. 1. DensityDensity

Densities (in g cm–3) of the s-block elements and the first series of the d-block elements at

20C

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• d-block > s-block

the atoms of the d-block elements 1. are generally smaller in size

2. are more closely packed

(fcc/hcp vs bcc in group 1)

3. have higher relative atomic masses


35

• The densities

generally increase across the first series of the d-block elements

1. general decrease in atomic radius across the

series

2. general increase in atomic mass across the series


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2. 2. Ionization Ionization

EnthalpyEnthalpy

ElementIonization enthalpy (kJ mol–1)

1st 2nd 3rd 4th

K

Ca

418

590

3 070

1 150

4 600

4 940

5 860

6 480

Sc

Ti

V

Cr

632

661

648

653

1 240

1 310

1 370

1 590

2 390

2 720

2 870

2 990

7 110

4 170

4 600

4 770

K Ca (sharp ) ; Ca Sc (slight )

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EnthalpyEnthalpy

ElementIonization enthalpy (kJ mol–1)

1st 2nd 3rd 4th

Cr

Mn

Fe

Co

Ni

Cu

Zn

653

716

762

757

736

745

908

1 590

1 510

1 560

1 640

1 750

1 960

1 730

2 990

3 250

2 960

3 230

3 390

3 550

3 828

4 770

5 190

5 400

5 100

5 400

5 690

5 980

Sc Cu (slight ) ; Cu Zn (sharp )

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• The first ionization enthalpies of thed-block elements

greater than those of the s-block elements in the same period of

the Periodic Table

1. The atoms of the d-block elements are smaller in size

2. greater effective nuclear charges


EnthalpyEnthalpy

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Sharp across periods 1, 2 and 3

Slight across the transition series

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• Going across the first transition series

the nuclear charge of the elements increases

additional electrons are added to the ‘inner’ 3d sub-shell


EnthalpyEnthalpy

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• The screening effect of the additional3d electrons is significant


EnthalpyEnthalpy

• The effective nuclear charge experienced by the 4s electrons increases very slightly across the series• For 2nd, 3rd, 4th… ionization enthalpies,

slight and gradual across the series are observed.

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Electron has to be removed from completely filled 3p subshell

3d5

3d5

3d5

3d10

d10/s2Cr+

Mn2

+

Fe3+

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• The first few successive ionization enthalpies for the d-block elements

do not show dramatic changes

4s and 3d energy levels are close to each other


EnthalpyEnthalpy

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3. 3. Melting Points and Melting Points and

HardnessHardness

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

d-block >> s-block

1. both 4s and 3d e- are involved in the formation of metal bonds

2. d-block atoms are smaller

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HardnessHardnessK has an exceptionally small m.p. because it has an more open b.c.c. structure.

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

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Unpaired electrons are relatively more involved in the sea of electrons

Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

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3d 4s

Sc

Ti

V

1.m.p. from Sc to V due to the of unpaired d-electrons (from d1 to d3)


1541 1668 1910 1907 1246 1538 1495 1455 1084 419

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2.m.p. from Fe to Zn due to the of unpaired d-electrons (from 4 to 0)


1541 1668 1910 1907 1246 1538 1495 1455 1084 419

3d 4s

Fe

Co

Ni

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1541 1668 1910 1907 1246 1538 1495 1455 1084 419

3. Cr has the highest no. of unpaired electrons but its m.p. is lower than V.

3d 4s

Cr

It is because the electrons in the half-filled d-subshell are relatively less involved in the sea of electrons.

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1541 1668 1910 1907 1246 1538 1495 1455 1084 419

4. Mn has an exceptionally low m.p. because it has the very open cubic structure.

Why is Hg a liquid at room conditions ?

All 5d and 6s electrons are paired up and the size of the atoms is much larger than that of Zn.

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• The metallic bonds of the d-block elements are stronger than those of the s-block elements

much harder than the s-block elements


HardnessHardness• The hardness of a metal depends on

the strength of the metallic bonds

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Mohs scale : - A measure of hardness

Talc Diamond

0 10 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn

0.5 1.5 3.0 4.5 6.1 9.0 5.0 4.5 -- -- 2.8 2.5

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• In general, the s-block elements

react vigorously with water to form metal hydroxides and hydrogen

4. 4. Reaction with WaterReaction with Water

• The d-block elements

react very slowly with cold water

react with steam to give metal oxides and hydrogen

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4. 4. Reaction with WaterReaction with Water

2K(s) + 2H2O(l) 2KOH(aq) + H2(g)2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)Zn(s) + H2O(g) ZnO(s) + H2(g)

3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

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d-block compounds vs s-block compoundsA Summary : -

Ions of d-block metals have higher charge density

more polarizing

1. more covalent in nature

2. less soluble in water

3. less basic (more acidic)

Basicity : Fe(OH)3 < Fe(OH)2 << NaOH

Charge density : Fe3+ > Fe2+ > Na+

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4. less thermally stable e.g. CuCO3 << Na2CO3

5. tend to exist as hydrated salts

e.g. CuSO4.5H2O, CoCl2.2H2O

6. hydrated ions undergo hydrolysis more easily

e.g. [Fe(H2O)6]3+(aq) + H2O [Fe(OH)(H2O)5]2+(aq) + H3O+

d-block compounds vs s-block compoundsA Summary : -

acidic

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• One of the most striking properties

variable oxidation states

Variable Oxidation StatesVariable Oxidation States

• The 3d and 4s electrons are

in similar energy levels

available for bonding

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• Elements of the first transition series

form ions of roughly the same stability by losing different

numbers of the 3d and 4s electrons

Variable Oxidation StatesVariable Oxidation States

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Oxidation

statesOxides / Chloride

+1Cu2O

Cu2Cl2

+2TiO VO CrO MnO FeO CoO NiO CuO ZnO

TiCl2 VCl2 CrCl2 MnCl2 FeCl2 CoCl2 NiCl2 CuCl2 ZnCl2

+3Sc2O3 Ti2O3 V2O3 Cr2O3 Mn2O3 Fe2O3 Ni2O3 • xH2O

ScCl3 TiCl3 VCl3 CrCl3 MnCl3 FeCl3

+4TiO2 VO2 MnO2

TiCl4 VCl4 CrCl4

+5 V2O5

+6 CrO3

+7 Mn2O7

Oxidation states of the elements of the first transition series in their oxides and chlorides

60

Oxidation states of the elements of the first transition series in their compounds

Element Possible oxidation state

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Element Possible oxidation state

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

+3

+1 +2 +3 +4

+1 +2 +3 +4 +5

+1 +2 +3 +4 +5 +6

+1 +2 +3 +4 +5 +6 +7

+1 +2 +3 +4 +5 +6

+1 +2 +3 +4 +5

+1 +2 +3 +4 +5

+1 +2 +3

+2

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1. Scandium and zinc do not exhibit variable oxidation states

• Scandium of the oxidation state +3

the stable electronic configuration of argon (i.e. 1s22s22p63s23p6)• Zinc of the oxidation state +2

the stable electronic configuration of [Ar] 3d10

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2. (a) All elements of the first transition series (except Sc) can show an oxidation state of +2

(b) All elements of the first transition series (except Zn) can show an oxidation state of +3

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3. Manganese has the highest oxidation state +7

E.g. MnO4-, Mn2O7

Mn7+ ions do not exist.

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The +7 state of Mn does not mean that all 3d and 4s electrons are removed from Mn to give Mn7+.

Instead, Mn forms covalent bonds with oxygen atoms by making use of its half filled orbitals

Mn

O

OO

O-

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Draw the structure of Mn2O7

Mn

O

OO

OMn

O

OO

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3. Manganese has the highest oxidation state +7

• The highest possible oxidation state

= the total no. of the 3d and 4s electrons

inner electrons (3s, 3p…) are not involved in covalent bond

formation

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4. For elements after manganese, there is a reduction in the number of possible oxidation states

• The 3d electrons are held more firmly

the decrease in the number of unpaired electrons

the increase in nuclear charge

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Stability : - Mn2+(aq) > Mn3+(aq)

[Ar] 3d5 [Ar] 3d4

5. The relative stability of various oxidation states is correlated with the stability of electronic configurations

ohydrationH : Fe3+ > Fe2+

Major factor

Major factor

Fe3+(aq) > Fe2+

(aq)

[Ar] 3d5 [Ar] 3d6

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Stability : -Zn2+(aq) > Zn+

(aq)

[Ar] 3d10 [Ar] 3d104s1

5. The relative stability of various oxidation states is correlated with the stability of electronic configurations

ohydrationH : Zn2+ > Zn+Major factor

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• The compounds of vanadium, vanadium

oxidation states of +2, +3, +4 and +5

forms ions of different oxidation states

show distinctive colours in aqueous solutions

1. 1. Variable Oxidation States of Variable Oxidation States of

Vanadium and their Vanadium and their

InterconversionsInterconversions

7B

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Ion

Oxidation state

of vanadium in

the ion

Colour in

aqueous

solution

V2+(aq)

V3+(aq)

VO2+(aq)

VO2+(aq)

+2

+3

+4

+5

Violet

Green

Blue

Yellow

Colours of aqueous ions of vanadium of different oxidation states

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• In an acidic medium

the vanadium(V) state usually occurs in the form of VO2

+

(aq) dioxovanadium(V) ion

the vanadium(IV) state occurs in the form of VO2+(aq)

oxovanadium(IV) ion




73

• In an alkaline medium

the stable form of the vanadium(V) state is




VO3–(aq), metavanadate(V) or

VO43–(aq), orthovanadate(V),

in strongly alkaline medium

74

• Compounds with vanadium in its highest oxidation state (i.e. +5)

strong oxidizing agents




75

• Vanadium of its lowest oxidation state(i.e. +2)

in the form of V2+(aq)

strong reducing agent

easily oxidized when exposed to air




76

• The most convenient starting material

ammonium metavanadate(V) (NH4VO3)

a white solid

the oxidation state of vanadium is +5



InterconversionsInterconversions• Interconversions of the common oxidation states of vanadium can be carried out readily in the laboratory

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1. Interconversions of Vanadium(V) species




VO2+(aq) V2O5(s) VO3

(aq) VO4

3(aq)

OH

H+

OH

H+

OH

H+

Yellow orange yellow colourless

Vanadium(V) can exist as cation as well as anion

78






(aq) VO4

3(aq)

OH

H+

OH

H+

OH

H+


In acidic medium

In alkaline mediumAmphoteric

79






(aq) VO4

3(aq)

OH

H+

OH

H+

OH

H+


In acidic medium

In alkaline mediumAmphotericGive the equation for the conversion : V2O5

VO2+

V2O5(s) + 2H+(aq) 2VO2+(aq) + H2O(l)

80






(aq) VO4

3(aq)

OH

H+

OH

H+

OH

H+


In acidic medium

In alkaline mediumAmphotericGive the equation for the conversion : V2O5

VO3

V2O5(s) + 2OH(aq) 2VO3(aq) + H2O(l)

81






(aq) VO4

3(aq)

OH

H+

OH

H+

OH

H+


In acidic medium

In alkaline medium

Give the equation for the conversion : VO3

VO2+

VO3(aq) + 2H+(aq) VO2

+(aq) + H2O(l)

Amphoteric

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V5+

H

O

H

H

O

H

H

O

H

H

O

H

VO43(aq) + 8H3O+

8H2O

O

H

H

V5+ ions does not exist in water since it undergoes vigorous hydrolysis to give VO4

3

The reaction is favoured in highly alkaline solution

orthovanadate(V) ion

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V VO43(aq) orthovanadate(V) ion

Cr CrO42(aq) chromate(VI) ion

Mn MnO4(aq) manganate(VII) ion

Draw the structures of VO43, CrO4

2 and MnO4

O

Cr

OO-

O-

O

Mn

OO

O-

O

V-O

OO-

84

V5+

H

O

H

H

O

H

H

O

H

H

O

H

VO3(aq) + 6H3O+

6H2O

O

H

H

The reaction is favoured in alkaline solution

VO3 is a polymeric anion like SiO3

2

Metavanadate(V) ion

85

Metavanadate(V) ion, (VO3)nn

86

V5+

H

O

H

H

O

H

H

O

H

H

O

H

VO2+(aq) + 4H3O+

4H2O

O

H

H

The reaction is favoured in acidic solution

87

2. The action of zinc powder and concentrated hydrochloric acid

vanadium(V) ions can be reduced sequentially to vanadium(II) ions




88



InterconversionsInterconversionsVO2

+(aq)

yellow

Zn

conc. HClVO2+(aq) blue

Zn

conc. HCl

V3+(aq) green

Zn

conc. HClV2+(aq)

violet

89

(a)

Colours of aqueous solutions of compounds containing vanadium in four different oxidation

states:(a) +5; (b) +4; (c) +3; (d) +2

(b) (c) (d)

VO2+(aq) VO2+(aq) V3+(aq) V2+(aq)

90

• The feasibility of the changes in oxidation state of vanadium

can be predicted using standard electrode potentials

Half reaction (V)

Zn2+(aq) + 2e– Zn(s)

VO2+(aq) + 2H+(aq) + e– VO2+(aq) + H2O(l)

VO2+(aq) + 2H+(aq) + e– V3+(aq) + H2O(l)

V3+(aq) + e– V2+(aq)

–0.76

+1.00

+0.34

–0.26

91

• Under standard conditions

zinc can reduce

1. VO2+(aq) to VO2+(aq)




> 0

> 0

> 0

2. VO2+(aq) to V3+(aq)

3. V3+(aq) to V2+(aq)

92



InterconversionsInterconversions2 × (VO2+(aq) + 2H+(aq) + e–

VO2+(aq) + H2O(l)) = +1.00 V–) Zn2+(aq) + 2e– Zn(s) = –0.76 V

2VO2+(aq) + Zn(s) + 4H+(aq)

2VO2+(aq) + Zn2+(aq) + 2H2O(l)

= +1.76 V

93



InterconversionsInterconversions2 × (VO2+(aq) + 2H+(aq) + e–

V3+(aq) + H2O(l)) = +0.34 V

–) Zn2+(aq) + 2e– Zn(s) = –0.76 V

2VO2+(aq) + Zn(s) + 4H+(aq)2V3+(aq) + Zn2+(aq) + 2H2O(l)

= +1.10 V

94



InterconversionsInterconversions2 × (V3+(aq) + e– V2+(aq)) = –

0.26 V–) Zn2+(aq) + 2e– Zn(s) = –0.76 V

2V3+(aq) + Zn(s) 2V2+(aq) + Zn2+

(aq)

= +0.50 V

95

• Manganese

show oxidation states of +2, +3, +4, +5, +6 and +7 in its compounds


Manganese and their Manganese and their


• The most common oxidation states

+2, +4 and +7

96

Ion

Oxidation state of

manganese in the

ion

Colour

Mn2+

Mn(OH)3

Mn3+

MnO2

MnO43

MnO42–

MnO4–

+2

+3

+3

+4

+5

+6

+7

Very pale pink

Dark brown

Red

Black

Bright blue

Green

Purple

Colours of compounds or ions of manganese in different oxidation states

97

(a)

Colours of compounds or ions of manganese in differernt oxidation states: (a) +2; (b) +3; (c) +4

(b) (c)

Mn2+(aq) Mn(OH)3(aq)

MnO2(s)

98

(e)(d)

Colours of compounds or ions of manganese in differernt oxidation states: (d) +6; (e) +7

MnO42–(aq) MnO4

–(aq)

99

• Manganese of the oxidation state +2

the most stable at pH 0


Manganese and their Manganese and their


Mn2+Mn3++1.50V Mn

1.18V

MnO4

+1.51V

MnO2

+1.23V

100

Mn(VII)

Explosive on heating and extremely oxidizing2KMnO4 K2MnO4 + MnO2 + O2

heat+7 +6 +42 0

in ON = 2(+2) = +4

in ON = (1) + (3) = 4

101

Mn(VII)

in ON = 6(+2) = +12

in ON = 4(3) = 12

2 0+4+7

4MnO4 + 4H+ 4MnO2 + 2H2O +

3O2

light

The reaction is catalyzed by light

Acidified KMnO4(aq) is stored in amber bottle

102

Oxidizing power of Mn(VII) depends on pH of the solution

In an acidic medium (pH 0)

MnO4–(aq) + 8H+(aq) + 5e– Mn2+(aq) +

4H2O(l) = +1.51 V

In a neutral or alkaline medium (up to pH 14)

MnO4–(aq) + 2H2O(l) + 3e– MnO2(s) +

4OH (aq) = +0.59 V

103

The reaction does not involve H+(aq) nor OH(aq)

Why is the Eo of MnO4 MnO4

2 Eo = +0.56V

not affected by pH ?MnO4

(aq) + e MnO42 Eo = +0.56V

104

MnO4(aq) + e MnO4

2 Eo = +0.56V

When [OH(aq)] > 1M

In an acidic medium (pH 0)

MnO4–(aq) + 8H+(aq) + 5e– Mn2+(aq) +

4H2O(l) = +1.51 V

In a neutral or alkaline medium (up to pH 14)MnO4

–(aq) + 2H2O(l) + 3e– MnO2(s) + 4OH (aq) = +0.59

VUnder what conditions is the following conversion favourable?

105

Predict if Mn(VI) Mn(VII) + Mn(IV) is feasible at (i) pH 0 and (ii) pH 14

At pH 0 (1) 2(3)

3MnO42(aq) + 4H+(aq) 2MnO4

(aq) + MnO2(s) + 2H2O(l)

Eocell = +1.70V (feasible)At pH 14 (2) 2(3)

3MnO42(aq) + 2H2O(l) 2MnO4

(aq) + MnO2(s) + 4OH(aq)

Eocell = +0.04V (much less feasible)

MnO42(aq) + 4H+(aq) + 2e MnO2(s) + 2H2O(l) Eo =

+2.26VMnO4

2(aq) + 2H2O(l) + 2e MnO2(s) + 4OH(aq) Eo = +0.60VMnO4

+ e MnO42 Eo = +0.56V

(1)

(2)

(3)

Mn(VI) is unstable in acidic medium

106

Mn(IV) Oxidizing in acidic medium

MnO2(s) + 4H+(aq) + 2e– Mn2+(aq) + 2H2O(l) = 1.23

V• Used in the laboratory production of

chlorine

MnO2(s) + 4HCl(aq) MnCl2(aq) + 2H2O(l) + Cl2(g)

107

Mn(IV) Reducing in alkaline medium

• Oxidized to Mn(VI) in alkaline medium

2MnO2 + 4OH + O2 2MnO42 + 2H2O

108

MnO2 is oxidized to MnO42 in alkaline medium

2MnO2 + 4OH + O2 2MnO42 + 2H2O

Suggest a scheme to prepare MnO4 from

MnO2

109

Cu+(aq) + e Cu(s) Eo = +0.52V

Cu2+(aq) + 2e Cu(s) Eo = +0.34V

Cu2+(aq) is more stable than Cu+(aq)

The only copper(I) compounds which can be stable in water are those which are

(i) insoluble (e.g. Cu2O, CuI, CuCl)

(ii) complexed with ligands other than water

e.g. [Cu(NH3)4]+ Cu+(aq) + e Cu(s)[Cu+(aq)] Equil. Position shifts to

left

110

Estimation of Cu2+ ions

2Cu2+(aq) + 4I(aq) 2CuI(s) + I2(aq)

I2(aq) + 2S2O32(aq) 2I(aq) + S4O6

2(aq)

unknown

excess white fixed

standard solution

111

• Another striking feature of the d-block elements is the formation of complexes

Formation of ComplexesFormation of Complexes

112


A complex is formed when a central metal atom or ion is surrounded by other molecules or ions which form dative covalent bonds with the central metal atom or ion.

The molecules or ions that donate lone pairs of electrons to form the dative covalent bonds are called ligands.

113

• A ligand

can be an ion or a molecule having at least one lone pair of electrons that can be donated to the central metal atom or ion to form a dative

covalent bond


114


electrically neutral Ni(CO)4

[Co(H2O)6]3+positively charged

[Fe(CN)6]3negatively charged

Complexes can be

115

A co-ordination compound is either

a neutral complex e.g. Ni(CO)4

or made of

a complex ion and another ion

e.g. [Co(H2O)6]Cl3 [Co(H2O)6]3+ + 3Cl

K3[Fe(CN)6] 3K+ + [Fe(CN)6]3

116

Criteria for complex formation

2. High charge density of the central metal ions.

1. Presence of vacant and low-energy 3d, 4s, 4p and 4d orbitals in the metal atoms or ions to accept lone pairs from ligands.

117

Diagrammatic representation of the formation of a complex

118

[Co(H2O)6]2+

Co :

3d 4s 4p 4d

Co2+ :

3d 4s 4p 4d

sp3d2 hybridisation

The six sp3d2 orbitals accept six lone pairs from six H2O.

Arranged octahedrally to minimize repulsion between dative bonds.

119

1. 1. Complexes with Monodentate Complexes with Monodentate

LigandsLigandsA ligand that forms one dative covalent bond only is called a monodentate ligand. • Examples:

neutral CO, H2O, NH3

anionic Cl–, CN–, OH–

120

121

In the formation of complexes, classify the transition metal ion and the ligand as a Lewis acid or base. Explain your answer briefly.

122

What is the oxidation state of the central metal ?

123


124


125

2. 2. Complexes with Bidentate LigandsComplexes with Bidentate Ligands

A ligand that can form two dative covalent bonds with a metal atom or ion is called a bidentate ligand.

A ligand that can form more than one dative covalent bond with a central metal atom or ion is called a chelating ligand.

126

Ethylenediamine (H2NCH2CH2NH2)

ethylenediamine

Oxalate (C2O42–)

oxalate ion

The term chelate is derived from Greek, meaning ‘claw’.The ligand binds with the metal like the great claw of the lobster.

127

ethylenediamine oxalate ion

128

3. 3. Complexes formed by Multidentate Complexes formed by Multidentate

LigandsLigandsLigands that can form more than two dative covalent bonds to a metal atom or ion are called multidentate ligands. Some ligands can form as many as six bonds to a metal atom or ion. • Example:

ethylenediaminetetraacetic acid (abbreviated as EDTA)

129

ethylenediaminetetraacetate ion

EDTA forms six dative covalent bonds with the metal ion through six atoms giving a very stable complex.

hexadentate ligand

130

Structure of the complex ion formed by iron(II) ions and EDTA

?

131

Uses of EDTA

1. Determining concentrations of metal ions by complexometric titrations

e.g. determination of water hardness

2. In chelation therapy for mercury poisoning and lead poisoning

Poisonous Hg2+ and Pb2+ ions are removed by forming stable complexes with EDTA.3. Preparing buffer solutions ( )

4aa K toK1

4. As preservative to prevent catalytic oxidation of food by metal ions.

132

The coordination number of the central metal atom or ion in a complex is the number of dative covalent bonds formed by the central metal atom or ion in a complex.

Complex

The central metal

atom or ion in the

complex

Coordinati

on

number

[Ag(NH3)2]+ Ag+ 2

[Cu(NH3)4]2+ Cu2+ 4

[Fe(CN)6]3– Fe3+ 6

133

4. 4. Nomenclature of Transition Metal Nomenclature of Transition Metal

Complexes with Monodentate Complexes with Monodentate

LigandsLigandsIUPAC conventions

1. (a)For any ionic compound

the cation is named before the anion

(b)If the complex is neutral

the name of the complex is the name of the compound

134

1. (c) In naming a complex (which may be neutral, a cation or an anion)

the ligands are named before the central metal atom or ion

the liqands are named in alphabetical order

(prefixes not counted)(d)The number of each type of ligands are specified by the Greek prefixes1 mono- 2 di 3 tri

4 tetra- 5 penta- 6 hexa-

135

1. (e)The oxidation number of the metal ion in the complex is indicated immediately after the name of the metal using Roman numerals

[CrCl2(H2O)4]Cltetraaquadichlorochromium(III) chloride

[CoCl3(NH3)3]triamminetrichlorocobalt(III)

K3[Fe(CN)6]potassium hexacyanoferrate(III)

136

2. (a)The root names of anionic ligands

always end in “-o”CN–cyano

Cl–

chloro

Br

bromo

I iodo

OH

hydroxo

NO2 nitro

SO42

sulphato

H

hydrido

(b)The names of neutral ligands are the names of the molecules

except NH3, H2O, CO and NO

137

Neutral ligand Name of ligand

Ammonia (NH3)

Water (H2O)

Carbon monoxide (CO)

Nitrogen monoxide (NO)

Ammine

Aqua

Carbonyl

Nitrosyl

138

3. (a)If the complex is anionic

the suffix “-ate” is added to the end of the name of the metal,

followed by the oxidation number of that metal

[CuCl4]2–

[Fe(CN)6]3

[CoCl4]2

Name of the complexFormula

139

Metal Name in anionic complex

Titanium

Vanadium

Chromium

Manganese

Iron

Cobalt

Nickel

Copper

Zinc

Platinum

Titanate

Vanadate

Chromate

Manganate

Ferrate

Cobaltate

Nickelate

Cuprate

Zincate

Platinate

Names of some common metals in anionic complexes

140

3. (b)If the complex is cationic or neutral

the name of the metal is unchanged

followed by the oxidation number of that metal

[CoCl3(NH3)3]

[CrCl2(H2O)4]+

Name of the complexFormula

141

(a) Write the names of the following compounds.

(i) [Fe(H2O)6]Cl2

(ii) [Cu(NH3)4]Cl2

(iii) [PtCl4(NH3)2]

(iv) K2[CoCl4]

(v) [Cr(NH3)4SO4]NO3

(vi) [Co(H2O)2(NH3)3Cl]Cl

(vii) K3[AlF6]

142

(i) [Fe(H2O)6]Cl2

(ii) [Cu(NH3)4]Cl2

(iii) [PtCl4(NH3)2]

(iv) K2[CoCl4]

(v) [Cr(NH3)4SO4]NO3

143

(a) (vi) [Co(H2O)2(NH3)3Cl]Cl

(vii) K3[AlF6]

144

(b) Write the formulae of the following compounds.

(i)Pentaamminechlorocobalt(III) chloride

(ii) Ammonium hexachlorotitanate(IV)

(iii)Tetraaquadihydroxoiron(II)

145

Coordination number

of the central metal

atom or ion

Shape of complex Example

2

linear

[Ag(NH3)2]+

[Ag(CN)2]–

Stereo-structures of complexes

146

[Cu(NH3)4]2+

[CuCl4]2–

Square planar

[Zn(NH3)4]2+

[CoCl4]2+

Tetrahedral4

ExampleShape of complexCoordination number


atom or ion


sp3

dsp2

147

Tetra-coordinated Complexes(a) Tetrahedral complexes

tetrahedral shape

148

(b) Square planar complexes

have a square planar structure

Tetra-coordinated Complexes

149

• Example:

Tetra-coordinated Complexes

150

Coordination number


atom or ion

Shape of complex Example

6

Octahedral

[Cr(NH3)6]3+

[Fe(CN)6]3–


sp3d2

151

Hexa-coordinated Complexes• Example:

152

6. 6. Displacement of Ligands and Displacement of Ligands and

Relative Stability of Complex IonsRelative Stability of Complex Ions

Different ligands have different tendencies to bind with the metal atom/ion

ligands compete with one another for the metal atom/ion.

A stronger ligand can displace a weaker ligand from a complex.

153

6. 6. Displacement of Ligands and Displacement of Ligands and

Relative Stability of Complex IonsRelative Stability of Complex Ions

[Fe(H2O)6]2+(aq) + 6CN–(aq)Hexaaquairon(II) ion

[Fe(CN)6]4–(aq) + 6H2O(l)

Hexacyanoferrate(II) ion

Stronger ligand

Weaker ligand

Reversible reaction

Equilibrium position lies to the right

Kst 1024 mol6 dm18

154

[Ni(H2O)6]2+(aq) + 6NH3(aq)Hexaaquanickel(II) ion

[Ni(NH3)6]2+(aq) + 6H2O(l)Hexaamminenickel(II) ion

Stronger ligand

Weaker ligand

The greater the equilibrium constant,the stronger is the ligand on the LHS andthe more stable is the complex on the RHS

The equilibrium constant is called the stability constant, Kst

155

Consider the general equilibrium system below,

[M(H2O)x]m+ + xLn [M(L)x](m-xn)+ + xH2O

xnmx2

xn)(mx

st ]][L]O)[[M(H]][[M(L)

K

Units = (mol dm3)-x

Kst measures the stability of the complex, [M(L)x](m-

xn)+, relative to the aqua complex, [M(H2O)x]m+

156

Relative strength of some ligands bonding with copper(II) ions

monodentate

bidentate

multidentate

157

Equilibrium Kst ((mol dm–3)–n)

[Cu(H2O)4]2+(aq) + 4Cl–(aq)

[CuCl4]2–(aq) + 4H2O(l)

[Cu(H2O)4]2+(aq) + 4NH3(aq)

[Cu(NH3)4]2+(aq) + 4H2O(l)

[Cu(H2O)4]2+(aq) + 2H2NCH2CH2NH2(aq)

[Cu(H2NCH2CH2NH2)2]2+(aq) + 4H2O(l)

[Cu(H2O)4]2+(aq) + EDTA4–(aq)

[CuEDTA]2–(aq) + 4H2O(l)

4.2 × 105

1.1 × 1013

1.0 × 1018.7

1.0 × 1018.8

What is the Kst of the formation of [Cu(H2O)4]2+(aq) ?

158

Factors affecting the stability of complexes

1. The charge density of the central ion

7.7 × 104

4.5 × 1033

[Co(H2O)6]2+(aq) + 6NH3(aq)

[Co(NH3)6]2+(aq) + 6H2O(l)

[Co(H2O)6]3+(aq) + 6NH3(aq)

[Co(NH3)6]3+(aq) + 6H2O(l)

Kst (mol6 dm18)Equilibrium

≈ 1024

≈ 1031

[Fe(H2O)6]2+(aq) + 6CN–(aq)

[Fe(CN)6]4–(aq) + 6H2O(l)

[Fe(H2O)6]3+(aq) + 6CN–(aq)

[Fe(CN)6]3–(aq) + 6H2O(l)

159


2. The nature of ligands

Ability to form complex : -

CN > NH3 > Cl > H2O

[Zn(CN)4]2 Kst = 5 1016 mol4 dm12

[Zn(NH3)4]2+ Kst = 3.8 109 mol4 dm12

[Cu(NH3)4]2+ Kst = 1.1 1013 mol4 dm12

[CuCl4]2+ Kst = 4.2 105 mol4 dm12

160


3. The pH of the solution

In acidic solution, the ligands are protonated

lone pairs are not available

the complex decomposes

[Cu(NH3)4]2+(aq) + 4H2O(l) [Cu(H2O)4]2+(aq) + 4NH3(aq)

NH4+(aq)

H+

(aq)Equilibrium position shifts to the right

161

Consider the stability constants of the following silver complexes:

Ag+(aq) + 2Cl–(aq) [AgCl2]–(aq) Kst = 1.1 × 105 mol–2 dm6

Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+(aq) Kst = 1.6 × 107 mol–2 dm6

Ag+(aq) + 2CN–(aq) [Ag(CN)2]–(aq) Kst = 1.0 × 1021 mol–2 dm6

What will be formed when CN–(aq) is added to a solution of [Ag(NH3)2]+?

162

What will be formed when NH3(aq) is added to a solution of [Ag(CN)2]–?

Consider the stability constants of the following silver complexes:

Ag+(aq) + 2Cl–(aq) [AgCl2]–(aq) Kst = 1.1 × 105 mol–2 dm6

Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+(aq) Kst = 1.6 × 107 mol–2 dm6

Ag+(aq) + 2CN–(aq) [Ag(CN)2]–(aq) Kst = 1.0 × 1021 mol–2 dm6

163

FeSO4(aq) is used as the antidote for cyanide poisoning

[Fe(H2O)6]2+(aq) + 6CN(aq) [Fe(CN)6]4 + 6H2O(l)

Kst 1 1024 mol6 dm18 Very stable

Why is Fe2(SO4)3(aq) not used as the antidote ?

Only free CN is poisonous

164

[Cu(H2O)4]2+(aq) + Cl(aq) [Cu(H2O)3Cl]+(aq) + H2O(l)K1 = 6.3102 mol1 dm3

[Cu(H2O)3Cl]+(aq) + Cl(aq) [Cu(H2O)2Cl2](aq) + H2O(l)K2 = 40 mol1 dm3

[Cu(H2O)2Cl2](aq) + Cl(aq) [Cu(H2O)Cl3](aq) + H2O(l)K3 = 5.4 mol1 dm3

[Cu(H2O)Cl3](aq) + Cl(aq) [CuCl4]2(aq) + H2O(l)

K1 = 3.1 mol1 dm3

[Cu(H2O)4]2+(aq) + 4Cl(aq) [CuCl4]2(aq) + 4H2O(l)

Kst = K1 K2 K3 K4 = 4.2 105 mol4 dm12

165

K1 > K2 > K3 > K4Reasons :

1. Statistical effect

On successive displacement, less water ligands are available to be displaced.

166

K1 > K2 > K3 > K4Reasons :

[Cu(H2O)Cl3] Cl repulsion

[Cu(H2O)4]2+ Cl attraction

2. Charge effect

On successive displacement, the Cl experiences more repulsion from the complex

167

Formula of copper(II)

complex

Colour of the

complex

[Cu(H2O)4]2+

[CuCl4]2–

[Cu(NH3)4]2+

[Cu(H2NCH2CH2NH2)]2+

[Cu(EDTA)]2–

Pale blue

Yellow

Deep blue

Violet

Sky blue

Colours of some copper(II) complexes

The displacement of ligands are usually accompanied with easily observable colour changes

168

The colours of many gemstones are due to the presence of small quantities of d-block metal

ions

Coloured IonsColoured Ions

169

• Most of the d-block metals

form coloured compounds


due to the presence of the incompletely filled d

orbitals in thed-block metal ionsWhich aqueous transition metal ion(s)

is/are not coloured ?

170

Number of

unpaired

electrons in 3d

orbitals

d-Block metal

ion

Colour in

aqueous

solution

0

Sc3+

Ti4+

Zn2+

Cu+

Colourless

Colourless

Colourless

Colourless

1

Ti3+

V4+

Cu2+

Purple

Blue

Blue

Colours of some d-block metal ions in aqueous solutions

171

Number of

unpaired

electrons in 3d

orbitals

d-Block metal

ion

Colour in

aqueous

solution

2V3+

Ni2+

Green

Green

3

V2+

Cr3+

Co2+

Violet

Green

Pink


172

Number of

unpaired

electrons in 3d

orbitals

d-Block metal

ion

Colour in

aqueous

solution

4

Cr2+

Mn3+

Fe2+

Blue

Violet

Green

5Mn2+

Fe3+

Very pale pink

Yellow


173


Co2+(aq) Fe3+(aq)Zn2+(aq)

174

Cu2+(aq)Fe2+(aq)Mn2+(aq)


175

A substance absorbs visible light of a certain wavelength

reflects or transmits visible light of other wavelengths

(complimentary colour)

appears coloured

Coloured ionLight

absorbed

Light reflected or

transmitted

[Cu(H2O)4]2+

(aq)Yellow Blue

[CuCl4]2(aq) Blue Yellow

176

Blue

Yellow

Magenta

Green

RedCyan

Violet

Greenish yellow

Complimentary colour chart

Blue light absorbed

Appears yellow

Yellow light absorbed

Appears blue

177

• The absorption of visible light is due to the d-d electronic transition

3d 3d

i.e. an electron jumping from a lower 3d orbital to a higher 3d orbital


178

In gaseous state,

the five 3d orbitals are degenerate

i.e. they are of the same energy level

In the presence of ligands,

The five 3d orbitals interact with the orbitals of ligands and split into two groups of orbitals with slightly different energy levels

179

The splitting of the degenerate 3d orbitals of a d-block metal ion in an octahedral

complex

ge

gt2

222 yxzd , d

yzxzxy d,d , d

distributes along x and y axesdistributes along z

axisInteract more strongly with the orbitals of ligands

180

181

• Criterion for d-d transition : -

presence of unpaired d electrons in the d-block metal atoms or ions

d-d transition is possible for

3d1 to 3d9 arrangements

d-d transition is NOT possible for

3d0 and 3d10 arrangements

182

3d9 : d-d transition is possible

Cu2

+

H2O as ligand

183


*Cu2+

Yellow light absorbed, appears blue

H2O as ligand

184


Fe2+

185


*Fe2+

Magenta light absorbed, appears green

186

3d10 : d-d transition NOT possible

Zn2+

187

3d0 : d-d transition NOT possible

Sc3+

188

E

E depends on

1. the nature and charge of metal ion

[Fe(H2O)6]2+ green,

[Fe(H2O)6]3+ yellow

[Cu(H2O)4]2+ blue,

[CuCl4]2 yellow 2. the nature of ligand

189

Why does Na+(aq) appear colourless ?


190

The EndThe End

191

• The d-block metals and their compounds

important catalysts in industry and biological systems

Catalytic Properties of Transition Catalytic Properties of Transition Metals and their CompoundsMetals and their Compounds

192

d-Block

metalCatalyst Reaction catalyzed

VV2O5 or

vanadate(V) (VO3–)

Contact process

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

Fe FeHaber process

N2(g) + 3H2(g) 2NH3(g)

The use of some d-block metals and their compounds as catalysts in industry

193

d-Block

metalCatalyst Reaction catalyzed

Ni Ni

Hardening of vegetable oil

(Manufacture of margarine)

RCH = CH2 + H2 RCH2CH3

Pt Pt

Catalytic oxidation of ammonia

(Manufacture of nitric(V) acid)

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l)

The use of some d-block metals and their compounds as catalysts in industry

194

• The d-block metals and their compounds exert their catalytic actions in either

heterogeneous catalysis

homogeneous catalysis


195

• Generally speaking, the function of a catalyst

provides an alternative reaction pathway of lower activation

energy

enables the reaction to proceed faster than the uncatalyzed one


196

1.1. Heterogeneous CatalysisHeterogeneous Catalysis

• The catalyst and reactants

exist in different states

• The most common heterogeneous catalysts

finely divided solids for gaseous reactions

197


A heterogeneous catalyst provides a suitable reaction surface for the reactants to come close together and react.

198


• Example:

The synthesis of gaseous ammonia from nitrogen and hydrogen (i.e. Haberprocess)

N2(g) + 3H2(g) 2NH3(g)

199


• In the absence of a catalyst

the formation of gaseous ammonia proceeds at an extremely low

rate• The probability of collision of four

gaseous molecules (i.e. one nitrogen and three hydrogen molecules)

very small

200


• The four reactant molecules

collide in proper orientation in order to form the product

• The bond enthalpy of the reactant (N N),

very large

the reaction has a high activation energy

201


• In the presence of iron as catalyst

the reaction proceeds much faster

provides an alternative reaction pathway of lower activation

energy

202


• Fe is a solid

• H2, N2 and NH3 are gases

• The catalytic action occurs at the interface between these two states

• The metal provides an active reaction surface for the reaction to occur

203


1. Gaseous nitrogen and hydrogen molecules

diffuse to the surface of the catalyst

2. The gaseous reactant molecules

adsorbed (i.e. adhered) on the surface of the catalyst

204


2. The iron metal

many 3d electrons and low-lying vacant 3d orbitals

form bonds with the reactant molecules

adsorb them on its surface

weakens the bonds present in the reactant molecules

205


2. The free nitrogen and hydrogen atoms

come into contact with each other

readily to react and form the product3. The weak interaction between the product and the iron surface

gaseous ammonia molecules desorb easily

206

The catalytic mechanism of the formation of gaseous ammonia from nitrogen and hydrogen

207


208


209


210


211

43.3 Characteristic Properties of the d-Block Elements and their compound (SB p.162)


• Sometimes, the reactants

in aqueous or liquid state

• Other example:

The decomposition of hydrogen peroxide

2H2O2(aq) 2H2O(l) + O2(g)

MnO2(s) as the catalyst

212

Energy profiles of the reaction of nitrogen and hydrogen to form gaseous ammonia in the presence and absence of

a heterogeneous catalyst

213

2.2. Homogeneous CatalysisHomogeneous Catalysis

• A homogeneous catalyst

the same state as the reactants and products

the catalyst forms an intermediate with the reactants in the

reaction

changes the reaction mechanism to an another one with a lower

activation energy

214


In homogeneous catalysis, the ability of the d-block metals to exhibit variableoxidation states enables the formation of the reaction intermediates.

• Example:

The reaction between peroxodisulphate(VI) ions (S2O8

2–) and iodide ions (I–)

215


• Peroxodisulphate(VI) ions

oxidize iodide ions to iodine in an aqueous solution

themselves being reduced to sulphate(VI) ions

S2O82–(aq) + 2I–(aq)

2SO42–(aq) + I2

(aq) V .Eocell 511

216


• Iron(III) ions

take part in the reaction by oxidizing

iodide ions to iodine

themselves being reduced to iron(II) ions2I–(aq) + 2Fe3+(aq)

I2(aq) + 2Fe2+(aq) = +0.23 V

• The reaction is very slow due to strong repulsion between like charges.

217


• Iron(II) ions

subsequently oxidized by peroxodisulphate(VI) ion

the original iron(III) ions are regenerated

2Fe2+(aq) + S2O82–(aq)

2Fe3+(aq) + 2SO42–(aq) =

+1.28 V

218


• The overall reaction:

2I–(aq) + 2Fe3+(aq) I2(aq) + 2Fe2+(aq) =

+0.23 V

S2O82–(aq) + 2I–(aq)

2SO42–(aq) + I2(aq) =

+1.51 V

2Fe2+(aq) + S2O82–(aq)

+) 2Fe3+(aq) + 2SO42–(aq) = +1.28 V

Feasible reaction

219

43.3 Characteristic Properties of the d-Block Elements and their compound (SB p.164)


• Iron(III) ions

catalyze the reaction

acting as an intermediate for the transfer of electrons between

peroxodisulphate(VI) ions and iodide ions

220


• Iodide ions

reduce Fe3+ to Fe2+

• Peroxodisulphate(VI) ions

oxidize Fe2+ to Fe3+

221

Energy profiles for the oxidation of iodide ions by peroxodisulphate(VI) ions in the presence and absence of a homogeneous catalyst

Check Point 43-3ECheck Point 43-3E

222

Besides iron(III) ions, iron(II) ions can also catalyze the reaction between peroxodisulphate(VI) ions and

iodide ions. Why?

Answer

223

Iron(II) ions catalyze the reaction by reacting with the

peroxodisulphate(VI) ions first.

2Fe2+(aq) + S2O82–(aq) 2Fe3+(aq) + 2SO4

2–(aq)

The iron(III) ions formed then oxidize the iodide ions.

2Fe3+(aq) + 2I–(aq) 2Fe2+(aq) + I2(aq)

In this way, the reaction between peroxodisulphate(VI) ions and

iodide ions is catalyzed.

Back

224

Which of the following redox systems might catalyze the oxidation of iodide ions by peroxodisulphate(VI) ions inan aqueous solution?

Cr2O72–(aq) + 14H+(aq) + 6e–

2Cr3+(aq) + 7H2O(l) = +1.33 V

MnO4–(aq) + 8H+(aq) + 5e–

Mn2+(aq) + 4H2O(l) = +1.52 V

Sn4+(aq) + 2e– Sn2+(aq) = +0.15 V

(Given: S2O82–(aq) + 2e– 2SO4

2–(aq) = +2.01 V

I2(aq) + 2e– 2I–(aq) = +0.54 V)

Answer

225

Back

Those redox systems with greater than +0.54 V and smaller than

+2.01 V are able to catalyze the oxidation of iodide ions by

peroxodisulphate(VI) ions in an aqueous solution. Therefore, the

following two redox systems are able to catalyze the reaction.

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

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

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The d -Block Elements

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