The s -Block Elements

1

The The ss-Block Elements-Block Elements

2


• Elements of Groups IA* (the alkali metals) and IIA* (the alkaline earth metals)

constitute the s-block elements

their outermost shell electrons are in the s orbital

*Note: In the following, Groups IA and IIA are abbreviated as Groups I and II respectively.

3 The s-block elements

4


• Similarities

1.highly reactive metals

2.strong reducing agents

3.form ionic compounds with fixed oxidation states of +1 for Group I

elements and +2 for Group II elements

5

[Rn] 7s2[Xe] 6s2[Kr] 5s2[Ar] 4s2[Ne] 3s2[He] 2s2Electronic configuration

*RaRadium

BaBarium

SrStrontium

CaCalcium

MgMagnesium

BeBeryllium

Group II

[Rn] 7s1[Xe] 6s1[Kr] 5s1[Ar] 4s1[Ne] 3s1[He] 2s1Electronic configuration

*FrFrancium

CsCaesium

RbRubidium

KPotassium

NaSodium

LiLithium

Group I

Q.1

6

Group I elementsGroup I elements

• Lithium

7


• Sodium

8


• Potassium

9


• Rubidium

10


• Francium - radioactive

11


• Beryllium

12


• Magnesium

13


• Calcium

14


• Strontium

15


• Barium

16


• Radium - radioactive

17

Characteristic Characteristic Properties of Properties of

thethes-Block s-Block

ElementsElements

18

Group I

element

Electronegativ

ity value

Group II

element

Electronegativ

ity value

Li

Na

K

Rb

Cs

Fr

1.00.90.80.80.7–

Be

Mg

Ca

Sr

Ba

Ra

1.5

1.2

1.0

1.0

0.9

–

All have low electronegativity.

electropositive

19

Group I

element

Electronegativ

ity value

Group II

element

Electronegativ

ity value

Li

Na

K

Rb

Cs

Fr

1.00.90.80.80.7–

Be

Mg

Ca

Sr

Ba

Ra

1.5

1.2

1.0

1.0

0.9

–

EN down the group

EN : Group II > Group I (∵ greater ENC)

20

Group I m.p.(C) b.p.(C) Group II m.p.(C) b.p.(C)

Li 181 1342 Be 1287 2469

Na 98 883 Mg 650 1090

K 63 760 Ca 850 1492

Rb 39 688 Sr 770 1367

Cs 29 690 Ba 714 1637

Fr - - Ra - -

BondingStrength of metallic bond : Group II > Group I

m.p./b.p. : Group II > Group I

21

Group I m.p.(C) b.p.(C) Group II m.p.(C) b.p.(C)

Li 181 1342 Be 1287 2469

Na 98 883 Mg 650 1090

K 63 760 Ca 850 1492

Rb 39 688 Sr 770 1367

Cs 29 690 Ba 714 1637

Fr - - Ra - -

Hardness : - Group I < Group II

Na/K…can be easily cut with a knife

22

Structure

Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba

Group I StructureDensity

(g cm3)Group II Structure

Density

(g cm3)

Li b.c.c. 0.53 Be h.c.p. 1.86

Na b.c.c. 0.97 Mg h.c.p. 1.74

K b.c.c. 0.86 Ca f.c.c. 1.55

Rb b.c.c. 1.53 Sr f.c.c. 2.54

Cs b.c.c. 1.90 Ba b.c.c. 3.59

Fr - - Ra - -

Density : Group II > Group I

23

Structure

Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba

Group I StructureDensity

(g cm3)Group II Structure

Density

(g cm3)

Li b.c.c. 0.53 Be h.c.p. 1.86

Na b.c.c. 0.97 Mg h.c.p. 1.74

K b.c.c. 0.86 Ca f.c.c. 1.55

Rb b.c.c. 1.53 Sr f.c.c. 2.54

Cs b.c.c. 1.90 Ba b.c.c. 3.59

Fr - - Ra - -

Density also depends on size and mass of the atoms

24

Metallic charater (Reactivity) : -

Mn+(aq) + ne M(s)

High tendency to lose electrons as shown by –ve E

Group I (V) Group II (V)

Li -3.04 Be -1.69

Na -2.72 Mg -2.37

K -2.92 Ca -2.87

Rb -2.99 Sr -2.89

Cs -3.02 Ba -2.90

oE oE

25

Metallic charater (Reactivity) : -

Group I > Group II

Group I (V) Group II (V)

Li -3.04 Be -1.69

Na -2.72 Mg -2.37

K -2.92 Ca -2.87

Rb -2.99 Sr -2.89

Cs -3.02 Ba -2.90

oE oE

down the groups

26

Sodium is stored under paraffin oil

sodium

27

Caesium and rubidium are stored in vacuum-sealed

ampoules

caesium

rubidium

28

Formation of Basic OxidesFormation of Basic Oxides

• All alkali metals form more than one type of oxide on burning in air (except lithium)

1. 1. Group I ElementsGroup I Elements

29

• Three types of oxides:

normal oxides

peroxides

superoxides


2O21

O2–

oxide ion

O22–

peroxide ion

2O2O2

–

superoxide ion

Abundant supply

30


Type of oxide formed depends on

1.supply of oxygen

2.reaction temperature

3.charge density of M+

31

• Lithium

when it is burnt in air, it forms normal oxide only


C180

4Li(s) + O2(g) 2Li2O(s)

lithium oxide

32

• Sodium

when it is burnt in an abundant supply of oxygen

forms both the normal oxide and the peroxide


C180

4Na(s) + O2(g) 2Na2O(s)

sodium oxide

C300

2Na2O(s) + O2(g) 2Na2O2(s)

sodium peroxide

excess

33

• Potassium, rubidium and caesium

form All three types of oxides when burnt in sufficient supply of

oxygen


34

• Potassium:

4K(s) + O2(g) 2K2O(s)potassium oxide

2K2O(s) + O2(g) 2K2O2(s)potassium peroxide

K2O2(s) + O2(g) 2KO2(s)potassium superoxide


35

• Rubidium:

4Rb(s) + O2(g) 2Rb2O(s)

2Rb2O(s) + O2(g) 2Rb2O2(s)

Rb2O2(s) + O2(g) 2RbO2(s)


36

• Caesium:

4Cs(s) + O2(g) 2Cs2O(s)

2Cs2O(s) + O2(g) 2Cs2O2(s)

Cs2O2(s) + O2(g) 2CsO2(s)


37

Group I

element

Normal

oxidePeroxide Superoxide

Li

Na

K

Rb

Cs

Li2O

Na2O

K2O

Rb2O

Cs2O

–

Na2O2

K2O2

Rb2O2

Cs2O2

–

–

KO2

RbO2

CsO2

Oxides formed by Group I elements

Cations with high charge densities (Li+ or Na+) tend to polarize the large electron clouds of peroxide ions and/or superoxide ions

Making them decompose to give oxide ions

38


The electron cloud of the superoxide ion is greatly distorted by the small lithium

ion

39

Group I

element

Normal

oxidePeroxide Superoxide

Li

Na

K

Rb

Cs

Li2O

Na2O

K2O

Rb2O

Cs2O

–

Na2O2

K2O2

Rb2O2

Cs2O2

–

–

KO2

RbO2

CsO2

Oxides formed by Group I elements

White solids

Slightly coloured

solids

Highly coloured

solids

40

KO2 used as oxygen generators and CO2 scrubbers in spacecrafts

4KO2 + 2H2O 4KOH + 3O2

2KOH + CO2 K2CO3 + H2O

41

• Beryllium, magnesium and calcium

form normal oxides only on burning in air

2Be(s) + O2(g) 2BeO(s)

2Mg(s) + O2(g) 2MgO(s)

2Ca(s) + O2(g) 2CaO(s)

2. 2. Group II ElementsGroup II Elements

42

Q.2(a)

Be2+, Mg2+ and Ba2+ have higher charge densities

more polarizing

distort the electron cloud of O22

O22 decomposes to give O2

43

Q.2(b)

2SrO(s) + O2(g) 2SrO2(s) strontium peroxide

Sr(s) + O2(g) SrO2(s)

2Sr(s) + O2(g) 2SrO(s)strontium oxide

44

2Ba(s) + O2(g) 2BaO(s) barium oxide

2BaO(s) + O2(g) 2BaO2(s)

barium peroxide

500C

700C

Q.2(b)

Ba(s) + O2(g) BaO2(s)

45

Group

II

elemen

t

Normal

oxidePeroxide

Superoxid

e

Be

Mg

Ca

Sr

Ba

BeO

MgO

CaO

SrO

BaO

–

–

–

SrO2

BaO2

–

–

–

–

–

Oxides formed by Group II elements

KO2 superoxid

e

46

Group

II

elemen

t

Normal

oxidePeroxide

Superoxid

e

Be

Mg

Ca

Sr

Ba

BeO

MgO

CaO

SrO

BaO

–

–

–

SrO2

BaO2

–

–

–

–

–

Oxides formed by Group II elements

All these oxides are basic in nature (except beryllium oxide which is amphoteric)

47

2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

2Rb(s) + 2H2O(l) 2RbOH(aq) + H2(g)

2Cs(s) + 2H2O(l) 2CsOH(aq) + H2(g)

1. 1. Group I Group I

hydroxideshydroxides

Formation of hydroxides

48

For normal oxides,

M2O(s) + H2O(l) 2MOH(aq)

1. 1. Group I Group I



For peroxides,

M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq)

For superoxides,

2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g)

49

Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)

Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g)

Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)Mg reacts with steam but not water.

Be does not react with water and steam.

Mg(s) + H2O(g) MgO(s) + H2(g)

2. Group II 2. Group II



50

2. Group II 2. Group II



CaO(s) + H2O(l) Ca(OH)2(aq)

SrO(s) + H2O(l) Sr(OH)2(aq)

BaO(s) + H2O(l) Ba(OH)2(aq)MgO(s) + H2O(l) Mg(OH)2(aq)slightly soluble

BeO(s) + H2O(l) No reaction

51

Ionic Bonding with FixedIonic Bonding with FixedOxidation State in their Oxidation State in their CompoundsCompounds

Group I : +1

Group II : +2

∵ Low 1st I.E. but very high 2nd I.E.

∵ Low 1st and 2nd I.E. but very high 3rd I.E.

Predominantly ionic

52

Group I

elemen

t

OxideHydrid

e

Chlorid

e

Oxidation state of

Group I element in the

compound

Li

Na

K

Rb

Cs

Li2O

Na2O2

KO2

RbO2

CsO2

LiH

NaH

KH

RbH

CsH

LiCl

NaCl

KCl

RbCl

CsCl

+1+1+1+1+1

Chemical formulae of some Group I compounds and the oxidation states of Group I elements in the compounds

53

Group

II

elemen

t

OxideHydrid

e

Chlorid

e

Oxidation state of

Group II element in

the compound

Be

Mg

Ca

Sr

Ba

BeO

MgO

CaO

SrO

BaO

BeH2

MgH2

CaH2

SrH2

BaH2

BeCl2

MgCl2

CaCl2

SrCl2

BaCl2

+2+2+2+2+2

Chemical formulae of some Group II compounds and the oxidation states of Group II elements in the compounds

54

Weak Tendency to Form Weak Tendency to Form ComplexesComplexesA complex is formed when a central metal atom or ion is surrounded by other molecules or ions (called ligands) which form dative covalent bonds with the central metal atom or ion using their lone pair.

55

Weak Tendency to Form Weak Tendency to Form ComplexesComplexes

Unlike transition metals, all s-block metals (except Be) show little tendency to form complexes

56

Weak Tendency to Form Weak Tendency to Form ComplexesComplexesReasons : -

1. Absence of low-lying vacant d-orbtals to accept lone pairs from ligands.

For Na+, 1s2, 2s2, 2p6, 3s, 3p, 3dHigh-lying relative to 2p

For Fe2+, 1s2, 2s2, 2p6, 3s2, 3p3, 3d6

Low-lying relative to 3p

57

Weak Tendency to Form Weak Tendency to Form ComplexesComplexesReasons : -

2. s-block cations (M+, M2+) have relatively low charge densities

less polarizing and less able to accept lone pairs from ligands.

58

Co3+

H3N

H3N NH3

NH3

NH3

NH3

All six bonds are strong dative covalent bonds

A complex ion, [Co(NH3)6]3+

Na+

OH2

OH2

H2O

OH2

OH2

H2O

A hydrated ion, Na+(aq)

Dipole-ion attraction

Weaker than dative bond

59

Weak Tendency to Form Weak Tendency to Form ComplexesComplexesOwing to its high charge density,

Be2+ can form complexes

60

Be2+ O

H

H O

H

H

Be(OH)+ + H3O+

[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+

(aq)[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+

(aq)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+

(aq)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq)

[Be(H2O)4]2+(aq) + 4H2O(l) [Be(OH)4]2(aq) + 4H3O+

(aq)

Overall reaction : (1) + (2) + (3) + (4)

61

[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+(aq) (1)

[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+(aq) (2)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+

(aq) (3)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq) (4)Overall reaction : (1) + (2) + (3) + (4) [Be(H2O)4]2+(aq) + 4H2O(l) [Be(OH)4]2(aq) + 4H3O+(aq)

pH equilibrium positions shifts to the right

62

[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+(aq) (1)

[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+(aq) (2)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+

(aq) (3)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq) (4)(1) + (2)

Be2+(aq) + 2OH(aq) Be(OH)2(s)

[Be(H2O)4]2+(aq) + 2H2O(l) [Be(H2O)2(OH)2](s) + 2H3O+

(aq) + 2OH(aq)

+ 2OH(aq)

[Be(H2O)4]2+(aq) + 2OH(aq) [Be(H2O)2(OH)2](s) + 2H2O

Or simply,

63

Characteristic Flame Colours of Characteristic Flame Colours of SaltsSaltsMost s-block elements and their compounds give a characteristic flame colour in the flame test

Group I element

Flame colour

Group II element

Flame colour

Li Crimson Be -

NaGolden yellow

Mg Bright white

K Lilac Ca Brick redRb Bluish red Sr Blood redCs Blue Ba Apple green

64

Mechanism : -1. In the hotter part of the flame,

2. In the cooler part of the flame,

Na(g) Na(g)*

heat

Na(g)* Na(g)

cool

[Ne] 3p1 [Ne] 3s1

Ground state

[Ne] 3s1

[Ne] 3p1

+ golden yellow light Visible

region

65

Mechanism : -For salts of s-block elements,the metal ions of the salts are first converted to metal atoms

Na+Cl Na(g) + Cl(g)heat

Na(g) Na(g)*

heat

Na(g)* Na(g)

cool + golden yellow light

Na2CO3(s) Na+Cl (more volatile)

Conc. HCl

66

Q.3

Na+(g) Na+

(g)*

heat

Na+(g)* Na+

(g)

cool + uv light

[He] 2s2 2p6

[He] 2s2 2p5 3s1

[He] 2s2 2p5 3s1

[He] 2s2 2p6

2p

3s

3pvisible

uv

67

Li Na K Ca

Pt or nichrome(an alloy of Ni and Cr) is suitable for making the wire because1.They have no reaction with conc. HCl2.They do not impart visible light when heated

68

Variation in Physical Properties Variation in Physical Properties of s-block Elementsof s-block Elements

1. 1. Atomic Radius and Ionic RadiusAtomic Radius and Ionic Radius

22 . . Ionization EnthalpiesIonization Enthalpies

3. Hydration Enthalpies3. Hydration Enthalpies

44 . . Melting PointsMelting Points

69


Group I

element

Atomic radius

(nm)

Group II

element

Atomic radius

(nm)

Li

Na

K

Rb

Cs

Fr

0.1520.1860.2310.2440.2620.270

Be

Mg

Ca

Sr

Ba

Ra

0.1120.1600.1970.2150.2170.220

down the groups ∵ the outermost electrons are further away from the nuclei

70


Group I

element

Atomic radius

(nm)

Group II

element

Atomic radius

(nm)

Li

Na

K

Rb

Cs

Fr

0.1520.1860.2310.2440.2620.270

Be

Mg

Ca

Sr

Ba

Ra

0.1120.1600.1970.2150.2170.220

Group II < Group I∵ ENC from left to right across the periods

71

On moving down the groups,first sharply (e.g. from Li to K)then slowly (e.g. from K to Fr)

72

1. There is a sharp in NC from 19K to 37RbOutermost e is drawn closer to the

nucleus

73

2. The inner d-electrons (of Rb, Cs, Sr, Ba) have poor shielding effect on the

outermost electrons transition contraction

74

2. 2. Ionization Ionization

EnthalpyEnthalpyGroup I

elemen

t

1st IE 2nd IEGroup II

element1st IE 2nd IE 3rd IE

Li

Na

K

Rb

Cs

Fr

519494418402376381

7 300

4 560

3 070

2 370

2 420

–

Be

Mg

Ca

Sr

Ba

Ra

900736590548502510

1 760

1 450

1 150

1 060

966

979

14 800

7 740

4 940

4 120

3 390

–Both atomic radius and ENC down the groupsAtomic radius is more importantIE down the groups

75



elemen

t



Li

Na

K

Rb

Cs

Fr

519494418402376381

7 300

4 560

3 070

2 370

2 420

–

Be

Mg

Ca

Sr

Ba

Ra

900736590548502510

1 760

1 450

1 150

1 060

966

979

14 800

7 740

4 940

4 120

3 390

–For Group I elements, 2nd IE >> 1st IE because1.the outer s-electron is well shielded by inner shell electrons

76



elemen

t



Li

Na

K

Rb

Cs

Fr

519494418402376381

7 300

4 560

3 070

2 370

2 420

–

Be

Mg

Ca

Sr

Ba

Ra

900736590548502510

1 760

1 450

1 150

1 060

966

979

14 800

7 740

4 940

4 120

3 390

–For Group I elements, 2nd IE >> 1st IE because2. the 2nd electron is closer to the nucleus

and is poorly shielded by other electrons in the same shell which is completely filled.

77



elemen

t



Li

Na

K

Rb

Cs

Fr

519494418402376381

7 300

4 560

3 070

2 370

2 420

–

Be

Mg

Ca

Sr

Ba

Ra

900736590548502510

1 760

1 450

1 150

1 060

966

979

14 800

7 740

4 940

4 120

3 390

–For Group II elements, 3rd IE >> 2nd IESimilar reasons can be applied

78

Variations in the first and second ionization enthalpies of Group I elements

79

Variations in the first, second and third ionization enthalpies of Group II elements

80



elemen

t



Li

Na

K

Rb

Cs

Fr

519494418402376381

7 300

4 560

3 070

2 370

2 420

–

Be

Mg

Ca

Sr

Ba

Ra

900736590548502510

1 760

1 450

1 150

1 060

966

979

14 800

7 740

4 940

4 120

3 390

–Group II > Group I ∵ The outer s-electrons of Group II atoms are closer to the nucleus and experience higher ENC

81

3. 3. Hydration Hydration

enthalpy enthalpy Hydration enthalpy (Hhyd) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions.

M+(g) + aq M+(aq) H = Hhyd

M2+(g) + aq M2+(aq)H = Hhyd

always has a negative value

82

Group I

ion

Hydration enthalpy (kJ

mol–1)

Group II

ion


mol–1)

Li+

Na+

K+

Rb+

Cs+

Fr+

–519

–406

–322

–301

–276

–

Be 2+

Mg2+

Ca2+

Sr2+

Ba2+

Ra2+

–2 450

–1 920

–1 650

–1 480

–1 360

–

down the groups ∵ charge density of metal ions down the groups

attraction between ions and water molecules +

H

O

H

83

Group I

ion


mol–1)

Group II

ion


mol–1)

Li+

Na+

K+

Rb+

Cs+

Fr+

–519

–406

–322

–301

–276

–

Be 2+

Mg2+

Ca2+

Sr2+

Ba2+

Ra2+

–2 450

–1 920

–1 650

–1 480

–1 360

–

Group II > Group I ∵ Group II ions have higher charge and small size

higher charge density stronger ion-dipole interaction

84

Variations in hydration enthalpy of the ions ofGroups I and II elements

85

The melting points of s-block elements depend on the metallic bond strength which in turn depends on

1.charge density of cations

2.number of valence electrons participating in the sea of electrons

3.packing efficiency of the crystal lattices

4. 4. Melting PointMelting Point

86

Group I

element

Melting Point

(C)

Group II

element

Melting Point

(C)

Li

Na

K

Rb

Cs

Fr

180

97.8

63.7

38.9

28.7

24

Be

Mg

Ca

Sr

Ba

Ra

1280

650

850

768

714

697

1. down the groups∵ ionic radii down the groups

charge density interaction between ions and electron

sea

87

Group I

element

Melting Point

(C)

Group II

element

Melting Point

(C)

Li

Na

K

Rb

Cs

Fr

180

97.8

63.7

38.9

28.7

24

Be

Mg

Ca

Sr

Ba

Ra

1280

650

850

768

714

697

2. Group II > Group I ∵ (a) Group II cations have higher charge density(b) More valence electrons are involved in the sea of electrons(c) Packing efficiency : Group II > Group I

88

Reason not known !!

89

Variation in Chemical PropertiesVariation in Chemical Properties

s-Block elements have strong reducing power

∵ low ionization enthalpies

low atomization enthalpies

90

Hydration enthalpy

Atomization enthalpy

M(g)

Ionization

enthalpy

M+

(g)

~Ea

M(s)

M+

(aq)

0ΔH reaction0

M(s) M+(aq) + e H < 0

91

Hydration enthalpy

Atomization enthalpy

M(g)

Ionization

enthalpy

M+

(g)

~Ea

M(s)

M+

(aq)

0ΔH reaction0

Reactivity : Na > Ca (depends on Ea)

Position in e.c.s. : Ca > Na (depends on Ho or Eo

92


The reactivity of s-block elements down the groups

∵ both I.E. and A.E. down the groups

Ea down the groups

Reaction rate down the groups

93


Reactivity : Group I > Group II

∵ both I.E. and A.E. across the periods

Ea across the periods

Reaction rate across the periods

94

1. 1. Reactions with Reactions with

hydrogenhydrogenGroup I

2M(s) + H2(g) 2MH(s)300C – 500C

Group II

M(s) + H2(g) MH2(s)600C – 700C

95

1. 1. Reactions with Reactions with

hydrogenhydrogen4LiH + AlCl3 LiAlH4 + 3LiCl

Dry ether

Reducing agent in organic syntheses

96

Most s-block elements

show a silvery white lustre when they are freshly cut

they tarnish rapidly upon exposure to the atmosphere

∵ they react with oxygen in the air to form an oxide layer

2. Reactions with 2. Reactions with

OxygenOxygen

97

Sodium shows a silvery white lustre when freshly cut

98

Group I (p.2)

Group II

2M(s) + O2(g) 2MO(s)heat

M(s) + O2(g) MO2(s)heat

99

3. Reactions with 3. Reactions with

ChlorineChlorineGroup I

2M(s) + Cl2(g) 2MCl(s)heat

Group II

M(s) + Cl2(g) MCl2(s)heat

100

4. Reactions with water or 4. Reactions with water or

steamsteamGroup I

2M(s) + H2O(l) 2MOH(aq) + H2(g)

heat

Group II

M(s) + 2H2O(l) M(OH)2(aq) + H2(g)

heat

Mg reacts with steam but not water

Be has no reaction with either water or steam

Mg(s) + H2O(g) MgO(s) + H2(g)

heat

101

Variation in chemical properties of Variation in chemical properties of the compounds of the compounds of ss-block elements-block elements

Reactions of Reactions of

oxidesoxidesReactions of Reactions of

hydrideshydridesReactions of Reactions of

chlorideschlorides

102


oxidesoxides

M2O(s) + H2O(l) 2MOH(aq)

M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq)

2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g)

Group I

1. Reactions with water

103

Na2O2 is used in qualitative analysis of Cr3+

2Cr(OH)3(s) + 3Na2O2(s)

2Na2CrO4(aq) + 2NaOH(aq) + 2H2O(l)

green

yellow

104


oxidesoxides

CaO(s) + H2O(l) Ca(OH)2(aq)

SrO(s) + H2O(l) Sr(OH)2(aq)

BaO(s) + H2O(l) Ba(OH)2(aq)MgO(s) + H2O(l) Mg(OH)2(aq)slightly soluble

BeO(s) + H2O(l) No reaction

Group II

increasing basicity

105


oxidesoxides

M2O(s) + 2HCl(aq) 2MCl(aq) + H2O(l)

M2O2(s) + 2HCl(aq) 2MCl(aq) + H2O2(aq)

2MO2(s) + 2HCl(aq) 2MCl(aq) + H2O2(aq) + O2(g)

Group I

2. Reactions with acids

Group II

MO(s) + 2HCl(aq) MCl2(aq) + H2O(l)

More vigorous than those with water

106


oxidesoxides3. Reactions with alkalis

Reaction with water instead except BeO

BeO(s) + 2OH(aq) + H2O(l) Be(OH)4

2(aq)amphoteric

107


hydrideshydrides

MH(s)H 2

O or NaOH(aq) MOH(aq) +

H2(g)

MCl(aq) + H2(g)

HCl(aq)

H (a strong base) tends to react with protonic reagents to release H2

Reactivity down the groups

More vigorous

108


chlorideschlorides

No significant reactions with water, acids or alkalis

Group I

Group II

Do not undergo significant hydrolysis except BeCl2 and MgCl2

BeCl2(aq) + 2H2O(l) Be(OH)2(aq) + 2HCl(aq)

MgCl2(aq) + H2O(l) Mg(OH)Cl(aq) + HCl(aq)Basic salt

More favoured in alkaline solutions

109

Relative Thermal Stability of the Relative Thermal Stability of the Carbonates and Hydroxides ofCarbonates and Hydroxides ofss-Block Elements-Block Elements

Thermal stability refers to the resistance of a compound to undergo decompositionon heating.

110

Thermal decomposition reactionsMetal carbonates

M2CO3(s) M2O(s) + CO2

heat

MCO3(s) MO(s) + CO2

heat

Metal hydroxides

2MOH(s) M2O(s) + H2O(g)

heat

M(OH)2(s) MO(s) + H2Oheat

111

Relative thermal stability can be measured in two ways

1. By comparing the decomposition temperatures

A higher decomposition temperature

a greater thermal stability

112

Metal carbonate

BeCO3 MgCO3 CaCO3 SrCO3 BaCO3

Decomposition

temperature /C

~100 540 900 1290 1360

Decomposition temperature is

the temperature at which the pressure of CO2 in equilibrium with the solid carbonate reaches 1 atm in a closed system.

Below the DT, some CO2 can still be detected but the pressure is less than 1 atm

113

• Example:

1. 1. The CarbonatesThe Carbonates

C100BeCO3(s) BeO(s) + CO2(g)

MgCO3(s) MgO(s) + CO2(g) C540

CaCO3(s) CaO(s) + CO2(g) C900

SrCO3(s) SrO(s) + CO2(g) C1290

BaCO3(s) BaO(s) + CO2(g) C1360

114

Relative thermal stability can be measured in two ways

2. By comparing the standard enthalpy changes of thermal decomposition reactions

A more positive H value

a thermally more stable compound

M(OH)2(s) MO(s) + H2O(g) H > 0

115

Metal hydroxide

Be(OH)2 Mg(OH)2

Ca(OH)

2

Sr(OH)2 Ba(OH)2

Ho / kJ mol1

+54 +81 +109 +127 +146

Trends : -

1. down the groups

2. Group I > Group II

3. Li resembles Mg more than the other group 1 elements (diagonal

relationship, pp.14-15)

116

2. 2. The HydroxidesThe Hydroxides

Be(OH)2(s) BeO(s) + H2O(g) H = +54 kJ

mol–1 Mg(OH)2(s) MgO(s) + H2O(g)

H = +81 kJ mol–1 Ca(OH)2(s) CaO(s) + H2O(g)

H = +109 kJ mol–1

Sr(OH)2(s) SrO(s) + H2O(g) H = +127 kJ

mol–1 Ba(OH)2(s) BaO(s) + H2O(g)

H = +146 kJ mol–1

117

Factors affecting thermal stability of carbonates and hydroxides

1. Polarizing power of cation

2. Polarizability of polyatomic anion

3. Lattice enthalpy of metal oxide produced

118

Interpretation of trends in thermal stability of carbonates and hydroxides1. Group I > Group II

(a) M2+ ions have higher charge densities than M+ ions

M2+ ions are more polarizing than M+ ions

Can polarize more the electron cloud of polyatomic anions

119

O C

O

O

M2+ MO + CO2

O H

O H

M2+

heat

heatMO + H2O

polarization

polarization

120

Polarizability as the size of anion

121

Polyatomic ion

Thermal decomposition

122

When a compound with large anions undergoes thermal decomposition, a compound with small anions will be formed since small anions are less

easily polarized

123

Simple ion

more stable compound with stronger bond

124

M2+ S2 M Spolarization

Stronger ionic bond with covalent character

Simple ion

125

Interpretation of trends in thermal stability of carbonates and hydroxides1. Group I > Group II

(b) M2+ ions have higher charge densities than M+ ions

Lattice enthalpy : MO > M2O

Energetic stability : MO > M2O

126

CaCO3(s) CaO(s) + CO2(g) heat

Na2CO3(s) Na2O(s) + CO2(g)

more favourable

less favourable

heat

more stable

less stable

Thermal stability of carbonates : -

Group I > Group II

127

Interpretation of trends in thermal stability of carbonates and hydroxides2. Thermal stability down the groups

∵ size of cations down the groups

∴ (a) charge density/polarizing power of cation down the groups

(b) lattice enthalpies of MO/M2O down the groups

128

MgCO3(s) MgO(s) + CO2(g) heat

more favourable

more stable

BaCO3(s) BaO(s) + CO2(g) heat

less favourable

less stable

more polarized

less polarized

Thermal stability of carbonates : -

down the groups

129

Effect of sizes of the cations on thermal stability of the carbonates and

hydroxides of both Groups I and II metals

130

Interpretation of trends in thermal stability of carbonates and hydroxides3. Li compounds resemble Mg compounds

(diagonal relationship)Charge density/polarizing power : -

Li+ Mg2+

131

Interpretation of trends in thermal stability of carbonates and hydroxides4. Thermal stability of nitrates follows similar patterns (Optional)

2MNO3(s) 2MNO2 + O2

heat

2M(NO3)2(s) 2MO + 4NO2 + O2

heat

132

Relative Solubility of the Relative Solubility of the Sulphates(VI) and Hydroxides ofSulphates(VI) and Hydroxides ofs-Block Elementss-Block Elements

In general,

Group I >> Group II

133

Compounds

Solubility / mol per 100 of water

Mg(OH)2 0.02 103

Ca(OH)2 1.5 103

Sr(OH)2 3.4 103

Ba(OH)2 15 103

Compounds


MgSO4 1800 104

CaSO4 11 104

SrSO4 0.71 104

BaSO4 0.009 104

Q.4

Size and/or charge of the anion

Polarizability of anion Covalent character Solubility in water

In general,

134

Compounds


Mg(OH)2 0.02 103

Ca(OH)2 1.5 103

Sr(OH)2 3.4 103

Ba(OH)2 15 103

down the group

Compounds


MgSO4 1800 104

CaSO4 11 104

SrSO4 0.71 104

BaSO4 0.009 104

down the group

135

• Two processes are

1. the breakdown of the ionic lattice

2. the subsequent stabilization of the ions by water molecules (this

process is called hydration)

11. Processes involved in . Processes involved in

Dissolution and their EnergeticsDissolution and their Energetics

136

1. the breakdown of the ionic lattice

2. the subsequent stabilization of the ions by water molecules (this process

is called hydration)

NaCl(s) Na+(g) + Cl(g)

Na+(g) + Cl(g) + aq Na+(aq) + Cl(aq)

H2 = (hydration enthalpy) < 0

H1 = (lattice enthalpy) > 0

137

NaCl(s) Na+(aq) + Cl-(aq)

Na+(g) + Cl-(g)

Hsolution

-HL =

+776 kJ m

ol 1

Hhy

drat

ion

= -7

72 k

J mol

1

olattice

ohydration

osolution ΔHΔHΔH

= (-772 +776) kJ mol1= +4 kJ mol1

138

osolution

osolution

osolution STΔHΔG

If , we expect the solids to dissolve in water

0ΔH osolution

Solubility as becomes more –ve (less +ve)

osolutionΔH

Solids (e.g. NaCl) with small +ve valuesare also soluble in water if the dissolution involves an increase in the entropy of the system.

osolutionΔH

139

osolution

osolution

osolution STΔHΔG

0ΔGosolution Spontaneous

dissolutionosolutionST is always

positiveosolutionHDissolution with slightly positive

can be spontaneous

140

Trends and Interpretations

1. The solubility of Group(II) sulphate decreases down the group

On moving down the group, cationic radius(r+) both and become less -ve

oLH o

hydrationH

However, less rapidly than oLH o

hydrationH

141


rr1

ΔH2

4SO

oL

rr 24SO

∵

constant

olattice

ohydration

osolution ΔHΔHΔH

less –ve down the group

+ve constant

less –ve down the group Solubility down the group

142


rr1

ΔH2

4SO

oL

rr 24SO

∵

constant

olattice

ohydration

osolution ΔHΔHΔH

more rapidly down the group

less rapidly down the group

less –ve down the group Solubility down the group

(-ve)(+ve)

143


2. The solubility of Group(II) hydroxides increases down the group

On moving down the group, cationic radius(r+) both and become less -ve

oLH o

hydrationH

However, more rapidly than oLH o

hydrationH

144


olattice

ohydration

osolution ΔHΔHΔH

less rapidly down the group

more rapidly down the group

more –ve down the group Solubility down the group

(-ve)(+ve)

less +ve down the group

145

For s-block compounds with small anions (e.g. OH, F),

solubility in water down the group

For s-block compounds with large anions (e.g. SO42,

CO32-),

solubility in water down the group

For s-block compounds with medium size anions (e.g. Br),

solubility in water exhibits irregular pattern down the group

146

Compounds


MgBr2 5.5 101

CaBr2 6.3 101

SrBr2 4.3 101

BaBr2 3.3 101

Irregular

Solublily : First and then

olattice

ohydration

osolution ΔHΔHΔH

(-ve)(+ve)

First more rapidly

Then more rapidly

First less +ve down the group

Then less -ve down the group

147

Group II compounds with doubly-charged anions (MX) are less soluble than those with singly-charged anions (MY2)

Reasons :

1. HL of MX > HL of MY2

2. HL is the major factor affecting solubility

Hsolution of MX is more positive

Solubility : MX < MY2

148

Solubility : Group I > Group II

Reasons :

For a given anions, both HL and Hhydration become more –ve from Group I to Group II

However, HL is the major factor affecting solubility

Hsolution : Group I is less positve than Group II

Solubility : Group I > Group II

149

increasing polarizing power increasing electronegativity

Diagonal relationship

150

ReactionOther Group I

elementsLithium Magnesium

Combination with O2Peroxides and superoxides

Li2O (normal oxide) MgO (normal oxide)

Combination with N2 No reaction Li3N Mg3N2

Action of heat on carbonate

No reaction (thermally stable)

Decomposes to give Li2O and CO2

Decomposes to give MgO and CO2

Action of heat on hydroxide

No reaction (thermally stable)

Decomposes to give Li2O and H2O

Decomposes to give MgO and H2O

Action of heat on nitrate

Decomposes to give MNO2 and O2

Decomposes to give Li2O, NO2 and O2

Decomposes to give MgO, NO2 and O2

Hydrogen carbonates Exist as solids Only exist in solution

Solubility of salts in water

Most salts are more soluble than those of

Li, Mg.

Fluoride, hydroxide, carbonate, phosphate, ethanedioate are sparingly soluble.

Solubility of salts in organic solvents.

Halides only slightly soluble in organic

solvents

Halides (with covalent character) dissolve in organic solvents

151

The END

152

Metals are sometimes referred to as electropositive elements. Why?

AnswerThey have low electronegativity values.

Back

40.1 Characteristic Properties of the s-Block Elements (SB p.40)

153

s-Block compounds give a characteristic flame colour in the flame test. Based on this, can you give

one use ofs-block compounds?

Answers-Block compounds can be used in fireworks.

Back


154

(a) Which ion has a greater ionic radius, potassium ion or calcium ion? Explain your answer. Answer

(a) Potassium ion (0.133 nm) has a greater ionic radius than calcium

ion (0.099 nm) . In fact, potassium ion and calcium ion are

isoelectronic and have the same number of electron shells.

However, calcium ion has one more proton than potassium ion,

the electron cloud of calcium ion will experience greater attractive

forces from the nucleus. This leads to a smaller ionic radius of

calcium ion.


155

(b) Explain why Group I elements show a fixed oxidation state of +1 in their compounds in terms of ionization enthalpies. Answer


156

(b) Group I elements form ions with an oxidation state of +1 only. It is

because they have only one outermost shell electron. Once this

outermost shell electron is removed, a stable fully-filled electronic

configuration is obtained. Therefore, the first ionization enthalpies

of Group I elements are low. The second ionization involves the

removal of an electron from an inner electron shell. Once this

electron is removed, the stable electronic configuration will be

disrupted. Therefore, their second ionization enthalpies are very

high. As a result, Group I elements form predominantly ionic

compounds with non-metals by losing their single outermost shell

electron, and they form ions having a fixed oxidation state of +1.


157

(c) Ions of Group I and Group II elements have a very low tendency to form complexes. Give one reason to explain your answer.Answer

(c) As ions of Group I and Group II elements do not have low-lying

vacant orbitals available for forming dative covalent bonds with the

lone pair electrons of surrounding ligands, they rarely form

complexes.


158

(d) Give one test which would enable you to distinguish a sodium compound from a potassium compound. Answer

(d) Sodium compounds and potassium compounds can be

distinguished by conducting a flame test. In the flame test, sodium

compounds give a golden yellow flame, while potassium

compounds give a lilac flame.

Back


159

What is a dative covalent bond? How is it formed?

AnswerA dative covalent bond is a covalent bond in which the shared pair

of electrons is supplied by only one of the bonded atoms. A dative

covalent bond is formed by the overlapping of an empty orbital of an

atom with an orbital occupied by a lone pair of electrons of another

atom.

Back


160

(a) (i) List the factors that affect the value of the ionization enthalpy of an atom.

Answer(a) (i) There are four main factors affecting the magnitude of the

ionization enthalpy of an atom. They are the electronic

configuration of an atom, the nuclear charge, the screening

effect, and the atomic radius.

40.2 Variation in Properties of the s-Block Elements (SB p.56)

161

(a) (ii) Why is ionization enthalpy of an atom always positive?

Answer(a) (ii) Ionization enthalpy of an atom always has a positive value

because energy is required to overcome the attractive forces

between the nucleus and the electron to be removed.


162

(a) (iii) Describe the general trend of the first and second ionization enthalpies down Group I of the Periodic Table.Answer


163


(a) (iii) The first ionization enthalpies of Group I elements are

relatively low. The outermost s electron is located in a new

electron shell. The attractive force between this s electron and

the nucleus is relatively weak. Also, this s electron is

effectively shielded from the attraction of the nucleus by the

fully-filled inner electron shells. Once this electron is removed,

a stable octet or duplet electronic configuration is obtained.

Consequently, this s electron is relatively easy to be removed,

and hence the first ionization enthalpies of Group I elements

are relatively low. However, the second ionization of Group I

elements involves the loss of an inner shell electron which is

closer to the nucleus. The removal of this electron disrupts the

stable electronic configuration. Therefore, the second

ionization enthalpies of Group I elements are extremely high.

164

(b) (i) List the factors that affect the value of the hydration enthalpy of an ion.

Answer(b) (i) The value of the hydration enthalpy of an ion depends on

the size and the charge of the ion.


165

(b) (ii) Why does hydration enthalpy of an ion always have a negative value?

Answer


(b) (ii) Hydration enthalpy of an ion always has a negative value

because it is the amount of energy released resulting from the

attraction between the ion and water molecules.

166

(b) (iii) Describe the general trend of the hydration enthalpy down Group II of the Periodic Table. Answer


(b) (iii) Going down Group II, the hydration enthalpy of the ions

decreases (becomes less negative). Since the ions get larger

in size on moving down the group, the charge density of the

ions falls. As a result, the electrostatic attraction between the

ions and water molecules becomes weaker, and the hydration

enthalpy becomes less negative down the group.

Back

167

The burning of lithium, sodium and potassium in oxygen gives different types of oxides. Why do the

metals behave differently?Answer


168

On burning in air, lithium forms only lithium oxide, and it does not form the

peroxide or superoxide. This is because the size of lithium ion is very small,

leading to its high polarizing power. When a peroxide ion or superoxide ion

approaches a lithium ion, the electron cloud of the peroxide ion or

superoxide ion (large in size) would be greatly distorted by the lithium ion.

The greater the distortion of the electron cloud, the lower the stability of the

compound. That is why lithium peroxide and lithium superoxide do not

exist. Sodium ion has a larger size than lithium ion. Its lower polarizing

power allows it to form the peroxide when sodium is burnt in air. Potassium

ion has a much larger size, so it has relatively low polarizing power. The

electron cloud of the peroxide ion or superoxide ion would not be seriously

distorted by potassium ion. This allows the peroxide ions or superoxide

ions to pack around potassium ion with a higher stability. As a result,

potassium is able to form stable peroxide or superoxide on burning in air.

Back


169

(a) Suggest a reason why the reaction of lithium with water is less vigorous than those of sodium and potassium.

Answer(a) The reactivity of Group I metals with water is related to the relative

ease of the metal atoms to lose the outermost shell electron. Going

down the group, as the atomic size increases, the outermost shell

electron becomes easier to be removed. Therefore, the reactivity of

Group I metals towards water increases down the group. Lithium

reacts with water vigorously. Sodium reacts with water violently and

moves on the water surface with a hissing sound.


170

(b) Which element is the strongest reducing agent, calcium, strontium or barium?

Answer(b) Barium is the strongest reducing agent. It is because the reducing

power of an element is related to the ease of the atom to lose the

outermost shell electron. Since barium has larger atomic sizes, its

outermost shell electrons are less firmly held by the nucleus.

Therefore, barium has a higher tendency to lose its outermost shell

electrons than both calcium and strontium.


Back

171

The value of Hsoln of a solid does not indicate whether the solid is soluble in water or not. So how

can we predict the solubility of a solid in water?Answer

Back

40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)

Generally speaking, for a solid to be soluble in water, its enthalpy

change of solution has to be a negative or a small positive value.

172

(a) Give balanced chemical equations for the following reactions:

(i) Thermal decomposition of barium carbonate

(ii) Reaction between sodium peroxide and water

(iii) Reaction between calcium oxide and dilute hydrochloric acid

Answer


(a) (i) BaCO3(s) BaO(s) + CO2(g)

(ii) Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(aq)

(iii) CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)

173

(b) Suggest a reason why barium sulphate(VI) is insoluble in water, while potassium sulphate(VI) is soluble inwater although they have cations of similar sizes and the same anion.

(The ionic radii of potassium ion and barium ion are 0.133 nm and 0.135 nm respectively.)

Answer


174

(b) When an ionic solid dissolves in water, two processes are taking

place. They are the breakdown of the ionic lattice and the

subsequent stabilization of the ions by water molecules. The

enthalpy change involved in the whole dissolution process is known

as the enthalpy change of solution, Hsoln, which is equal to Hsoln =

Hhyd – Hlattice. For an ionic compound to be soluble in water, the

enthalpy change of solution has to be a negative or a small positive

value. The reason why barium sulphate(VI) is insoluble in water

while potassium sulphate(VI) is soluble in water is that potassium

ion has a smaller charge than barium ion. The Hlattice of potassium

sulphate(VI) is smaller in magnitude (less negative) than that of

barium sulphate(VI). As a result, the enthalpy change of solution of

potassium sulphate(VI) is more negative, and hence it is soluble in

water while barium sulphate(VI) is not.


175

(c) Compare the solubility of calcium sulphate(VI) and barium sulphate(VI) in water. Explain your answer. Answer

(c) Calcium sulphate(VI) is expected to be more soluble than barium

sulphate(VI). It is because calcium ion has a smaller size than

barium ion. This causes the Hhyd of calcium sulphate(VI) to be more

negative than that of barium sulphate(VI). As a result, the Hsoln of

calcium sulphate(VI) becomes more negative than that of barium

sulphate(VI), and hence calcium sulphate(VI) is more soluble in

water than barium sulphate(VI).


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The s -Block Elements

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