New Way Chemistry for Hong Kong A-Level Book 41 1 The s-Block Elements 40.1Characteristic Properties...

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

1

The The ss-Block Elements-Block Elements

40.140.1 Characteristic Properties of the Characteristic Properties of the ss-Block -Block

ElementsElements

40.240.2 Variation in Properties of the Variation in Properties of the ss-Block -Block ElementsElements

40.340.3 Variation in Properties of the Variation in Properties of the Compounds of the Compounds of the ss-Block Elements-Block Elements

4400


2

The Syllabus

• 8.1 Characteristic Properties

• Metallic character• Low electronegativity• Formation of basic oxides and hydroxides• Fixed Oxidation state in their compounds• Weak tendency to form complexes• Flame colours of salts – flame test


3

The Syllabus• 8.2 Variation in properties of the s-block

elements and their compounds

Variations in atomic radii, ionisation enthalpies, hydration enthalpies and melting points.

Interpretation of these variations in terms of structure and bonding.

Reactions of the elements with oxygen and water. Reactions of the oxides with water, dilute acids and dilute alkalis.

Relative thermal stability of the carbonates and hydroxides. Relative solubility of the sulphates(VI) and hydroxides


4

s-Block elements:

• Consists of Group IA and Group IIA elements

• Outermost electron shell:ns1 ns2

• Highly reactive metals

• Good reducing agents

• Fixed oxidation states +1 for Group I elements+2 for Group II elements

Notes p. 1


5

40.40.11Characteristic Characteristic Properties of Properties of

thethes-Block s-Block

ElementsElements


6

Group I elements:

• Silvery in colour, tarnish rapidly in air

∴ keep immersed under paraffin oil or in vacuum sealed tubes

• Soft, low boiling and melting points

∵ weak metallic bond due to only 1 e– is contributed to form bonds

• Low density

∵ body-centred cubic structure -- have more spaces

Metallic Character (not mentioned in notes)Metallic Character (not mentioned in notes)

40.1 Characteristic Properties of the s-Block Elements (SB p.38)

Cutting Rubidium


7

Group I elements:

Lithium Sodium Potassium

Rubidium

Caesium


8

Group I metal

Atomic radius (nm)

Ionic radius (nm)

Crystal structure

Melting point (C)

Boiling point (C)

Density (g cm–3)

Abundance on earth

(%)

Li

Na

K

Rb

Cs

Fr

0.152

0.186

0.231

0.244

0.262

0.270

0.060

0.095

0.133

0.148

0.169

0.176

b

b

b

b

b

—

180.5

97.8

63.7

39.1

28.4

27

1330

890

774

688

690

680

0.53

0.97

0.86

1.53

1.87

—

0.0020

2.36

2.09

0.009 0

0.000 10

Trace

“b” denotes body-centred cubic structure

Some information about Group I elements



9

Group II elements:

• silvery in colour

• harder and higher boiling and melting points than Gro

up I counterparts

∵ stronger metallic bond due to 2e– are contributed to for

m bond and smaller atomic sizes

• show different crystal structures



10

Group II elements:Beryllium

Magnesium

Calcium

Strontium

BariumRadium


11

Group II metal

Atomic radius (nm)

Ionic radius (nm)

Crystal structure

Melting point (C)

Boiling point (C)

Density (g cm–3)

Abundance on earth

(%)

Be

Mg

Ca

Sr

Ba

Ra

0.112

0.160

0.197

0.215

0.217

0.220

0.031

0.065

0.099

0.113

0.135

0.140

h

h

f

f

b

—

1278

648.8

839

769

729

697

2477

1100

1480

1380

1640

1140

1.85

1.75

1.55

2.54

3.60

5.0

0.000 28

2.33

4.15

0.038

0.042

Trace

“h”, “f” and “b” denote hexagonal close-packed, face-centred cubic and body-centred cubic structures respectively

Some information about Group II elements



12

Atomic Radius and Ionic Radius (notes p. 1)

Variation in Physical PropertiesVariation in Physical Properties


13

Question:

The atomic and ionic radii increase down the Groups, why?

∵ outermost shell electrons become further away, and

more inner shells shielding the outermost shell electrons

attraction between the nucleus and the outermost shell

electrons decreases

atomic and ionic radii increase

41.3 Variation in Properties of the s-Block Elements (SB p.52)


14

Question:

Atomic and ionic radii decrease when going from

Group I to II in each period, why?

∵ Group II elements have 1 more proton and electron

than Group I elements. Increase in nuclear charge outweighs

the increase in shielding effect of additional electron of the

same shell.

atomic and ionic radii decrease


15


Question:

Ionic radius of any Group I or II element is smaller than

the atomic radius, why?

∵ after losing the outermost shell electron(s), there is one

electron shell less in the cation than in the atom.

Increase in p/e ratio


16


Ionization Enthalpy (notes p. 2)


17


Variations in the 1st, 2nd and 3rd ionization enthalpies of Group II elements


18

1st I.E. is much smaller than 2nd I.E. for Gp. I elements

For the 1st I.E., electron is further away from the nucleus and shield

ing effect of inner shell electrons

small 1st I.E.

For 2nd I.E., electron is removed from stable noble gas configuratio

n and higher effective nuclear charge

large 2nd I.E.


19

The ionization enthalpies decrease down the Groups

Reason:

• atomic sizes increase down the group

the outermost shell electron(s) is/are further away from

the nucleus, they will be better shielded by inner electron

shells.

less attractive force experienced

less energy is required to remove the electrons

Because of the high I.E., Li and Be forms a few covalent compounds

instead of forming Li+ and Be2+ respectively.


20

Low ElectronegativityLow Electronegativity

• All have low electronegativity values

∵ the outermost electron shell is effectively shielded by inner

electron shells.

- Low effective nuclear charge.

• Decrease when going down the group

∵ the outermost electron shell are further away from nucleus

- increase in shielding effect.

40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 3))


21

Group II elements are relatively more electronegative than Group I counterparts

∵ higher nuclear charge, stronger attraction to outermost shell electrons

Group I element

Electro-negativity

Group II element

Electro-negativity

Li

Na

K

Rb

Cs

Fr

1.0

0.9

0.8

0.8

0.7

—

Be

Mg

Ca

Sr

Ba

Ra

1.5

1.2

1.0

1.0

0.9

—



22

Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts

• The outermost shell electrons of Group I & II elements are we

akly held

The electrons can be excited to higher energy levels on

heating

When electrons return to ground state, radiations are

emitted

The radiations fall into the visible light region

The flame colour is a characteristic property of the

element



23

Flame Test


24



25

Flame colours


26

Weak tendency to form complexes (not mentioned in notes)Weak tendency to form complexes (not mentioned in notes)

Complex: Polyatomic ion or neutral molecule formed when

molecular or ionic gropups (called ligands) form da

tive covalent bonds with a central ion.


Group I & II elements seldom form complex:- s-block ions do not have low energy vacant orbitals available f

or dative covalent bonds. - Low ionic charge


27


Variations in melting points of Groups I and II elements

Melting Point (notes p. 4)


28

Observations:

• melting point decreases as going down Groups I and II

Reason:

• the ionic size of the elements increases

attraction between ions and electrons becomes weaker

metallic bond is weaker


29

Observations:

• melting points of Group II elements are much higher than

those of Group I elements

Reason:

• no. of valence electrons per mole contributed to the

delocalized electron sea is greater.

• Group II elements have higher ionic charge

the attractive force between ions and electrons are stronger

metallic bond is stronger


30

Observations:

• irregularity in the general decrease in

melting point down Group II elements

Reason:

• different metallic crystal structures of

the Group II elements


Group II metal

Crystal structure

Be

Mg

Ca

Sr

Ba

Ra

h

h

f

f

b

—


31

Extraction of sodium (not in syllabus)

Downs Cell


32

Manufacture of sodium hydroxidegraphite anodes

chlorine

used brine

mercury alloyed with sodium

flow of mercury flowing mercury (as cathode)

saturated brine

Flowing mercury cell

WaterMercury (recycle)


33

During electrolysis, chlorine is liberated at the anode and

sodium at the cathode.

At anode (graphite): 2Cl(aq) Cl2(g) + 2e

At cathode (mercury): Na+(aq) + e Na(s);

Na(s) + Hg(l) Na/Hg(l)

sodium amalgam


34

Flowing mercury cell

Q. 1b; Q.8


35

40.3 Variation in Properties of the s-Block Elements (SB p.56, notes p. 8)

Hydration Enthalpy

Hydration enthalpy (Hhyd) is the amount of energy rele

ased when one mole of aqueous ions is formed from its ga

seous ions. Hhyd must be negative value.

Hhyd depends on charge density charge/size

Higher the charge, stronger the attraction, more

energy released

Smaller the size, stronger the attraction, more

energy released

Xn+(g) + aq Xn+(aq)


36

Variations in hydration enthalpy of Groups I and II elements

M+


37

• magnitude of hydration enthalpies become smaller (less

negative) as going down the Groups

Reason:

• the ionic size of the elements increases down the group,

the charge density decreases

the attractive force between water molecules and ions

becomes weaker

the hydration enthalpy becomes less negative

Down the group, fewer molecules of water of

crystallizationNa2CO3.10H2O MgSO4.7H2O MgCl2.6H2OK2CO3.2H2OCaSO4.2H2OCaCl2.6H2O SrSO4 BaCl2.2H2O


38

Observations:

• hydration enthalpies of Group II ions are more negative

than those of Group I ions

Reason:

• Group II ions have higher charge and smaller size

charge density is much higher that of Group I ions

the attractive force would be much stronger


39

Lattice Enthalpies of Group I Halides (p.10)


40

Lattice Enthalpies of Group I Halides (p.10)

• Lattice Enthalpies decrease down the group:

Reasons:

Size increase

Internuclear distance increase

Attractive force between opposite ions decrease

• Good agreement between calculated and measured value. Why?


41

Lattice Enthalpies of Group II Halides (p.11)• Discrepancies occurred between calculated and measured

values.

Reason:

Covalent charactersCovalent characters occurred in small cations.

• Group II Halides have a higher lattice enthalpies than Group I Halides.

Reason:

Higher charge; smaller size.


42

Formation of Hydroxides – reactions with waterFormation of Hydroxides – reactions with water

• All Group I metals react with H2O to form metal

hydroxides and H2 gas

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

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

H2(g)

40.1 Characteristic Properties of the s-Block Elements (SB p.43, notes p. 13)

Li+H2O Na +H2O


43

K+H2O Rb+H2O

Cs+H2O


44

• All Group II metals (except Be) react with H2O to form me

tal hydroxides and H2 gas (Mg reacts with hot water).

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

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

• Be does not react with H2O(l or g)



45

Strontium + water Barium + water


46

Formation of Basic OxidesFormation of Basic Oxides

Group I elements

• Produce more than one type of oxides (except Li(except Li)

• All are ionic

• Three types of oxides: normal oxides (monoxides), peroxides, superoxides

• Relationship between three oxides:

O2– O22– 2O2

–

monoxide peroxide superoxide

2O2

12O


O O-


47

• Li forms the Li forms the monoxidemonoxide only only

4Li(s) + O2(g) 2Li2O(s)180C

• Na forms the monoxide and peroxide when O2 is abundant

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

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

180C

300C



48

• K forms the monoxide, peroxide and superoxide

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

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

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

180C

300C

3000C

• Rb, Cs also forms superoxides

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

Cs2O2(s) + O2(g) 2CsO2(s)3000C

3000C



49


Group I element

Monoxide Peroxide Superoxide

Li

Na

K

Rb

Cs

Li2O

Na2O

K2O

Rb2O

Cs2O

—

Na2O2

K2O2

Rb2O2

Cs2O2

—

—

KO2

RbO2

CsO2


50

• Li does not form peroxides or superoxides

Reason:

Li+ is small

high polarizing power

serious distortion on electron cloud of peroxide or superoxide (large polyatomic anions)

more distortion , more unstable

Li2O2 and LiO2 do not exist

• K+, Rb+ and Cs+ ions are large

Low polarizing power peroxides and superoxides are relatively stable

40.1 Characteristic Properties of the s-Block Elements (SB p.42 notes p. 14)


51

• Form normal oxides only, except Sr, Ba which can form peroxides.

• All are basic (except BeO which is amphoteric), why?


Group II Elements

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

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

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

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

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


52

Strontium + air Barium + air


53

Group II

element

Normal oxide

Peroxide Superoxide

Be

Mg

Ca

Sr

Ba

BeO

MgO

CaO

SrO

BaO

—

—

—

SrO2

BaO2

—

—

—

—

—

Be, Mg, Ca peroxide do not exist, why?

Reason:

High charge density high polarizing power

serious distortion on electron cloud of the peroxide ion



54

40.3 Variation in Properties of the compounds of the s-Block Elements ( p.59) notes p. 14 2(e)

Reaction with Water

Reactions of Oxides of s-Block ElementsReactions of Oxides of s-Block Elements

• Group I oxides react with H2O to form hydroxides

• Normal oxides:e.g. Li2O(s) + H2O(l) 2LiOH(aq)

• Peroxides:e.g. Na2O2(s) + 2H2O(l)

2NaOH(aq) + H2O2(aq)

• Superoxides:e.g. 2KO2(s) + 2H2O(l) 2KOH(aq) + H2O2(aq) + O2(g)

Dissolution of Na2O2 in H2O containing phenolphthalein


55

• Group II oxides (except BeO, MgO) react with H2O to form a

weakly alkaline solution

e.g. CaO(s) + H2O(l) Ca(OH)2(aq) (weakly alkalin

e)

• The basicity of all Group II oxides increases down the group

• MgO is slightly soluble in water, but dissolves in acids to

form salts

• BeO is amphoteric

BeO(s) + 2H+(aq) Be2+(aq) + H2O(l)

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

hot

hot

• BaO2(s) + 2H2O(l) Ba(OH)2(aq) + H2O2(aq)


56

Reaction with Acids

• All oxides of s-Block elements are basic except BeO which i

s amphoteric

• Normal oxides:

e.g. CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)

• Peroxides:

e.g. Na2O2(s) + 2HCl(aq) 2NaCl(aq) + H2O2(aq)

• Superoxides:

e.g. 2KO2(s) + 2HCl(aq) 2KCl(aq) + H2O2(aq) + O2(g)

40.3 Variation in Properties of the compounds of the s-Block Elements (p.60, not mentioned in notes)


57

Reaction with Alkalis

• No reaction between the oxides of s-block elements with al

kalis except BeO

• BeO is amphoteric, it reacts with NaOH to give Na2Be(O

H)4

BeO(s) + 2NaOH(aq) + H2O(l) Na2Be(OH)4(aq)

40.3 Variation in Properties of the compounds of the s-Block Elements (p.60)


58

Relative Thermal Stability of the Carbonates and HydroxidesRelative Thermal Stability of the Carbonates and Hydroxides

Thermal stability refers to the resistance of a compound to decomposition on heating

• The higher the thermal stability of a compound, the

higher is the temperature needed to decompose it

• The thermal stability of ionic compounds depends on:

(1) charges &

(2) sizes of ions

40.3 Variation in Properties of the compounds of the s-Block Elements (p.60) notes p. 15, 18


59

• Compound with large polarizable polyatomic anion (larg

e electron cloud, as shown in notes), the thermal stability d

epends on the polarizing power (charge density) of cation

s

The stronger the polarizing power, the electron

cloud of anion will be distorted to greater extent

The compound tends to be less thermal stable

40.3 Variation in Properties of the compounds of the s-Block Elements (p.61) notes p. 18


60

• Group II ions are smaller and have a higher charge than

Group I ions in the same period

Greater polarizing power

The carbonates and hydroxides of Group II metals are

less stable on heating

e.g. K2CO3 is stable upon heating while CaCO3 decomposes

on heating


Group II carbonates/hydroxides are less stable than Group I


61

• Most carbonates and hydroxides of Group II metals read

ily undergo decomposition on heating to give oxides (m

ore stable)

e.g. MgCO3(s) MgO(s) + CO2(g)

Ca(OH)2(s) CaO(s) + H2O(g)


Mg2+

-O H

-O HMgO + H2O

Mg2+ C O

-O

-O MgO + CO2


62

• Down the group, the size of cations increases

polarizing power decreases

compound with large anion become more stable

∴ thermal stability of carbonates & hydroxides of Groups I and II metals increases down the group

Effect of sizes of cations on thermal stability of compounds


Do Q. 2b on p. 73


63

Q. Explain briefly why lithium hydrogencarbonate does not exist as a solid while other Group I hy

drogencarbonates can be found in solid state.

A. In solid form, the cation and anion are close to each other. Due to small size of Li+, it has a high polarizing power. This distorts the electron cloud of HCO3

-, making the anion unstable.As the size of cations increases down the group, the pol

arizing power decreases, therefore, solid hydrogencarbonates can be formed.


64

Effect of Heat on s-block carbonates and hydroxides (p.19)

i. CarbonatesCarbonates

Group I: AllAll are thermally stable except Lithium.

Group II: AllAll decompose on heating forming metal oxides and carbon dioxide.

ii. Hydroxides Hydroxides (p.21)

Group I: AllAll are thermally stable except Lithium.

Group II: AllAll decompose on heating forming metal oxides and water.


65

Relative Solubility of the Sulphates(VI) and HydroxidesRelative Solubility of the Sulphates(VI) and Hydroxides

• When an ionic solid is dissolved in water, two processes are taken

place:

1. Breakdown of the ionic solid (-ve lattice enthalpy)

2. Stabilization of ions by water molecules

(hydration enthalpy released)

Processes involved in Dissolution and their Energetics

40.3 Variation in Properties of the compounds of the s-Block Elements (p.63) notes p. 21


66

Dissolution of NaCl


67

Solubility of s-block Sulphates and Hydroxides (p.23)

MX(s) M+(aq) + X-(aq)

M+(g) + X-(g)

Hs

U Hhyd

A low modulus of lattice enthalpy and a high modulus of hydration enthalpy favour the dissolving process.


68

Effect of charge and size of ions on Hhyd and Hlattice

rr

ZZlatticeH

rr

11Hhyd


69

Solubility of s-block Sulphates and Hydroxides

i. For large anions, like sulphatesFor large anions, like sulphates

When moving down the group, the decrease in size of the cation does not cause a significant change of U. However, Hhyd become less negative and has a significant change the solubility of sulphates decreases down the grothe solubility of sulphates decreases down the group.up.

SO42-

MgSO4

SO42-

SrSO4


70

ii. For smaller anions, like hydroxidesFor smaller anions, like hydroxides

When moving down the group, the increase in size of the cation causes a significant change of U but Hhyd change a little because of the great hydration energy of the anion. Therefore the solubility of hydroxide increases down the solubility of hydroxide increases down the group.the group.

iii. Group I sulphates and hydroxides are more Group I sulphates and hydroxides are more ssoluble than that of Group II. Why?oluble than that of Group II. Why?

Mg(OH)2 Sr(OH)2


71

• The sulphates(VI) and hydroxides of Group I metals are mor

e soluble in water than those of Group II metals

∵ Group I metals has a smaller charge and larger size than Gr

oup II metals in the same period

The lattice enthalpies of Group I compounds are smaller

in magnitude than those of Group II compounds

The enthalpy changes of solution are more –ve

Relative Solubility of the Sulphates(VI) and Hydroxides –Trend and Interpretation



72

The END

Do Q. 6, 10 and Q. 7 on p. 74

Date post:	01-Jan-2016
Category:	Documents
Upload:	roxanne-snow
View:	267 times
Download:	5 times

New Way Chemistry for Hong Kong A-Level Book 41 1 The s-Block Elements 40.1Characteristic Properties...

Documents