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Chapter 41
The s-Block Elements
41.1 Introduction
41.2 Characteristic Properties of the s-Block Elements
41.3 Variation in Properties of the s-Block Elements
41.4 Variation in Properties of the Compounds of the
s-Block Elements
41.5 Uses of the Compounds of the s-Block Elements
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41.1 Introduction (SB p.40)
s-Block elements:
• Consists of Group IA and
Group IIA elements
• Outermost shell electrons
in s orbitals
• Highly reactive metals
• Good reducing agents
• Fixed oxidation states
+1 for Group I elements
+2 for Group II elements
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41.1 Introduction (SB p.41)
Group I elements:
Lithium
Rubidium
3
Sodium
Potassium
Caesium
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41.1 Introduction (SB p.41)
Calcium
Group II elements:
Beryllium
Strontium
4
Magnesium
Barium
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Radium
4
41.2 Characteristic Properties of the s-Block Elements (SB p.42)
Some characteristic properties of Group I metals
Group I Electronic
Electrometal configuration negativity
Oxidation
no. in
compound
Oxide
formed
Hydroxide
formed
Flame
colour
deep red
yellow
lilac
bluish
red
blue
—
Li
Na
K
Rb
[He]2s1
[Ne]3s1
[Ar]4s1
[Kr]5s1
1.0
0.9
0.8
0.8
+1
+1
+1
+1
Li2O
Na2O2
K2O2, KO2
Rb2O2,RBO2
LiOH
NaOH
KOH
RbOH
Cs
Fr
[Xe]6s1
[Rn]7s1
0.7
—
+1
—
Cs2O2,CsO2
—
CsOH
—
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41.2 Characteristic Properties of the s-Block Elements (SB p.42)
Some characteristic properties of Group II elements
Group II Electronic
Electrometal configuration negativity
Oxidation
no. in
compound
Be
Mg
Ca
Sr
[He]2s2
[Ne]3s2
[Ar]4s2
[Kr]5s2
1.5
1.2
1.0
1.0
+2
+2
+2
+2
BeO
MgO
CaO
SrO, SrO2
Ba
Ra
[Xe]6s2
[Rn]7s2
0.9
—
+2
—
BaO, BaO2 Ba(OH)2
—
—
6
Oxide
formed
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Hydroxide
formed
Be(OH)2
Mg(OH)2
Ca(OH)2
Sr(OH)2
Flame
colour
none
none
brick red
blood red
or
crimson
green
—
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41.2 Characteristic Properties of the s-Block Elements (SB p.42)
Metallic Character
Group I elements:
•
Silvery in colour, tarnish rapidly in air
∴ keep immersed under paraffin oil or in vacuum sealed ampoules
•
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
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41.2 Characteristic Properties of the s-Block Elements (SB p.43)
Some information about Group I elements
Atomic Ionic
Group
radius radius
I metal
(nm)
(nm)
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
Crystal
structure
b
b
b
b
b
—
Melting Boiling
point
point
(C)
(C)
180.5
97.8
63.7
39.1
28.4
27
1330
890
774
688
690
680
Density Abundance
(g cm–3) on earth (%)
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
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41.2 Characteristic Properties of the s-Block Elements (SB p.43)
Group II elements:
• greyish in colour
• harder and higher boiling and melting points
(compared to Group I counterparts)
∵ stronger metallic bond due to 2e– are contributed to
form bond and smaller atomic sizes
• show different crystal structures
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41.2 Characteristic Properties of the s-Block Elements (SB p.44)
Some information about Group II elements
Atomic Ionic
Melting
Group
Crystal
radius radius
point
II metal
structure
(nm)
(nm)
(C)
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
Boiling
point
(C)
Density Abundance
(g cm–3) on earth (%)
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
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41.2 Characteristic Properties of the s-Block Elements (SB p.44)
Low Electronegativity
• All have low electronegativity values
∵ the outermost s electrons are effectively shielded by inner
electron shells. The tendency of losing electrons is relatively
high.
• Electronegativity values decrease when going down the
group
∵ the outermost shell electrons are further and further away
from nucleus
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41.2 Characteristic Properties of the s-Block Elements (SB p.44)
Group II elements are more electronegative than Group I
counterparts
∵
higher nuclear charge, stronger attraction to outermost
shell electrons
i.e. more difficult to remove the electrons
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Group I
element
Electronegativity
Group II
element
Electronegativity
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
—
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41.2 Characteristic Properties of the s-Block Elements (SB p.45)
Formation of Basic Oxides
Group I elements
• Produce more than one type of oxides (except Li)
• All are ionic
• Three types of oxides: normal oxides (monoxides),
peroxides, superoxides
• Relationship between three oxides:
O2–
 O2
monoxide
13
1
O2
2
2–
peroxide
O2
 2O2–
superoxide
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41.2 Characteristic Properties of the s-Block Elements (SB p.45)
• Li forms the monoxide only
180C
4Li(s) + O2(g)  2Li2O(s)
• Na forms the monoxide and peroxide when O2 is abundant
180C
4Na(s) + O2(g)  2Na2O(s)
300C
2Na2O(s) + O2(g) 
2Na2O2(s)
• K forms the monoxide, peroxide and superoxide
180C
4K(s) + O2(g) 
2K2O(s)
300C
2K2O(s) + O2(g) 
2K2O2(s)
3000C
K2O2(s) + O2(g)  2KO2(s)
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41.2 Characteristic Properties of the s-Block Elements (SB p.45)
• Rb, Cs also forms superoxides
3000C
Rb2O2(s) + O2(g) 
2RbO2(s)
Cs2O2(s) + O2(g) 3000C
 2CsO2(s)
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Group I
element
Monoxide
Peroxide
Superoxide
Li
Na
K
Rb
Cs
Li2O
Na2O
K2O
Rb2O
Cs2O
—
Na2O2
K2O2
Rb2O2
Cs2O2
—
—
KO2
RbO2
CsO2
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41.2 Characteristic Properties of the s-Block Elements (SB p.46)
•
Li does not form peroxides or superoxides
Reason:
 Li+ is small
 high polarizing power
 serious distortion on electron cloud of peroxide or
superoxide
 more distortion , more unstable
 Li2O2 and LiO2 do not exist
•
K+, Rb+ and Cs+ ions are large
 Low polarizing power  peroxides and superoxides are
stable
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41.2 Characteristic Properties of the s-Block Elements (SB p.46)
Group II Elements
• Form normal oxides only, except Sr, Ba which can form
peroxides
• All are basic (except BeO which is amphoteric)
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)
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2BaO2(s)
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41.2 Characteristic Properties of the s-Block Elements (SB p.46)
Group II
element
Normal
oxide
Peroxide
Superoxide
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
—
—
—
SrO2
BaO2
—
—
—
—
—
Beryllium peroxide (BeO2) does not exist
Reason:
 Be2+ is small  high polarizing power
 serious distortion on electron cloud of the peroxide ion
 polarizing power of Be2+ > Li+, due to smaller size, higher charge
 BeO2 does not exist
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41.2 Characteristic Properties of the s-Block Elements (SB p.47)
Ba cannot form superoxide while K can
Reason:
 polarizing power of Ba2+ > K+  high polarizing power
 more serious distortion on electron cloud of the
superoxide ion
  Ba(O2)2 does not exist
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41.2 Characteristic Properties of the s-Block Elements (SB p.47)
Formation of Hydroxides
•
All Group I metals (except Li) react with H2O to
form metal hydroxides and H2 gas
e.g.
H2(g)
•
2Na(s) + 2H2O(l)  2NaOH(aq) +
+ 2H

+ H2(g)
All Group2K(s)
I oxides
react
with
H22KOH(aq)
O to form metal
hydroxides
2O(l)
General equation:
M2O(s) + H2O(l)  2MOH(aq)
M2O2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq)
2MO2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq) + O2(g)
20
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41.2 Characteristic Properties of the s-Block Elements (SB p.47)
•
All Group II metals (except Be & Mg) react with
H2O to form metal hydroxides and H2 gas
e.g.
H2(g)
Ca(s) + 2H2O(l)  Ca(OH)2(aq) +
Sr(s) + 2H2O(l)  Sr(OH)2(aq) + H2(g)
•
Be does not react with H2O(l or g)
•
Mg does not react with H2O(l) but with
H2O(g)
Mg(s) + H2O(g)  MgO(s) + H2(g)
21
Ca reacts with H2O readily
at room temperature
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41.2 Characteristic Properties of the s-Block Elements (SB p.48)
•
The reactivity increases down the group.
•
The oxides of Ca, Sr, Ba react with H2O(l) to give
hydroxides
CaO(s) + H2O(l)  Ca(OH)2(aq)
SrO(s) + H2O(l)  Sr(OH)2(aq)
BaO(s) + H2O(l)  Ba(OH)2(aq)
22
•
MgO dissolves in acids to form salts but is
slightly soluble in water
•
BeO is insoluble in both acids and water
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41.2 Characteristic Properties of the s-Block Elements (SB p.48)
Bonding and Oxidation State in Compounds
•
s-Block elements form compounds that are predominantly
ionic in nature
• Oxidation state of Group I metals must be +1
Reason:
• only 1 electron in the outermost s orbitals  once this e- is
removed, a stable electronic configuration is obtained  1st
I.E. is low
• The 2nd e- is removed from the stable octet  2nd I.E. is
very high
∴ Group I metals predominantly form ions with a fixed
oxidation number +1
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41.2 Characteristic Properties of the s-Block Elements (SB p.48)
Chemical formulae of some Group I compounds
Oxidation number of
Group I element in the
compounds
Group I
element
Oxide
Li
Li2O
LiH
LiCl
+1
Na
Na2O2
NaH
NaCl
+1
K
KO2
KH
KCl
+1
Rb
RbO2
RbH
RbCl
+1
Cs
CsO2
CsH
CsCl
+1
24
Hydride Chloride
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41.2 Characteristic Properties of the s-Block Elements (SB p.49)
• Oxidation state of Group II metals must be +2
Reason:
• only 2 electrons in the outermost s orbitals  once these 2eare removed, a stable electronic configuration is obtained
 sum of 1st I.E. and 2nd I.E. is low
• the 3rd e- is removed from the stable octet  3rd I.E. is
very high
∴ Group II metals predominantly form ions with a fixed
oxidation number +2
25
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41.2 Characteristic Properties of the s-Block Elements (SB p.49)
Chemical formulae of some Group II compounds
Group II
Oxide Hydride Chloride
element
26
Oxidation number of
Group II element in the
compounds
Be
BeO
BeH2
BeCl2
+2
Mg
MgO
MgH2
MgCl2
+2
Ca
CaO
CaH2
CaCl2
+2
Sr
SrO
SrH2
SrCl2
+2
Ba
BaO
BaH2
BaCl2
+2
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41.2 Characteristic Properties of the s-Block Elements (SB p.49)
Weak Tendency to Form Complexes
A complex is a polyatomic ion or neutral molecule formed
when molecular or ionic groups (called ligands) form dative
covalent bonds with a central metal atom or cation.
e.g.
27
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41.2 Characteristic Properties of the s-Block Elements (SB p.50)
•
Complex formation is common in d-block elements
∵ d-block metal ions utilize their low-lying vacant d-orbitals
to accept the lone pair electrons from the surrounding
ligands
 a complex is formed
28
•
Since s-block metal ions do not possess low-lying vacant
d-orbitals, they do not form complexes
•
When s-block metal ions are surrounded by polar molecules,
there is only electrostatic attraction between the positive
ion and the negative ends of the dipoles
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41.2 Characteristic Properties of the s-Block Elements (SB p.50)
Characteristic Flame Colours of Salts
•
Many s-block elements can give characteristic flame
colours in the flame test
• The outermost shell electrons of Group I & II elements are
weakly 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
 As the energy is quantized, the flame colour is a
characteristic property of the element
29
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41.2 Characteristic Properties of the s-Block Elements (SB p.50)
Group I
element
Li
Na
K
Rb
Cs
(a)
(b)
Flame colour
Deep red
Golden yellow
Lilac
Bluish red
Blue
(c)
Group II
element
Ca
Sr
Ba
Flame colour
Brick red
Blood red or
crimson
Green
(d)
(a) lithium
(b) sodium
(c) potassium
(d) calcium
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41.2 Characteristic Properties of the s-Block Elements (SB p.51)
Check Point 41-1
(a) Which ion has a greater ionic radius, potassium ion or
calcium ion? Give reasons for your choice.
Answer
(a) K+ ion (0.133 nm) has a greater ionic radius than Ca2+
ion (0.099 nm). In fact, K+ and Ca2+ ions are isoelectronic
and have the same number of electron shells. However,
because Ca2+ ion has one more proton than K+ ion, the
electrons of Ca2+ ion will experience a greater attractive
force from the nucleus. This leads to a smaller ionic
radius of Ca2+ ion.
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41.2 Characteristic Properties of the s-Block Elements (SB p.51)
Check Point 41-1 (cont’d)
(b) Explain why s-block elements are highly electropositive.
Answer
(b) All s-block elements are highly electropositive. This
means that they lose their electrons easily. The reason is
that the outermost shell electrons of the s-block elements
are effectively shielded from the nucleus by the fullyfilled inner electron shells. So the electrons are less firmly
held by the nucleus, and hence easily to be removed.
32
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41.2 Characteristic Properties of the s-Block Elements (SB p.51)
Check Point 41-1 (cont’d)
(c) Explain why alkali metals show a fixed oxidation state of
+1 in(c)
their
compounds in terms of ionization enthalpies.
Alkali metals have the [ ] ns1 electronic configuration. The
last s electron enters a new electron shell which is much Answer
further
away from the nucleus. Hence, the attractive force from the
nucleus holding this electron is relatively weak. Also, this s
electron is shielded from the attraction of the nucleus by the fully-filled
inner electron shells. Moreover, once this electron is removed, a stable
electronic configuration (octet) is attained. Consequently, the first
ionization enthalpies of alkali metals are relatively low. Besides, as their
second ionization enthalpies involve the removal of an electron
from a fully-filled electron shell, their second ionization enthalpies
are relatively large. As a result, alkali metals show a fixed
oxidation state of +1 in all their compounds.
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41.2 Characteristic Properties of the s-Block Elements (SB p.51)
Check Point 41-1 (cont’d)
(d) Give one test which would enable you to distinguish a
sodium compound from a potassium compound.
Answer
(d) By conducting the flame test, sodium compounds
will give a golden yellow flame colour, whereas
potassium compounds will give a lilac flame colour.
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41.2 Characteristic Properties of the s-Block Elements (SB p.51)
Check Point 41-1 (cont’d)
(e) Ions of alkali metals and alkaline earth metals have very
low tendency to form complexes. Give one reason to
account for this.
Answer
(e) Since ions of alkali metals and alkaline earth metals
do not have low-lying vacant orbitals for forming
dative covalent bonds with the lone pair electrons of
surrounding ligands, they rarely form complexes.
35
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41.3 Variation in Properties of the s-Block Elements (SB p.51)
Variation in Physical Properties
Atomic Radius and Ionic Radius
Group I
element
Atomic
radius
(nm)
Ionic
radius
(nm)
Li
0.152
0.060
Be
0.112
0.031
Na
0.186
0.095
Mg
0.160
0.065
K
0.231
0.133
Ca
0.197
0.099
Rb
0.244
0.148
Sr
0.215
0.113
Cs
0.262
0.169
Ba
0.217
0.135
36
Atomic
Group II
radius
element
(nm)
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Ionic
radius
(nm)
36
41.3 Variation in Properties of the s-Block Elements (SB p.52)
Variations in atomic radius and ionic radius of Groups
I and II elements
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41.3 Variation in Properties of the s-Block Elements (SB p.52)
Observations:
• ionic radius of any Group I or II element is smaller than
the atomic radius
∵
after losing the outermost shell electron(s), there is one
electron shell less in the cation than in the atom,
electrons are hold more strongly by nucleus
 electron cloud contracts
38
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41.3 Variation in Properties of the s-Block Elements (SB p.52)
•
the atomic and ionic radii increase down the Groups
∵ more and more electron shells occupied, 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
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41.3 Variation in Properties of the s-Block Elements (SB p.52)
• atomic and ionic radii decrease when going from Group I
to II in each period
∵
Group II elements have 1 more proton and electron
than Group I elements, increase in the number of
protons leads to greater attractive force to hold
electrons more strongly, but increase in the number of
electrons does not lead to the increase in repulsive
force between
electrons and shielding effect
 atomic and ionic radii decrease
40
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41.3 Variation in Properties of the s-Block Elements (SB p.53)
Ionization Enthalpy
1st
2nd
Group I ionization ionization
element enthalpy enthalpy
(kJ mol–1) (kJ mol–1)
Li
Na
K
Rb
Cs
Fr
41
519
494
418
402
376
381
7 300
4 560
3 070
2 370
2 420
—
1st
2nd
Group II ionization ionization
element enthalpy enthalpy
(kJ mol–1) (kJ mol–1)
Be
Mg
Ca
Sr
Ba
Ra
900
736
590
548
502
510
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1 760
1 450
1 150
1 060
966
979
3rd
ionization
enthalpy
(kJ mol–1)
14 800
7 740
4 940
4 120
3 390
—
41
41.3 Variation in Properties of the s-Block Elements (SB p.54)
Variations in the 1st and 2nd ionization enthalpies of
Group I elements
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41.3 Variation in Properties of the s-Block Elements (SB p.54)
Variations in the 1st, 2nd and
3rd ionization enthalpies of
Group II elements
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
Observations:
• 1st I.E. of Group I elements are low and the 2nd I.E. are extremely
high
Reason:
• The outermost s electron enters a new electron shell which is
further away from nucleus, and is shielded by inner electron shells
 the attractive force is weak, easy to be removed
• The second electron to be removed is from the inner electron shell
 removal of 2nd electron will disrupt the stable electronic
configuration
 2nd I.E. are extremely high
44
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
• 1st and 2nd I.E. of Group II elements are low but the 3rd
I.E. are much higher
Reason:
• The outermost s electrons are further away from the
nucleus and effectively shielded by inner electron shells
 attractive force is weak, easy to be removed
• The 3rd electron to be removed is from inner electron shells
 removal of 3rd electron will disrupt the stable
electronic configuration
 3rd I.E. are much higher
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
• the ionization enthalpies decrease down the Groups
Reason:
• atomic sizes increase down the group
 the outermost shell electron(s) will be better shielded
by inner electron shells
 less attractive force experienced
 less energy is required to remove the electrons
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
Melting Point
•
Melting points of Groups I and II elements depend on
strength of metallic bonds
• The stronger the bond, the higher is the melting point
•
Metallic bond strength depends on:
(1) ionic radius,
(2) no. of valence electrons
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
Melting points of Groups I and II elements
48
Group I
element
Melting
point (C)
Group II
element
Melting
point (C)
Li
180
Be
1 280
Na
97.8
Mg
350
K
63.7
Ca
850
Rb
38.9
Sr
768
Cs
28.7
Ba
714
Fr
27
Ra
697
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48
41.3 Variation in Properties of the s-Block Elements (SB p.55)
Variations in melting points of Groups I and II elements
49
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49
41.3 Variation in Properties of the s-Block Elements (SB p.56)
Observations:
• melting point decreases as going down Groups I and II
Reason:
• the ionic size of the elements increases
• the no. of electrons in the delocalized electron sea
remains unchanged
 charge density decreases
• attraction between ions and electrons becomes weaker
• metallic bond is weaker
50
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50
41.3 Variation in Properties of the s-Block Elements (SB p.56)
• melting points of Group II elements are much higher than
those of Group I elements
Reason:
• no. of electrons per mole contributed to the delocalized
electron sea by Group II elements is greater than those by
Group I elements
• Group II elements have shorter ionic radii
• charge density of Group II elements is higher
• the attractive force between ions and electrons are stronger
• metallic bond is stronger
51
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51
41.3 Variation in Properties of the s-Block Elements (SB p.56)
• irregularity in the general decrease in melting point down
Group II elements
• melting point of Mg is lower than that of Ca
Reason:
• different metallic crystal structures of the Group II
elements
52
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52
41.3 Variation in Properties of the s-Block Elements (SB p.56)
Hydration Enthalpy
Hydration enthalpy (Hhyd) is the amount of energy
released when one mole of aqueous ions is formed from
its gaseous ions.
53
•
Hhyd must be negative value as it is the energy
released resulting from the attraction between water
molecules and ions
•
Hhyd depends on charge density
 Higher the charge, stronger the attraction, more
energy released
 Smaller the size, stronger the attraction, more
energy released
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53
41.3 Variation in Properties of the s-Block Elements (SB p.57)
Hydration enthalpies of Groups I and II metal ions
54
Group I
metal ion
Hydration
enthalpy
(kJ mol–1)
Group II
metal ion
Hydration
enthalpy
(kJ mol–1)
Li+
– 519
Be2+
– 2 450
Na+
– 406
Mg2+
– 1 920
K+
– 322
Ca2+
– 1 650
Rb+
– 301
Sr2+
– 1 480
Cs+
– 276
Ba2+
– 1 360
Fr+
—
Ra2+
—
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54
41.3 Variation in Properties of the s-Block Elements (SB p.57)
Variations in hydration
enthalpy of Groups I and II
elements
55
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55
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Observations:
• hydration enthalpies become smaller 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 and less negative
56
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56
41.3 Variation in Properties of the s-Block Elements (SB p.58)
• 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
• the hydration enthalpies of Group II ions are more negative
than those of Group I ions
57
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57
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Check Point 41-2
(a) Why is hydration enthalpy always negative?
Answer
(a) Hydration enthalpy is a negative value as it is the
amount of energy released resulting from the attraction
between ions and water molecules.
58
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58
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Check Point 41-2 (cont’d)
(b) List the factors that affect the value of the hydration
enthalpy of an ion.
Answer
(b) The value of hydration enthalpy of an ion depends on
the charge and size of the ion.
59
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59
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Check Point 41-2 (cont’d)
(c) How does hydration enthalpy affect the solubility of an
ionic compound?
Answer
(c) Ionic compounds with a large negative value of
hydration enthalpy are expected to be more soluble in
water. It is because the more negative the value of
hydration enthalpy, the stronger is the attraction between
the ions and water molecules. This means that the ions
have a high solubility in water.
60
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60
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Variation in Chemical Properties
• s-Block elements are good reducing agents
∵ they have low I.E.
 The lower the I.E., the stronger is the reducing power
• The reducing power increases down the group
∵ atomic size increases down the group, ionization
enthalpy decreases down the group
• Group I metals react readily with water, dilute acids and most
non-metals
• Group II metals are less reactive than Group I metals because
of their greater I.E.
61
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61
41.3 Variation in Properties of the s-Block Elements (SB p.58)
Reaction with Hydrogen
•
All Group I metals react with H2(g) between 300C and
500C to form white crystalline compounds, called metal
hydrides
e.g. 2Na(s) + H2(g)  2NaH(s)

• All Group II metals (except Be) react with H2(g) between
600C and 700C to form metal hydrides
e.g. Ca(s) + H2(g)  CaH2(s)

62
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62
41.3 Variation in Properties of the s-Block Elements (SB p.58)
•
Reactivity of Groups I and II metals increases down
the group
•
Most of s-block metal hydrides are ionic except BeH2
and MgH2 are predominantly covalent
63
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63
41.3 Variation in Properties of the s-Block Elements (SB p.59)
Reaction with Oxygen
64
•
s-Block elements show a silvery white lustre when they
are freshly cut, but tarnish upon exposure to air
•
All s-block elements burn in air to form one or more of
the following oxides:
•
Li forms Li2O ; Na forms Na2O, Na2O2 ; K, Rb and Cs
can form normal oxides, peroxides and superoxides
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64
41.3 Variation in Properties of the s-Block Elements (SB p.59)
•
Group II metals usually form normal oxides
•
Be forms BeO; Mg forms MgO; Sr and Ba can form
normal oxides and peroxides
Burning magnesium
65
Magnesium oxide
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65
41.3 Variation in Properties of the s-Block Elements (SB p.60)
Oxides of the s-block elements
66
Metals that form oxides
Type of oxide
Formula
Normal oxide
O2–
All Groups I and II elements
Peroxide
O22–
Na, K, Rb, Cs, Sr, Ba
Superoxide
O2–
K, Rb, Cs
when exposed or burnt in air
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66
41.3 Variation in Properties of the s-Block Elements (SB p.60)
Check Point 41-3
The products of burning lithium, sodium and potassium in
oxygen are Li2O, Na2O2 and KO2 respectively. How do you
down Group
I, the sizes of
of the
increase. As the
explain Going
the different
behaviour
thecations
metals?
charge to radius ratios of the cations decrease, they become less
Answer
polarizing. Among the lithium, sodium and potassium ions, the
polarizing power of lithium ion is the highest as it has the highest charge to
radius ratio. Besides, among the three types of oxide ions (i.e. normal oxide,
peroxide and superoxide ions), the polarizability of superoxide ion is the
highest as it has the largest size. When a cation of high polarizing power
approaches an anion of high polarizability, the electron cloud of the anion
will be greatly distorted by the cation, and thus the compound formed
will be unstable. As a result, Li tends to form the normal oxide Li2O,
Na tends to form the peroxide Na2O2, and K tends to
form the superoxide KO2.
67
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41.3 Variation in Properties of the s-Block Elements (SB p.60)
Reaction with Chlorine
•
All Group I metals react with Cl2 to
form ionic solids with high melting
point
e.g. 2Na(s) + Cl2(g)  2NaCl(s)
• All Group II metals (except Be)
directly combine with Cl2 to form ionic
chlorides
e.g. Mg(s) + Cl2(g)  MgCl2(s)
• Be react with Cl2 when heated to form
covalent chloride due to high charge
density
68
Reaction between Na
and Cl
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41.3 Variation in Properties of the s-Block Elements (SB p.60)
Reaction with Water
•
All Group I metals react with cold water, forming
hydroxides and hydrogen
e.g. 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
•
The reduction of water by Group I metals involves the
following half reaction:
2H2O(l) + 2e–  2OH– (aq) + H2(g)
69
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41.3 Variation in Properties of the s-Block Elements (SB p.61)
•
The reactivity of Group I metals with water depends on
the relative ease of the metals to donate the outermost
shell electrons
•
Down the group  atomic size increases  easier to
lose the outermost shell electron
70
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41.3 Variation in Properties of the s-Block Elements (SB p.61)
Li reacts with
H2O vigorously
71
Na reacts with
H2O violently
K reacts with
H2O almost
explosively
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41.3 Variation in Properties of the s-Block Elements (SB p.61)
72
•
Group II elements are less reactive than Group I elements
•
e.g. Be does not react with water
Mg(s) + H2O(g)  MgO(s) + H2(g)
(vigorous)
Mg(s) + 2H2O(l)  Mg(OH)2(aq) + H2(g)
(very slow)
Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g)
(moderate)
Sr(s) + 2H2O(l)  Sr(OH)2(aq) + H2(g)
(vigorous)
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41.3 Variation in Properties of the s-Block Elements (SB p.61)
Check Point 41-4
(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
corresponds to the relative ease of the metals to donate
the outermost shell electrons. Among all Group I metals,
Li has the smallest atomic size, so its outermost shell
electron is the most firmly held by the nucleus. It has a
relatively low tendency to donate its outermost shell
electron. Hence, Li reacts gently with water.
73
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41.3 Variation in Properties of the s-Block Elements (SB p.61)
Check Point 41-4 (cont’d)
(b) Which element is the best reducing agent, Ca, Sr or Ba?
Answer
(b) Ba is the best reducing agent among the three elements.
It is because the reducing power of an element depends on
the ease of donating its outermost shell electrons. As Ba
has the largest size among the three elements, its outermost
shell electrons are less firmly held by the nucleus.
Therefore, Ba has a higher tendency to donate its outermost
shell electrons than the other two, and it is the best
reducing agent among the three elements.
74
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74
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.62)
Reactions of Oxides of s-Block Elements
Reaction with Water
•
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)
Dissolution of
Na2O2 in H2O
containing
phenolphthalein
•
Superoxides:
e.g. 2KO2(s) + 2H2O(l)  2KOH(aq) + H2O2(aq) + O2(g)
•
The basicity of all Group I oxides increases down the group
75
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75
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.62)
•
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 alkaline)
•
The basicity of all Group II oxides increases down the group
•
BeO is amphoteric and is insoluble in water or acids
BeO(s) + 2H+(aq)  Be2+(aq) + H2O(l)
hot
BeO(s) + 2OH–(aq) + H2O(l)  [Be(OH)4]2–(aq)
• MgO is slightly soluble in water, but dissolves in acids to
form salts
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76
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.63)
•
The reactivity of Group II peroxides increases down the group
e.g. BaO2(s) + 2H2O(l)  Ba(OH)2(aq) + H2O2(aq)
•
Metal peroxides and metal superoxides are powerful
oxidizing agents because they give hydrogen peroxide as
product
• Metal peroxides and metal superoxides are useful for
qualitative and quantitative analysis
e.g. 2Cr(OH)3(s) + 3Na2O2(s)
 2Na2CrO4(aq) + 2NaOH(aq) + 2H2O(l)
77
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77
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.63)
Reaction with Acids
78
•
All oxides of s-Block elements are basic except BeO which
is 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)
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78
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.63)
Reaction with Alkalis
•
No reaction between the oxides of s-block elements with
alkalis except BeO
•
BeO is amphoteric, it reacts with NaOH to give
Na2Be(OH)4
BeO(s) + 2NaOH(aq) + H2O(l)  Na2Be(OH)4(aq)
79
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79
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.63)
Reactions of Hydrides of s-Block Elements
•
All Groups I and II metal hydrides react
with H2O to form metal hydroxides and
hydrogen gas
•
Their reactivity increases down the group
e.g. NaH(s) + H2O(l)
 NaOH(aq) + H2(g)
CaH2(s) + 2H2O(l)
 Ca(OH)2(aq) + 2H2(g)
•
The reaction is due to highly reactive
hydride ion (H–)
CaH2 reacts with H2O to
give H2 and OH-
H–(aq) + H2O(l)  OH–(aq) + H2(g)
80
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80
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.64)
Reactions of Chlorides of s-Block Elements
•
The chlorides of Group I metals are ionic, soluble in water,
no hydrolysis occur
e.g.
•
LiCl(s)  Li+(aq) + Cl–(aq)
LiCl is the only deliquescent chloride, while other Group I
chlorides are anhydrous
•
Group II metal chlorides exhibit some covalent character
∵
the electronegativity values of Group II metals are
higher than those of Group I metals
 The degree of ionic character increases down the group
81
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81
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.64)
• BeCl2 is covalent and hydrolyzed by water readily to form the
oxide
BeCl2(s) + H2O(l)  BeO(s) + 2HCl(g)
• MgCl2 is slightly hydrolyzed in water
MgCl2(s) + H2O(l)
Mg(OH)Cl(s) + HCl(aq)
• Heating the hydrated crystal will give off HCl and result in a
basic salt
MgCl2 • 6H2O(s)  Mg(OH)Cl(s) + HCl(g) + 5H2O(l)

82
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82
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.64)
Relative 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 &
•
83
(2) sizes of ions
The greater the charge to size ratios of ions, the higher
is the thermal stability of the compound
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83
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.65)
• Compound with large polarizable anion, the thermal
stability depends on the polarizing power of cations
 The stronger the polarizing power, the electron
cloud of anion will be distorted to greater extent
 The compound tends to be less thermal stable
84
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84
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.65)
•
Most carbonates and hydroxides of Group I and II
metals readily undergo decomposition on heating to give
oxides
e.g.
MgCO3(s)  MgO(s) + CO2(g)
Ca(OH)2(s)  CaO(s) + H2O(g)
85
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85
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.65)
• Oxide ion is smaller than CO32– and OH–, the charge
density is higher
 Oxide ion is less polarizable
 The attraction between cation and anion is stronger
 The compound is more stable
 Groups I and II metal carbonates and hydroxides
decompose to form metal oxides
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86
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.65)
• 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
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.65)
• 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
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88
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.66)
The Carbonates
•
All Group I carbonates (except Li2CO3) can withstand a
temperature around 800°C
700°C
Li2CO3(s)  Li2O(s) + CO2(g)
•
Li2CO3 is less stable than other Group I carbonates
∵ Li is the smallest and charge density is the highest
 Electron cloud of the carbonate ion is distorted to
greater extent
∴ decompose more easily
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.66)
•
Group II metal ions has higher polarizing power
 their carbonates are less stable
e.g.
100°C
BeCO3(s)  BeO(s) + CO2(g)
900°C
CaCO3(s)  CaO(s) + CO2(g)
1 360°C
BaCO3(s)  BaO(s) + CO2(g)
•
Going down the group, size of the cation increases
 polarizing power decreases
 compounds with large cations are more stable
90
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90
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.66)
The Hydroxides
•
All Group I hydroxides (except LiOH) are thermal stable
2LiOH(s)  Li2O(s) + H2O(g)

•
The Group II metal hydroxides are less stable than those of
Group I metals
Be(OH)2(s)  BeO(s) + H2O(g) H = +54 kJ mol–1

•
91
Ca(OH)2(s) 
CaO(s) + H2O(g)

H = +109 kJ mol–1
Ba(OH)2(s) 
BaO(s) + H2O(g)

H = +146 kJ mol–1
The thermal stability of Group II metal hydroxides
increases down the group
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91
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.67)
Relative Solubility of the Sulphates(VI) and Hydroxides
Processes involved in Dissolution and their Energetics
•
When an ionic solid is dissolved in water, two processes are taken
place:
1. Breakdown of the ionic solid
2. Stabilization of ions by water molecules (called hydration)
•
1st process involves an input of energy to breakdown the lattice
•
2nd process involves a release of energy when ions are hydrated
(i.e. new bonds are formed between water molecules and ions)
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.67)
•
Take NaCl as an example:
NaCl(s)  Na+(aq) + Cl–(aq)
•
Hsoln = +4 kJ mol–1
This change involves 2 processes:
1st : NaCl(s)  Na+(g) + Cl–(g)
H = +776 kJ mol–1
The enthalpy change involved in this process is the reverse
of lattice enthalpy. The lattice enthalpy is –776 kJ mol–1.
2nd : Na+(g) + Cl–(g)  Na+(aq) + Cl–(aq)
Hhyd = –772 kJ mol–1
This is the enthalpy change resulted from hydration of
one mole of both gaseous ions
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93
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.68)
According to Hess’s law,
The enthalpy change of solution is :
Hsoln = Hhyd – Hlattice
For a salt to be soluble in water, the Hsoln has to be a –ve
or small positive value
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94
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.68)
Relative Solubility of the Sulphates(VI) and
Hydroxides – Trend and Interpretation
•
The sulphates(VI) and hydroxides of Group I metals are
more soluble in water than those of Group II metals
∵
Group I metals has a larger size and smaller charge than
Group 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
95
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
• For Group II metal sulphates(VI), cations are much smaller
than anions
• The Hlattice is determined by the reciprocal of the sum of
cationic and anionic radii (i.e.
1
r  r
)
 Large anionic radius makes the relatively small cationic
radius insignificant w.r.t the sum of r+ and r–
 Down the group, increase in cationic size does not make
a significant change in the Hlattice
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
•
However, an increase in cationic size causes Hhyd to
become less and less negative
 Hsoln becomes less and less exothermic
 The solubility of Group II metal sulphates(VI)
decreases down the group
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97
41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
For Group II metal hydroxides, the sizes of cation and anion are
in the same order of magnitude
 As H lattice 
1
r  r
 Going down the group, cationic size increases
 Less energy is required to break the lattice
 change in Hhyd is comparatively small
 Hsoln becomes more and more negative
 The solubility of Group II metal hydroxides increases down
the group
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
Check Point 41-5
(a) Give balanced chemical equations for the following
reactions:
(i) decomposition of barium carbonate;
(ii) reaction between sodium hydride and water;
(iii) reaction between sodium peroxide and water.
Answer
(a) (i) BaCO3(s)  BaO(s) + CO2(g)
(ii) NaH(s) + H2O(l)  NaOH(aq) + H2(g)
(iii) Na2O2(s) + 2H2O(l)  2NaOH(aq) + H2O2(aq)
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
Check Point 41-5 (cont’d)
(b) Suggest a reason why barium sulphate(VI) is insoluble in
water while potassium sulphate(VI) is soluble in water
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
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
(b) When an ionic solid is dissolved in water, two processes are
taking place. They are the break down of the ionic solid (this is related to
the lattice enthalpy, Hlattice), and subsequent stabilization of the ions by
water molecules (this is related to the hydration enthalpy, Hhyd). 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 whereas potassium sulphate(VI) is soluble in water is
that potassium ion has a smaller charge than barium ion, and thus 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 whereas barium sulphate(VI) is not.
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41.4 Variation in Properties of the compounds of the
s-Block Elements (SB p.69)
Check Point 41-5
(c) Compare the solubility of CaSO4 and BaSO4. Explain your
answer.
Answer
(c) CaSO4 is expected to be more soluble than BaSO4. It is
because Ca2+ has a smaller size than Ba2+, this causes the
Hhyd of CaSO4 to be more negative than that of BaSO4. As a
result, the Hsoln of CaSO4 becomes more negative than that
of BaSO4 and hence CaSO4 is more soluble in water than
BaSO4.
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41.5 Uses of the Compounds of the s-Block Elements (SB p.69)
Sodium Carbonate
• Fusing sodium carbonate with
calcium carbonate and silica to
form silicates at 1500°C
• Soda glass is a mixture of sodium
silicate and calcium silicate
Na2CO3(s) + SiO2(s)
 Na2SiO3(s) + CO2(g)
CaCO3(s) + SiO2(s)
 CaSiO3(s) + CO2(g)
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Soft glass is made by
fusing SiO2 with Na2CO3
and CaCO3
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41.5 Uses of the Compounds of the s-Block Elements (SB p.70)
• Na2CO3 is used in water softening
∵ CO32– can precipitate Mg2+(aq) and Ca2+(aq) in hard water
Na2CO3(aq) + Mg2+(aq)  MgCO3(s) + 2Na+(aq)
Na2CO3(aq) + Ca2+(aq)  CaCO3(s) + 2Na+(aq)
• A large amount of Na2CO3 is used
in sewage treatment
• Na2CO3 is also used in paper
industry and in making soap
and caustic alkalis
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Washing powder contains
Na2CO3
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41.5 Uses of the Compounds of the s-Block Elements (SB p.70)
Sodium Hydrogencarbonate
•
NaHCO3 is found in baking powder which also contains a solid acid
•
On adding water, the acid reacts with NaHCO3 to give CO2(g)
•
NaHCO3 decomposes at high temperatures to give off CO2(g)
•
The CO2(g) makes the cake rise
and spongy
HCO3–(s) + H+(aq)
 H2O(l) + CO2(g)

2NaHCO3(s)
 Na2CO3(s) + CO2(g) + H2O(l)
•
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Soft drinks are also made from NaHCO3
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41.5 Uses of the Compounds of the s-Block Elements (SB p.70)
Sodium Hydroxide
•
In saponification, fats and oils are hydrolyzed by NaOH
to form soap
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
•
NaOH is used to make
soaps, detergents,
dyes, paper and drugs
Products manufactured
from NaOH
•
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NaOH is also used in the manufacture of rayon and
some very important organic compounds such as
phenol and naphthol
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
Magnesium Hydroxide
•
•
Mg(OH)2 is a weak base and slightly
soluble in water
Mg(OH)2 is a good antacid (Milk
of magnesia) to cure gastric pain by
neutralizing excess HCl
Mg(OH)2(s) + 2HCl(aq)
 MgCl2(aq) + 2H2O(l)
Advantages:
1. Relatively insoluble, not absorbed by body but remain to act
whatever acid is present
2. No burping due to no CO2 produced whereas hydrogencarbonate
antacids do
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
Calcium Oxide and Calcium Carbonate
•
CaCO3 decomposes to CaO (quicklime) on heating
•
Addition of water to quicklime produces Ca(OH)2
(slaked lime)
CaCO3(s) 
CaO(s) + CO2(g)

CaO(s) + H2O(l)  Ca(OH)2(s)
•
Slaked lime is used to neutralize the acids in industrial
effluents
Ca(OH)2(s) + 2H+(aq)  Ca2+(aq) + 2H2O(l)
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
Strontium Compounds
•
The main use of strontium
compounds is to make fireworks
because they give a persistent and
intense red flame on burning
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
Check Point 41-6
(a) How can sodium hydroxide be obtained on a large scale in
industry? State two important industrial applications of
sodium hydroxide.
Answer
(a) Sodium hydroxide can be obtained by electrolysis of
saturated sodium chloride solution. Sodium hydroxide is used
to manufacture soaps, detergents, dyes, rayon and some
important organic compounds such as phenol and naphthol.
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41.5 Uses of the Compounds of the s-Block Elements (SB p.71)
Check Point 41-6 (cont’d)
(b) Suggest compounds of cations to be used together with
magnesium powder in fireworks to give crimson, brick red,
yellow, green and lilac colours respectively.
Answer
(b) Fireworks should include lithium ion, calcium ion,
sodium ion, barium ion and potassium ion.
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The END
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