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40
The s-Block Elements
40.1
Characteristic Properties of the s-Block
Elements
40.2
Variation in Properties of the s-Block
Elements
Variation in Properties of the
Compounds of the s-Block Elements
40.3
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The Syllabus
• 8.1 Characteristic Properties
•
•
•
•
•
•
2
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
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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
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Notes p. 1
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
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40.1
Characteristic
Properties of the
s-Block Elements
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40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Metallic Character (not mentioned in notes)
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
Cutting Rubidium
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Group I elements:
Lithium
Rubidium
7
Sodium
Potassium
Caesium
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40.1 Characteristic Properties of the s-Block Elements (SB p.39)
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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40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Group II elements:
• silvery in colour
• harder and higher boiling and melting points than
Group I counterparts
∵ stronger metallic bond due to 2e– are contributed to
form bond and smaller atomic sizes
• show different crystal structures
9
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Calcium
Group II elements:
Beryllium
Strontium
10
Magnesium
Barium
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Radium
10
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
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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Variation in Physical Properties
Atomic Radius and Ionic Radius (notes p. 1)
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41.3 Variation in Properties of the s-Block Elements (SB p.52)
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
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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.

14
atomic and ionic radii decrease
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41.3 Variation in Properties of the s-Block Elements (SB p.52)
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
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41.3 Variation in Properties of the s-Block Elements (SB p.53)
Ionization Enthalpy (notes p. 2)
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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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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
shielding effect of inner shell electrons
 small 1st I.E.
For 2nd I.E., electron is removed from stable noble gas
configuration and higher effective nuclear charge
 large 2nd I.E.
18
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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.
19
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40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 3))
Low 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.
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40.1 Characteristic Properties of the s-Block Elements (SB p.41)
Group II elements are relatively more electronegative
than Group I counterparts
∵
21
higher nuclear charge, stronger attraction to outermost
shell electrons
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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40.1 Characteristic Properties of the s-Block Elements (SB p.43)
Characteristic Flame Colours of Salts
• 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
 The flame colour is a characteristic property of the
element
22
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Flame Test
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40.1 Characteristic Properties of the s-Block Elements (SB p.43)
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Flame colours
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40.1 Characteristic Properties of the s-Block Elements (SB p.43)
Weak tendency to form complexes (not mentioned in notes)
Complex:
Polyatomic ion or neutral molecule formed when
molecular or ionic gropups (called ligands) form
dative covalent bonds with a central ion.
Group I & II elements seldom form complex:
- s-block ions do not have low energy vacant orbitals available
for dative covalent bonds.
- Low ionic charge
26
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41.3 Variation in Properties of the s-Block Elements (SB p.55)
Melting Point (notes p. 4)
Variations in melting points of Groups I and II elements
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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
28
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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
29
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41.3 Variation in Properties of the s-Block Elements (SB p.56)
Observations:
• irregularity in the general decrease in
melting point down Group II elements
Reason:
• different metallic crystal structures of
the Group II elements
30
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Group Crystal
II metal structure
Be
Mg
Ca
Sr
Ba
Ra
h
h
f
f
b
—
30
Extraction of sodium (not in syllabus)
Downs Cell
31
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Manufacture of sodium hydroxide
graphite anodes
chlorine
+
used
brine
saturated
brine
mercury alloyed
with sodium
flow of mercury
Water
flowing mercury
(as cathode)
Mercury (recycle)
Flowing mercury cell
32
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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
33
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Flowing mercury cell
34
Q. 1b; Q.8
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40.3 Variation in Properties of the s-Block Elements (SB p.56, notes p. 8)
Hydration Enthalpy
Xn+(g) + aq  Xn+(aq)
Hydration enthalpy (Hhyd) is the amount of energy
released when one mole of aqueous ions is formed from
its gaseous ions.
35
•
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
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M+
Variations in hydration
enthalpy of Groups I and II
elements
36
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• 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 crystallization
Na2CO3.10H2O
K2CO3.2H2O

37
MgSO4.7H2O MgCl2.6H2O
CaSO4.2H2O CaCl2.6H2O
SrSO4
BaCl2.2H2O
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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
38
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Lattice Enthalpies of Group I Halides (p.10)
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Lattice Enthalpies of Group I Halides (p.10)
• Good agreement between calculated and measured
value. Why?
• Lattice Enthalpies decrease down the group:
Reasons:
Size increase
Internuclear distance increase
Attractive force between opposite ions decrease
40
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Lattice Enthalpies of Group II Halides (p.11)
• Discrepancies occurred between calculated and measured
values.
Reason:
Covalent characters occurred in small cations.
• Group II Halides have a higher lattice enthalpies than
Group I Halides.
Reason:
Higher charge; smaller size.
41
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40.1 Characteristic Properties of the s-Block Elements (SB p.43, notes p. 13)
Formation 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)
Li+H2O
42
Na +H2O
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Rb+H2O
K+H2O
Cs+H2O
43
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40.1 Characteristic Properties of the s-Block Elements (SB p.43)
•
All Group II metals (except Be) react with H2O to form metal
hydroxides and H2 gas (Mg reacts with hot water).
Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g)
e.g.
Sr(s) + 2H2O(l)  Sr(OH)2(aq) + H2(g)
•
44
Be does not react with H2O(l or g)
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Strontium + water
45
Barium + water
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40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 14)
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
46
1
O2
2
2–
peroxide
O O
O2
 2O2–
superoxide
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40.1 Characteristic Properties of the s-Block Elements (SB p.41)
• 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)
47
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40.1 Characteristic Properties of the s-Block Elements (SB p.41)
• 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)
• Rb, Cs also forms superoxides
3000C
Rb2O2(s) + O2(g) 
2RbO2(s)
Cs2O2(s) + O2(g) 3000C
 2CsO2(s)
48
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41.2 Characteristic Properties of the s-Block Elements (SB p.45)
49
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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40.1 Characteristic Properties of the s-Block Elements (SB p.42 notes p. 14)
•
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
50
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41.2 Characteristic Properties of the s-Block Elements (SB p.46, notes p. 14)
Group II Elements
• Form normal oxides only, except Sr, Ba which can form
peroxides.
• All are basic (except BeO which is amphoteric), why?
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)
51
2BaO2(s)
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Strontium + air
52
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Barium + air
52
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
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
53
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40.3 Variation in Properties of the compounds of the s-Block Elements ( p.59)
notes p. 14 2(e)
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)
•
54
Dissolution of
Na2O2 in H2O
containing
phenolphthalein
Superoxides:
e.g. 2KO2(s) + 2H2O(l)  2KOH(aq) + H2O2(aq) + O2(g)
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•
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
BeO(s) + 2H+(aq)  Be2+(aq) + H2O(l)
hot
BeO(s) + 2OH–(aq) + H2O(l)  [Be(OH)4]2–(aq)
hot
• MgO is slightly soluble in water, but dissolves in acids to
form salts
•
55
BaO2(s) + 2H2O(l)  Ba(OH)2(aq) + H2O2(aq)
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40.3
Variation in Properties of the compounds of the s-Block Elements
(p.60, not mentioned in notes)
Reaction with Acids
56
•
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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40.3 Variation in Properties of the compounds of the s-Block Elements (p.60)
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)
57
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.60)
notes p. 15, 18
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 &
(2) sizes of ions
58
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.61)
notes p. 18
• Compound with large polarizable polyatomic anion
(large electron cloud, as shown in notes), the thermal
stability depends on the polarizing power (charge density)
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
59
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.61)
Group II carbonates/hydroxides are less stable than Group I
• 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
60
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.61)
•
Most carbonates and hydroxides of Group II metals
readily undergo decomposition on heating to give
oxides (more stable)
MgCO3(s)  MgO(s) + CO2(g)
e.g.
Ca(OH)2(s)  CaO(s) + H2O(g)
-
O H
2+
Mg
MgO
-
+
O H
-
Mg2+
61
-
O
O
C O
MgO
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+
H2O
CO2
61
40.3 Variation in Properties of the compounds of the s-Block Elements (p.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
Do Q. 2b on p. 73
Effect of sizes of
cations on thermal
stability of
compounds
62
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Q.
Explain briefly why lithium hydrogencarbonate
does not exist as a solid while other Group I
hydrogencarbonates 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 polarizing
power decreases, therefore, solid hydrogencarbonates can be
formed.
63
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Effect of Heat on s-block carbonates and hydroxides (p.19)
i. Carbonates
Group I:
All are thermally stable except Lithium.
Group II:
All decompose on heating forming metal
oxides and carbon dioxide.
ii. Hydroxides (p.21)
64
Group I:
All are thermally stable except Lithium.
Group II:
All decompose on heating forming metal
oxides and water.
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.63)
notes p. 21
Relative Solubility of the Sulphates(VI) and Hydroxides
Processes involved in Dissolution and their Energetics
•
65
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)
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Dissolution of NaCl
66
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Solubility of s-block Sulphates and Hydroxides (p.23)
MX(s)
Hs
M+(aq) + X-(aq)
-U
Hhyd
M+(g) + X-(g)
A low modulus of lattice enthalpy and a high modulus of
hydration enthalpy favour the dissolving process.
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Effect of charge and size of ions on Hhyd and Hlattice
+
-
Z Z
H lattice  +
r +r
H hyd
68
1 1
 ++ r
r
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Solubility of s-block Sulphates and Hydroxides
i. For 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
group.
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SO42-
SO42-
MgSO4
SrSO4
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ii. For 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 group.
Mg(OH)2
iii.
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Sr(OH)2
Group I sulphates and hydroxides are more
soluble than that of Group II. Why?
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40.3 Variation in Properties of the compounds of the s-Block Elements (p.23)
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 smaller charge and larger size 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
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Do Q. 6, 10 and Q. 7 on p. 74
The END
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