the s -block elements
DESCRIPTION
The s -Block Elements. The s -Block Elements. Elements of Groups IA * (the alkali metals) and IIA * (the alkaline earth metals) constitute the s -block elements their outermost shell electrons are in the s orbital - PowerPoint PPT PresentationTRANSCRIPT
1
The The ss-Block Elements-Block Elements
2
The The ss-Block Elements-Block Elements
• Elements of Groups IA* (the alkali metals) and IIA* (the alkaline earth metals)
constitute the s-block elements
their outermost shell electrons are in the s orbital
*Note: In the following, Groups IA and IIA are abbreviated as Groups I and II respectively.
3 The s-block elements
4
The The ss-Block Elements-Block Elements
• Similarities
1.highly reactive metals
2.strong reducing agents
3.form ionic compounds with fixed oxidation states of +1 for Group I
elements and +2 for Group II elements
5
[Rn] 7s2[Xe] 6s2[Kr] 5s2[Ar] 4s2[Ne] 3s2[He] 2s2Electronic configuration
*RaRadium
BaBarium
SrStrontium
CaCalcium
MgMagnesium
BeBeryllium
Group II
[Rn] 7s1[Xe] 6s1[Kr] 5s1[Ar] 4s1[Ne] 3s1[He] 2s1Electronic configuration
*FrFrancium
CsCaesium
RbRubidium
KPotassium
NaSodium
LiLithium
Group I
Q.1
6
Group I elementsGroup I elements
• Lithium
7
Group I elementsGroup I elements
• Sodium
8
Group I elementsGroup I elements
• Potassium
9
Group I elementsGroup I elements
• Rubidium
10
Group I elementsGroup I elements
• Francium - radioactive
11
Group I elementsGroup I elements
• Beryllium
12
Group I elementsGroup I elements
• Magnesium
13
Group I elementsGroup I elements
• Calcium
14
Group I elementsGroup I elements
• Strontium
15
Group I elementsGroup I elements
• Barium
16
Group I elementsGroup I elements
• Radium - radioactive
17
Characteristic Characteristic Properties of Properties of
thethes-Block s-Block
ElementsElements
18
Group I
element
Electronegativ
ity value
Group II
element
Electronegativ
ity value
Li
Na
K
Rb
Cs
Fr
1.00.90.80.80.7–
Be
Mg
Ca
Sr
Ba
Ra
1.5
1.2
1.0
1.0
0.9
–
All have low electronegativity.
electropositive
19
Group I
element
Electronegativ
ity value
Group II
element
Electronegativ
ity value
Li
Na
K
Rb
Cs
Fr
1.00.90.80.80.7–
Be
Mg
Ca
Sr
Ba
Ra
1.5
1.2
1.0
1.0
0.9
–
EN down the group
EN : Group II > Group I (∵ greater ENC)
20
Group I m.p.(C) b.p.(C) Group II m.p.(C) b.p.(C)
Li 181 1342 Be 1287 2469
Na 98 883 Mg 650 1090
K 63 760 Ca 850 1492
Rb 39 688 Sr 770 1367
Cs 29 690 Ba 714 1637
Fr - - Ra - -
BondingStrength of metallic bond : Group II > Group I
m.p./b.p. : Group II > Group I
21
Group I m.p.(C) b.p.(C) Group II m.p.(C) b.p.(C)
Li 181 1342 Be 1287 2469
Na 98 883 Mg 650 1090
K 63 760 Ca 850 1492
Rb 39 688 Sr 770 1367
Cs 29 690 Ba 714 1637
Fr - - Ra - -
Hardness : - Group I < Group II
Na/K…can be easily cut with a knife
22
Structure
Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba
Group I StructureDensity
(g cm3)Group II Structure
Density
(g cm3)
Li b.c.c. 0.53 Be h.c.p. 1.86
Na b.c.c. 0.97 Mg h.c.p. 1.74
K b.c.c. 0.86 Ca f.c.c. 1.55
Rb b.c.c. 1.53 Sr f.c.c. 2.54
Cs b.c.c. 1.90 Ba b.c.c. 3.59
Fr - - Ra - -
Density : Group II > Group I
23
Structure
Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba
Group I StructureDensity
(g cm3)Group II Structure
Density
(g cm3)
Li b.c.c. 0.53 Be h.c.p. 1.86
Na b.c.c. 0.97 Mg h.c.p. 1.74
K b.c.c. 0.86 Ca f.c.c. 1.55
Rb b.c.c. 1.53 Sr f.c.c. 2.54
Cs b.c.c. 1.90 Ba b.c.c. 3.59
Fr - - Ra - -
Density also depends on size and mass of the atoms
24
Metallic charater (Reactivity) : -
Mn+(aq) + ne M(s)
High tendency to lose electrons as shown by –ve E
Group I (V) Group II (V)
Li -3.04 Be -1.69
Na -2.72 Mg -2.37
K -2.92 Ca -2.87
Rb -2.99 Sr -2.89
Cs -3.02 Ba -2.90
oE oE
25
Metallic charater (Reactivity) : -
Group I > Group II
Group I (V) Group II (V)
Li -3.04 Be -1.69
Na -2.72 Mg -2.37
K -2.92 Ca -2.87
Rb -2.99 Sr -2.89
Cs -3.02 Ba -2.90
oE oE
down the groups
26
Sodium is stored under paraffin oil
sodium
27
Caesium and rubidium are stored in vacuum-sealed
ampoules
caesium
rubidium
28
Formation of Basic OxidesFormation of Basic Oxides
• All alkali metals form more than one type of oxide on burning in air (except lithium)
1. 1. Group I ElementsGroup I Elements
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• Three types of oxides:
normal oxides
peroxides
superoxides
1. 1. Group I ElementsGroup I Elements
2O21
O2–
oxide ion
O22–
peroxide ion
2O2O2
–
superoxide ion
Abundant supply
30
1. 1. Group I ElementsGroup I Elements
Type of oxide formed depends on
1.supply of oxygen
2.reaction temperature
3.charge density of M+
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• Lithium
when it is burnt in air, it forms normal oxide only
1. 1. Group I ElementsGroup I Elements
C180
4Li(s) + O2(g) 2Li2O(s)
lithium oxide
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• Sodium
when it is burnt in an abundant supply of oxygen
forms both the normal oxide and the peroxide
1. 1. Group I ElementsGroup I Elements
C180
4Na(s) + O2(g) 2Na2O(s)
sodium oxide
C300
2Na2O(s) + O2(g) 2Na2O2(s)
sodium peroxide
excess
33
• Potassium, rubidium and caesium
form All three types of oxides when burnt in sufficient supply of
oxygen
1. 1. Group I ElementsGroup I Elements
34
• Potassium:
4K(s) + O2(g) 2K2O(s)potassium oxide
2K2O(s) + O2(g) 2K2O2(s)potassium peroxide
K2O2(s) + O2(g) 2KO2(s)potassium superoxide
1. 1. Group I ElementsGroup I Elements
35
• Rubidium:
4Rb(s) + O2(g) 2Rb2O(s)
2Rb2O(s) + O2(g) 2Rb2O2(s)
Rb2O2(s) + O2(g) 2RbO2(s)
1. 1. Group I ElementsGroup I Elements
36
• Caesium:
4Cs(s) + O2(g) 2Cs2O(s)
2Cs2O(s) + O2(g) 2Cs2O2(s)
Cs2O2(s) + O2(g) 2CsO2(s)
1. 1. Group I ElementsGroup I Elements
37
Group I
element
Normal
oxidePeroxide Superoxide
Li
Na
K
Rb
Cs
Li2O
Na2O
K2O
Rb2O
Cs2O
–
Na2O2
K2O2
Rb2O2
Cs2O2
–
–
KO2
RbO2
CsO2
Oxides formed by Group I elements
Cations with high charge densities (Li+ or Na+) tend to polarize the large electron clouds of peroxide ions and/or superoxide ions
Making them decompose to give oxide ions
38
1. 1. Group I ElementsGroup I Elements
The electron cloud of the superoxide ion is greatly distorted by the small lithium
ion
39
Group I
element
Normal
oxidePeroxide Superoxide
Li
Na
K
Rb
Cs
Li2O
Na2O
K2O
Rb2O
Cs2O
–
Na2O2
K2O2
Rb2O2
Cs2O2
–
–
KO2
RbO2
CsO2
Oxides formed by Group I elements
White solids
Slightly coloured
solids
Highly coloured
solids
40
KO2 used as oxygen generators and CO2 scrubbers in spacecrafts
4KO2 + 2H2O 4KOH + 3O2
2KOH + CO2 K2CO3 + H2O
41
• Beryllium, magnesium and calcium
form normal oxides only on burning in air
2Be(s) + O2(g) 2BeO(s)
2Mg(s) + O2(g) 2MgO(s)
2Ca(s) + O2(g) 2CaO(s)
2. 2. Group II ElementsGroup II Elements
42
Q.2(a)
Be2+, Mg2+ and Ba2+ have higher charge densities
more polarizing
distort the electron cloud of O22
O22 decomposes to give O2
43
Q.2(b)
2SrO(s) + O2(g) 2SrO2(s) strontium peroxide
Sr(s) + O2(g) SrO2(s)
2Sr(s) + O2(g) 2SrO(s)strontium oxide
44
2Ba(s) + O2(g) 2BaO(s) barium oxide
2BaO(s) + O2(g) 2BaO2(s)
barium peroxide
500C
700C
Q.2(b)
Ba(s) + O2(g) BaO2(s)
45
Group
II
elemen
t
Normal
oxidePeroxide
Superoxid
e
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
–
–
–
SrO2
BaO2
–
–
–
–
–
Oxides formed by Group II elements
KO2 superoxid
e
46
Group
II
elemen
t
Normal
oxidePeroxide
Superoxid
e
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
–
–
–
SrO2
BaO2
–
–
–
–
–
Oxides formed by Group II elements
All these oxides are basic in nature (except beryllium oxide which is amphoteric)
47
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
2K(s) + 2H2O(l) 2KOH(aq) + H2(g)
2Rb(s) + 2H2O(l) 2RbOH(aq) + H2(g)
2Cs(s) + 2H2O(l) 2CsOH(aq) + H2(g)
1. 1. Group I Group I
hydroxideshydroxides
Formation of hydroxides
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For normal oxides,
M2O(s) + H2O(l) 2MOH(aq)
1. 1. Group I Group I
hydroxideshydroxides
Formation of hydroxides
For peroxides,
M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq)
For superoxides,
2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g)
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Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)
Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g)
Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)Mg reacts with steam but not water.
Be does not react with water and steam.
Mg(s) + H2O(g) MgO(s) + H2(g)
2. Group II 2. Group II
hydroxideshydroxides
Formation of hydroxides
50
2. Group II 2. Group II
hydroxideshydroxides
Formation of 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)MgO(s) + H2O(l) Mg(OH)2(aq)slightly soluble
BeO(s) + H2O(l) No reaction
51
Ionic Bonding with FixedIonic Bonding with FixedOxidation State in their Oxidation State in their CompoundsCompounds
Group I : +1
Group II : +2
∵ Low 1st I.E. but very high 2nd I.E.
∵ Low 1st and 2nd I.E. but very high 3rd I.E.
Predominantly ionic
52
Group I
elemen
t
OxideHydrid
e
Chlorid
e
Oxidation state of
Group I element in the
compound
Li
Na
K
Rb
Cs
Li2O
Na2O2
KO2
RbO2
CsO2
LiH
NaH
KH
RbH
CsH
LiCl
NaCl
KCl
RbCl
CsCl
+1+1+1+1+1
Chemical formulae of some Group I compounds and the oxidation states of Group I elements in the compounds
53
Group
II
elemen
t
OxideHydrid
e
Chlorid
e
Oxidation state of
Group II element in
the compound
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
BeH2
MgH2
CaH2
SrH2
BaH2
BeCl2
MgCl2
CaCl2
SrCl2
BaCl2
+2+2+2+2+2
Chemical formulae of some Group II compounds and the oxidation states of Group II elements in the compounds
54
Weak Tendency to Form Weak Tendency to Form ComplexesComplexesA complex is formed when a central metal atom or ion is surrounded by other molecules or ions (called ligands) which form dative covalent bonds with the central metal atom or ion using their lone pair.
55
Weak Tendency to Form Weak Tendency to Form ComplexesComplexes
Unlike transition metals, all s-block metals (except Be) show little tendency to form complexes
56
Weak Tendency to Form Weak Tendency to Form ComplexesComplexesReasons : -
1. Absence of low-lying vacant d-orbtals to accept lone pairs from ligands.
For Na+, 1s2, 2s2, 2p6, 3s, 3p, 3dHigh-lying relative to 2p
For Fe2+, 1s2, 2s2, 2p6, 3s2, 3p3, 3d6
Low-lying relative to 3p
57
Weak Tendency to Form Weak Tendency to Form ComplexesComplexesReasons : -
2. s-block cations (M+, M2+) have relatively low charge densities
less polarizing and less able to accept lone pairs from ligands.
58
Co3+
H3N
H3N NH3
NH3
NH3
NH3
All six bonds are strong dative covalent bonds
A complex ion, [Co(NH3)6]3+
Na+
OH2
OH2
H2O
OH2
OH2
H2O
A hydrated ion, Na+(aq)
Dipole-ion attraction
Weaker than dative bond
59
Weak Tendency to Form Weak Tendency to Form ComplexesComplexesOwing to its high charge density,
Be2+ can form complexes
60
Be2+ O
H
H O
H
H
Be(OH)+ + H3O+
[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+
(aq)[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+
(aq)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+
(aq)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq)
[Be(H2O)4]2+(aq) + 4H2O(l) [Be(OH)4]2(aq) + 4H3O+
(aq)
Overall reaction : (1) + (2) + (3) + (4)
61
[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+(aq) (1)
[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+(aq) (2)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+
(aq) (3)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq) (4)Overall reaction : (1) + (2) + (3) + (4) [Be(H2O)4]2+(aq) + 4H2O(l) [Be(OH)4]2(aq) + 4H3O+(aq)
pH equilibrium positions shifts to the right
62
[Be(H2O)4]2+(aq) + H2O(l) [Be(H2O)3(OH)]+(aq) + H3O+(aq) (1)
[Be(H2O)3(OH)]+(aq) + H2O(l) [Be(H2O)2(OH)2](s) + H3O+(aq) (2)[Be(H2O)2 (OH)2](s) + H2O(l) [Be(H2O)(OH)3](aq) + H3O+
(aq) (3)[Be(H2O)(OH)3 ] (aq) + H2O(l) [Be(OH)4]2(aq) + H3O+(aq) (4)(1) + (2)
Be2+(aq) + 2OH(aq) Be(OH)2(s)
[Be(H2O)4]2+(aq) + 2H2O(l) [Be(H2O)2(OH)2](s) + 2H3O+
(aq) + 2OH(aq)
+ 2OH(aq)
[Be(H2O)4]2+(aq) + 2OH(aq) [Be(H2O)2(OH)2](s) + 2H2O
Or simply,
63
Characteristic Flame Colours of Characteristic Flame Colours of SaltsSaltsMost s-block elements and their compounds give a characteristic flame colour in the flame test
Group I element
Flame colour
Group II element
Flame colour
Li Crimson Be -
NaGolden yellow
Mg Bright white
K Lilac Ca Brick redRb Bluish red Sr Blood redCs Blue Ba Apple green
64
Mechanism : -1. In the hotter part of the flame,
2. In the cooler part of the flame,
Na(g) Na(g)*
heat
Na(g)* Na(g)
cool
[Ne] 3p1 [Ne] 3s1
Ground state
[Ne] 3s1
[Ne] 3p1
+ golden yellow light Visible
region
65
Mechanism : -For salts of s-block elements,the metal ions of the salts are first converted to metal atoms
Na+Cl Na(g) + Cl(g)heat
Na(g) Na(g)*
heat
Na(g)* Na(g)
cool + golden yellow light
Na2CO3(s) Na+Cl (more volatile)
Conc. HCl
66
Q.3
Na+(g) Na+
(g)*
heat
Na+(g)* Na+
(g)
cool + uv light
[He] 2s2 2p6
[He] 2s2 2p5 3s1
[He] 2s2 2p5 3s1
[He] 2s2 2p6
2p
3s
3pvisible
uv
67
Li Na K Ca
Pt or nichrome(an alloy of Ni and Cr) is suitable for making the wire because1.They have no reaction with conc. HCl2.They do not impart visible light when heated
68
Variation in Physical Properties Variation in Physical Properties of s-block Elementsof s-block Elements
1. 1. Atomic Radius and Ionic RadiusAtomic Radius and Ionic Radius
22 . . Ionization EnthalpiesIonization Enthalpies
3. Hydration Enthalpies3. Hydration Enthalpies
44 . . Melting PointsMelting Points
69
1. 1. Atomic Radius and Ionic RadiusAtomic Radius and Ionic Radius
Group I
element
Atomic radius
(nm)
Group II
element
Atomic radius
(nm)
Li
Na
K
Rb
Cs
Fr
0.1520.1860.2310.2440.2620.270
Be
Mg
Ca
Sr
Ba
Ra
0.1120.1600.1970.2150.2170.220
down the groups ∵ the outermost electrons are further away from the nuclei
70
1. 1. Atomic Radius and Ionic RadiusAtomic Radius and Ionic Radius
Group I
element
Atomic radius
(nm)
Group II
element
Atomic radius
(nm)
Li
Na
K
Rb
Cs
Fr
0.1520.1860.2310.2440.2620.270
Be
Mg
Ca
Sr
Ba
Ra
0.1120.1600.1970.2150.2170.220
Group II < Group I∵ ENC from left to right across the periods
71
On moving down the groups,first sharply (e.g. from Li to K)then slowly (e.g. from K to Fr)
72
1. There is a sharp in NC from 19K to 37RbOutermost e is drawn closer to the
nucleus
73
2. The inner d-electrons (of Rb, Cs, Sr, Ba) have poor shielding effect on the
outermost electrons transition contraction
74
2. 2. Ionization Ionization
EnthalpyEnthalpyGroup I
elemen
t
1st IE 2nd IEGroup II
element1st IE 2nd IE 3rd IE
Li
Na
K
Rb
Cs
Fr
519494418402376381
7 300
4 560
3 070
2 370
2 420
–
Be
Mg
Ca
Sr
Ba
Ra
900736590548502510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–Both atomic radius and ENC down the groupsAtomic radius is more importantIE down the groups
75
2. 2. Ionization Ionization
EnthalpyEnthalpyGroup I
elemen
t
1st IE 2nd IEGroup II
element1st IE 2nd IE 3rd IE
Li
Na
K
Rb
Cs
Fr
519494418402376381
7 300
4 560
3 070
2 370
2 420
–
Be
Mg
Ca
Sr
Ba
Ra
900736590548502510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–For Group I elements, 2nd IE >> 1st IE because1.the outer s-electron is well shielded by inner shell electrons
76
2. 2. Ionization Ionization
EnthalpyEnthalpyGroup I
elemen
t
1st IE 2nd IEGroup II
element1st IE 2nd IE 3rd IE
Li
Na
K
Rb
Cs
Fr
519494418402376381
7 300
4 560
3 070
2 370
2 420
–
Be
Mg
Ca
Sr
Ba
Ra
900736590548502510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–For Group I elements, 2nd IE >> 1st IE because2. the 2nd electron is closer to the nucleus
and is poorly shielded by other electrons in the same shell which is completely filled.
77
2. 2. Ionization Ionization
EnthalpyEnthalpyGroup I
elemen
t
1st IE 2nd IEGroup II
element1st IE 2nd IE 3rd IE
Li
Na
K
Rb
Cs
Fr
519494418402376381
7 300
4 560
3 070
2 370
2 420
–
Be
Mg
Ca
Sr
Ba
Ra
900736590548502510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–For Group II elements, 3rd IE >> 2nd IESimilar reasons can be applied
78
Variations in the first and second ionization enthalpies of Group I elements
79
Variations in the first, second and third ionization enthalpies of Group II elements
80
2. 2. Ionization Ionization
EnthalpyEnthalpyGroup I
elemen
t
1st IE 2nd IEGroup II
element1st IE 2nd IE 3rd IE
Li
Na
K
Rb
Cs
Fr
519494418402376381
7 300
4 560
3 070
2 370
2 420
–
Be
Mg
Ca
Sr
Ba
Ra
900736590548502510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–Group II > Group I ∵ The outer s-electrons of Group II atoms are closer to the nucleus and experience higher ENC
81
3. 3. Hydration Hydration
enthalpy enthalpy Hydration enthalpy (Hhyd) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions.
M+(g) + aq M+(aq) H = Hhyd
M2+(g) + aq M2+(aq)H = Hhyd
always has a negative value
82
Group I
ion
Hydration enthalpy (kJ
mol–1)
Group II
ion
Hydration enthalpy (kJ
mol–1)
Li+
Na+
K+
Rb+
Cs+
Fr+
–519
–406
–322
–301
–276
–
Be 2+
Mg2+
Ca2+
Sr2+
Ba2+
Ra2+
–2 450
–1 920
–1 650
–1 480
–1 360
–
down the groups ∵ charge density of metal ions down the groups
attraction between ions and water molecules +
H
O
H
83
Group I
ion
Hydration enthalpy (kJ
mol–1)
Group II
ion
Hydration enthalpy (kJ
mol–1)
Li+
Na+
K+
Rb+
Cs+
Fr+
–519
–406
–322
–301
–276
–
Be 2+
Mg2+
Ca2+
Sr2+
Ba2+
Ra2+
–2 450
–1 920
–1 650
–1 480
–1 360
–
Group II > Group I ∵ Group II ions have higher charge and small size
higher charge density stronger ion-dipole interaction
84
Variations in hydration enthalpy of the ions ofGroups I and II elements
85
The melting points of s-block elements depend on the metallic bond strength which in turn depends on
1.charge density of cations
2.number of valence electrons participating in the sea of electrons
3.packing efficiency of the crystal lattices
4. 4. Melting PointMelting Point
86
Group I
element
Melting Point
(C)
Group II
element
Melting Point
(C)
Li
Na
K
Rb
Cs
Fr
180
97.8
63.7
38.9
28.7
24
Be
Mg
Ca
Sr
Ba
Ra
1280
650
850
768
714
697
1. down the groups∵ ionic radii down the groups
charge density interaction between ions and electron
sea
87
Group I
element
Melting Point
(C)
Group II
element
Melting Point
(C)
Li
Na
K
Rb
Cs
Fr
180
97.8
63.7
38.9
28.7
24
Be
Mg
Ca
Sr
Ba
Ra
1280
650
850
768
714
697
2. Group II > Group I ∵ (a) Group II cations have higher charge density(b) More valence electrons are involved in the sea of electrons(c) Packing efficiency : Group II > Group I
88
Reason not known !!
89
Variation in Chemical PropertiesVariation in Chemical Properties
s-Block elements have strong reducing power
∵ low ionization enthalpies
low atomization enthalpies
90
Hydration enthalpy
Atomization enthalpy
M(g)
Ionization
enthalpy
M+
(g)
~Ea
M(s)
M+
(aq)
0ΔH reaction0
M(s) M+(aq) + e H < 0
91
Hydration enthalpy
Atomization enthalpy
M(g)
Ionization
enthalpy
M+
(g)
~Ea
M(s)
M+
(aq)
0ΔH reaction0
Reactivity : Na > Ca (depends on Ea)
Position in e.c.s. : Ca > Na (depends on Ho or Eo
92
Variation in Chemical PropertiesVariation in Chemical Properties
The reactivity of s-block elements down the groups
∵ both I.E. and A.E. down the groups
Ea down the groups
Reaction rate down the groups
93
Variation in Chemical PropertiesVariation in Chemical Properties
Reactivity : Group I > Group II
∵ both I.E. and A.E. across the periods
Ea across the periods
Reaction rate across the periods
94
1. 1. Reactions with Reactions with
hydrogenhydrogenGroup I
2M(s) + H2(g) 2MH(s)300C – 500C
Group II
M(s) + H2(g) MH2(s)600C – 700C
95
1. 1. Reactions with Reactions with
hydrogenhydrogen4LiH + AlCl3 LiAlH4 + 3LiCl
Dry ether
Reducing agent in organic syntheses
96
Most s-block elements
show a silvery white lustre when they are freshly cut
they tarnish rapidly upon exposure to the atmosphere
∵ they react with oxygen in the air to form an oxide layer
2. Reactions with 2. Reactions with
OxygenOxygen
97
Sodium shows a silvery white lustre when freshly cut
98
Group I (p.2)
Group II
2M(s) + O2(g) 2MO(s)heat
M(s) + O2(g) MO2(s)heat
99
3. Reactions with 3. Reactions with
ChlorineChlorineGroup I
2M(s) + Cl2(g) 2MCl(s)heat
Group II
M(s) + Cl2(g) MCl2(s)heat
100
4. Reactions with water or 4. Reactions with water or
steamsteamGroup I
2M(s) + H2O(l) 2MOH(aq) + H2(g)
heat
Group II
M(s) + 2H2O(l) M(OH)2(aq) + H2(g)
heat
Mg reacts with steam but not water
Be has no reaction with either water or steam
Mg(s) + H2O(g) MgO(s) + H2(g)
heat
101
Variation in chemical properties of Variation in chemical properties of the compounds of the compounds of ss-block elements-block elements
Reactions of Reactions of
oxidesoxidesReactions of Reactions of
hydrideshydridesReactions of Reactions of
chlorideschlorides
102
Reactions of Reactions of
oxidesoxides
M2O(s) + H2O(l) 2MOH(aq)
M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq)
2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g)
Group I
1. Reactions with water
103
Na2O2 is used in qualitative analysis of Cr3+
2Cr(OH)3(s) + 3Na2O2(s)
2Na2CrO4(aq) + 2NaOH(aq) + 2H2O(l)
green
yellow
104
Reactions of Reactions of
oxidesoxides
CaO(s) + H2O(l) Ca(OH)2(aq)
SrO(s) + H2O(l) Sr(OH)2(aq)
BaO(s) + H2O(l) Ba(OH)2(aq)MgO(s) + H2O(l) Mg(OH)2(aq)slightly soluble
BeO(s) + H2O(l) No reaction
Group II
increasing basicity
105
Reactions of Reactions of
oxidesoxides
M2O(s) + 2HCl(aq) 2MCl(aq) + H2O(l)
M2O2(s) + 2HCl(aq) 2MCl(aq) + H2O2(aq)
2MO2(s) + 2HCl(aq) 2MCl(aq) + H2O2(aq) + O2(g)
Group I
2. Reactions with acids
Group II
MO(s) + 2HCl(aq) MCl2(aq) + H2O(l)
More vigorous than those with water
106
Reactions of Reactions of
oxidesoxides3. Reactions with alkalis
Reaction with water instead except BeO
BeO(s) + 2OH(aq) + H2O(l) Be(OH)4
2(aq)amphoteric
107
Reactions of Reactions of
hydrideshydrides
MH(s)H 2
O or NaOH(aq) MOH(aq) +
H2(g)
MCl(aq) + H2(g)
HCl(aq)
H (a strong base) tends to react with protonic reagents to release H2
Reactivity down the groups
More vigorous
108
Reactions of Reactions of
chlorideschlorides
No significant reactions with water, acids or alkalis
Group I
Group II
Do not undergo significant hydrolysis except BeCl2 and MgCl2
BeCl2(aq) + 2H2O(l) Be(OH)2(aq) + 2HCl(aq)
MgCl2(aq) + H2O(l) Mg(OH)Cl(aq) + HCl(aq)Basic salt
More favoured in alkaline solutions
109
Relative Thermal Stability of the Relative Thermal Stability of the Carbonates and Hydroxides ofCarbonates and Hydroxides ofss-Block Elements-Block Elements
Thermal stability refers to the resistance of a compound to undergo decompositionon heating.
110
Thermal decomposition reactionsMetal carbonates
M2CO3(s) M2O(s) + CO2
heat
MCO3(s) MO(s) + CO2
heat
Metal hydroxides
2MOH(s) M2O(s) + H2O(g)
heat
M(OH)2(s) MO(s) + H2Oheat
111
Relative thermal stability can be measured in two ways
1. By comparing the decomposition temperatures
A higher decomposition temperature
a greater thermal stability
112
Metal carbonate
BeCO3 MgCO3 CaCO3 SrCO3 BaCO3
Decomposition
temperature /C
~100 540 900 1290 1360
Decomposition temperature is
the temperature at which the pressure of CO2 in equilibrium with the solid carbonate reaches 1 atm in a closed system.
Below the DT, some CO2 can still be detected but the pressure is less than 1 atm
113
• Example:
1. 1. The CarbonatesThe Carbonates
C100BeCO3(s) BeO(s) + CO2(g)
MgCO3(s) MgO(s) + CO2(g) C540
CaCO3(s) CaO(s) + CO2(g) C900
SrCO3(s) SrO(s) + CO2(g) C1290
BaCO3(s) BaO(s) + CO2(g) C1360
114
Relative thermal stability can be measured in two ways
2. By comparing the standard enthalpy changes of thermal decomposition reactions
A more positive H value
a thermally more stable compound
M(OH)2(s) MO(s) + H2O(g) H > 0
115
Metal hydroxide
Be(OH)2 Mg(OH)2
Ca(OH)
2
Sr(OH)2 Ba(OH)2
Ho / kJ mol1
+54 +81 +109 +127 +146
Trends : -
1. down the groups
2. Group I > Group II
3. Li resembles Mg more than the other group 1 elements (diagonal
relationship, pp.14-15)
116
2. 2. The HydroxidesThe Hydroxides
Be(OH)2(s) BeO(s) + H2O(g) H = +54 kJ
mol–1 Mg(OH)2(s) MgO(s) + H2O(g)
H = +81 kJ mol–1 Ca(OH)2(s) CaO(s) + H2O(g)
H = +109 kJ mol–1
Sr(OH)2(s) SrO(s) + H2O(g) H = +127 kJ
mol–1 Ba(OH)2(s) BaO(s) + H2O(g)
H = +146 kJ mol–1
117
Factors affecting thermal stability of carbonates and hydroxides
1. Polarizing power of cation
2. Polarizability of polyatomic anion
3. Lattice enthalpy of metal oxide produced
118
Interpretation of trends in thermal stability of carbonates and hydroxides1. Group I > Group II
(a) M2+ ions have higher charge densities than M+ ions
M2+ ions are more polarizing than M+ ions
Can polarize more the electron cloud of polyatomic anions
119
O C
O
O
M2+ MO + CO2
O H
O H
M2+
heat
heatMO + H2O
polarization
polarization
120
Polarizability as the size of anion
121
Polyatomic ion
Thermal decomposition
122
When a compound with large anions undergoes thermal decomposition, a compound with small anions will be formed since small anions are less
easily polarized
123
Simple ion
more stable compound with stronger bond
124
M2+ S2 M Spolarization
Stronger ionic bond with covalent character
Simple ion
125
Interpretation of trends in thermal stability of carbonates and hydroxides1. Group I > Group II
(b) M2+ ions have higher charge densities than M+ ions
Lattice enthalpy : MO > M2O
Energetic stability : MO > M2O
126
CaCO3(s) CaO(s) + CO2(g) heat
Na2CO3(s) Na2O(s) + CO2(g)
more favourable
less favourable
heat
more stable
less stable
Thermal stability of carbonates : -
Group I > Group II
127
Interpretation of trends in thermal stability of carbonates and hydroxides2. Thermal stability down the groups
∵ size of cations down the groups
∴ (a) charge density/polarizing power of cation down the groups
(b) lattice enthalpies of MO/M2O down the groups
128
MgCO3(s) MgO(s) + CO2(g) heat
more favourable
more stable
BaCO3(s) BaO(s) + CO2(g) heat
less favourable
less stable
more polarized
less polarized
Thermal stability of carbonates : -
down the groups
129
Effect of sizes of the cations on thermal stability of the carbonates and
hydroxides of both Groups I and II metals
130
Interpretation of trends in thermal stability of carbonates and hydroxides3. Li compounds resemble Mg compounds
(diagonal relationship)Charge density/polarizing power : -
Li+ Mg2+
131
Interpretation of trends in thermal stability of carbonates and hydroxides4. Thermal stability of nitrates follows similar patterns (Optional)
2MNO3(s) 2MNO2 + O2
heat
2M(NO3)2(s) 2MO + 4NO2 + O2
heat
132
Relative Solubility of the Relative Solubility of the Sulphates(VI) and Hydroxides ofSulphates(VI) and Hydroxides ofs-Block Elementss-Block Elements
In general,
Group I >> Group II
133
Compounds
Solubility / mol per 100 of water
Mg(OH)2 0.02 103
Ca(OH)2 1.5 103
Sr(OH)2 3.4 103
Ba(OH)2 15 103
Compounds
Solubility / mol per 100 of water
MgSO4 1800 104
CaSO4 11 104
SrSO4 0.71 104
BaSO4 0.009 104
Q.4
Size and/or charge of the anion
Polarizability of anion Covalent character Solubility in water
In general,
134
Compounds
Solubility / mol per 100 of water
Mg(OH)2 0.02 103
Ca(OH)2 1.5 103
Sr(OH)2 3.4 103
Ba(OH)2 15 103
down the group
Compounds
Solubility / mol per 100 of water
MgSO4 1800 104
CaSO4 11 104
SrSO4 0.71 104
BaSO4 0.009 104
down the group
135
• Two processes are
1. the breakdown of the ionic lattice
2. the subsequent stabilization of the ions by water molecules (this
process is called hydration)
11. Processes involved in . Processes involved in
Dissolution and their EnergeticsDissolution and their Energetics
136
1. the breakdown of the ionic lattice
2. the subsequent stabilization of the ions by water molecules (this process
is called hydration)
NaCl(s) Na+(g) + Cl(g)
Na+(g) + Cl(g) + aq Na+(aq) + Cl(aq)
H2 = (hydration enthalpy) < 0
H1 = (lattice enthalpy) > 0
137
NaCl(s) Na+(aq) + Cl-(aq)
Na+(g) + Cl-(g)
Hsolution
-HL =
+776 kJ m
ol 1
Hhy
drat
ion
= -7
72 k
J mol
1
olattice
ohydration
osolution ΔHΔHΔH
= (-772 +776) kJ mol1= +4 kJ mol1
138
osolution
osolution
osolution STΔHΔG
If , we expect the solids to dissolve in water
0ΔH osolution
Solubility as becomes more –ve (less +ve)
osolutionΔH
Solids (e.g. NaCl) with small +ve valuesare also soluble in water if the dissolution involves an increase in the entropy of the system.
osolutionΔH
139
osolution
osolution
osolution STΔHΔG
0ΔGosolution Spontaneous
dissolutionosolutionST is always
positiveosolutionHDissolution with slightly positive
can be spontaneous
140
Trends and Interpretations
1. The solubility of Group(II) sulphate decreases down the group
On moving down the group, cationic radius(r+) both and become less -ve
oLH o
hydrationH
However, less rapidly than oLH o
hydrationH
141
Trends and Interpretations
rr1
ΔH2
4SO
oL
rr 24SO
∵
constant
olattice
ohydration
osolution ΔHΔHΔH
less –ve down the group
+ve constant
less –ve down the group Solubility down the group
142
Trends and Interpretations
rr1
ΔH2
4SO
oL
rr 24SO
∵
constant
olattice
ohydration
osolution ΔHΔHΔH
more rapidly down the group
less rapidly down the group
less –ve down the group Solubility down the group
(-ve)(+ve)
143
Trends and Interpretations
2. The solubility of Group(II) hydroxides increases down the group
On moving down the group, cationic radius(r+) both and become less -ve
oLH o
hydrationH
However, more rapidly than oLH o
hydrationH
144
Trends and Interpretations
olattice
ohydration
osolution ΔHΔHΔH
less rapidly down the group
more rapidly down the group
more –ve down the group Solubility down the group
(-ve)(+ve)
less +ve down the group
145
For s-block compounds with small anions (e.g. OH, F),
solubility in water down the group
For s-block compounds with large anions (e.g. SO42,
CO32-),
solubility in water down the group
For s-block compounds with medium size anions (e.g. Br),
solubility in water exhibits irregular pattern down the group
146
Compounds
Solubility / mol per 100 of water
MgBr2 5.5 101
CaBr2 6.3 101
SrBr2 4.3 101
BaBr2 3.3 101
Irregular
Solublily : First and then
olattice
ohydration
osolution ΔHΔHΔH
(-ve)(+ve)
First more rapidly
Then more rapidly
First less +ve down the group
Then less -ve down the group
147
Group II compounds with doubly-charged anions (MX) are less soluble than those with singly-charged anions (MY2)
Reasons :
1. HL of MX > HL of MY2
2. HL is the major factor affecting solubility
Hsolution of MX is more positive
Solubility : MX < MY2
148
Solubility : Group I > Group II
Reasons :
For a given anions, both HL and Hhydration become more –ve from Group I to Group II
However, HL is the major factor affecting solubility
Hsolution : Group I is less positve than Group II
Solubility : Group I > Group II
149
increasing polarizing power increasing electronegativity
Diagonal relationship
150
ReactionOther Group I
elementsLithium Magnesium
Combination with O2Peroxides and superoxides
Li2O (normal oxide) MgO (normal oxide)
Combination with N2 No reaction Li3N Mg3N2
Action of heat on carbonate
No reaction (thermally stable)
Decomposes to give Li2O and CO2
Decomposes to give MgO and CO2
Action of heat on hydroxide
No reaction (thermally stable)
Decomposes to give Li2O and H2O
Decomposes to give MgO and H2O
Action of heat on nitrate
Decomposes to give MNO2 and O2
Decomposes to give Li2O, NO2 and O2
Decomposes to give MgO, NO2 and O2
Hydrogen carbonates Exist as solids Only exist in solution
Solubility of salts in water
Most salts are more soluble than those of
Li, Mg.
Fluoride, hydroxide, carbonate, phosphate, ethanedioate are sparingly soluble.
Solubility of salts in organic solvents.
Halides only slightly soluble in organic
solvents
Halides (with covalent character) dissolve in organic solvents
151
The END
152
Metals are sometimes referred to as electropositive elements. Why?
AnswerThey have low electronegativity values.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
153
s-Block compounds give a characteristic flame colour in the flame test. Based on this, can you give
one use ofs-block compounds?
Answers-Block compounds can be used in fireworks.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
154
(a) Which ion has a greater ionic radius, potassium ion or calcium ion? Explain your answer. Answer
(a) Potassium ion (0.133 nm) has a greater ionic radius than calcium
ion (0.099 nm) . In fact, potassium ion and calcium ion are
isoelectronic and have the same number of electron shells.
However, calcium ion has one more proton than potassium ion,
the electron cloud of calcium ion will experience greater attractive
forces from the nucleus. This leads to a smaller ionic radius of
calcium ion.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
155
(b) Explain why Group I elements show a fixed oxidation state of +1 in their compounds in terms of ionization enthalpies. Answer
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
156
(b) Group I elements form ions with an oxidation state of +1 only. It is
because they have only one outermost shell electron. Once this
outermost shell electron is removed, a stable fully-filled electronic
configuration is obtained. Therefore, the first ionization enthalpies
of Group I elements are low. The second ionization involves the
removal of an electron from an inner electron shell. Once this
electron is removed, the stable electronic configuration will be
disrupted. Therefore, their second ionization enthalpies are very
high. As a result, Group I elements form predominantly ionic
compounds with non-metals by losing their single outermost shell
electron, and they form ions having a fixed oxidation state of +1.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
157
(c) Ions of Group I and Group II elements have a very low tendency to form complexes. Give one reason to explain your answer.Answer
(c) As ions of Group I and Group II elements do not have low-lying
vacant orbitals available for forming dative covalent bonds with the
lone pair electrons of surrounding ligands, they rarely form
complexes.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
158
(d) Give one test which would enable you to distinguish a sodium compound from a potassium compound. Answer
(d) Sodium compounds and potassium compounds can be
distinguished by conducting a flame test. In the flame test, sodium
compounds give a golden yellow flame, while potassium
compounds give a lilac flame.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
159
What is a dative covalent bond? How is it formed?
AnswerA dative covalent bond is a covalent bond in which the shared pair
of electrons is supplied by only one of the bonded atoms. A dative
covalent bond is formed by the overlapping of an empty orbital of an
atom with an orbital occupied by a lone pair of electrons of another
atom.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
160
(a) (i) List the factors that affect the value of the ionization enthalpy of an atom.
Answer(a) (i) There are four main factors affecting the magnitude of the
ionization enthalpy of an atom. They are the electronic
configuration of an atom, the nuclear charge, the screening
effect, and the atomic radius.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
161
(a) (ii) Why is ionization enthalpy of an atom always positive?
Answer(a) (ii) Ionization enthalpy of an atom always has a positive value
because energy is required to overcome the attractive forces
between the nucleus and the electron to be removed.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
162
(a) (iii) Describe the general trend of the first and second ionization enthalpies down Group I of the Periodic Table.Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
163
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(a) (iii) The first ionization enthalpies of Group I elements are
relatively low. The outermost s electron is located in a new
electron shell. The attractive force between this s electron and
the nucleus is relatively weak. Also, this s electron is
effectively shielded from the attraction of the nucleus by the
fully-filled inner electron shells. Once this electron is removed,
a stable octet or duplet electronic configuration is obtained.
Consequently, this s electron is relatively easy to be removed,
and hence the first ionization enthalpies of Group I elements
are relatively low. However, the second ionization of Group I
elements involves the loss of an inner shell electron which is
closer to the nucleus. The removal of this electron disrupts the
stable electronic configuration. Therefore, the second
ionization enthalpies of Group I elements are extremely high.
164
(b) (i) List the factors that affect the value of the hydration enthalpy of an ion.
Answer(b) (i) The value of the hydration enthalpy of an ion depends on
the size and the charge of the ion.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
165
(b) (ii) Why does hydration enthalpy of an ion always have a negative value?
Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(b) (ii) Hydration enthalpy of an ion always has a negative value
because it is the amount of energy released resulting from the
attraction between the ion and water molecules.
166
(b) (iii) Describe the general trend of the hydration enthalpy down Group II of the Periodic Table. Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(b) (iii) Going down Group II, the hydration enthalpy of the ions
decreases (becomes less negative). Since the ions get larger
in size on moving down the group, the charge density of the
ions falls. As a result, the electrostatic attraction between the
ions and water molecules becomes weaker, and the hydration
enthalpy becomes less negative down the group.
Back
167
The burning of lithium, sodium and potassium in oxygen gives different types of oxides. Why do the
metals behave differently?Answer
40.2 Variation in Properties of the s-Block Elements (SB p.57)
168
On burning in air, lithium forms only lithium oxide, and it does not form the
peroxide or superoxide. This is because the size of lithium ion is very small,
leading to its high polarizing power. When a peroxide ion or superoxide ion
approaches a lithium ion, the electron cloud of the peroxide ion or
superoxide ion (large in size) would be greatly distorted by the lithium ion.
The greater the distortion of the electron cloud, the lower the stability of the
compound. That is why lithium peroxide and lithium superoxide do not
exist. Sodium ion has a larger size than lithium ion. Its lower polarizing
power allows it to form the peroxide when sodium is burnt in air. Potassium
ion has a much larger size, so it has relatively low polarizing power. The
electron cloud of the peroxide ion or superoxide ion would not be seriously
distorted by potassium ion. This allows the peroxide ions or superoxide
ions to pack around potassium ion with a higher stability. As a result,
potassium is able to form stable peroxide or superoxide on burning in air.
Back
40.2 Variation in Properties of the s-Block Elements (SB p.57)
169
(a) Suggest a reason why the reaction of lithium with water is less vigorous than those of sodium and potassium.
Answer(a) The reactivity of Group I metals with water is related to the relative
ease of the metal atoms to lose the outermost shell electron. Going
down the group, as the atomic size increases, the outermost shell
electron becomes easier to be removed. Therefore, the reactivity of
Group I metals towards water increases down the group. Lithium
reacts with water vigorously. Sodium reacts with water violently and
moves on the water surface with a hissing sound.
40.2 Variation in Properties of the s-Block Elements (SB p.58)
170
(b) Which element is the strongest reducing agent, calcium, strontium or barium?
Answer(b) Barium is the strongest reducing agent. It is because the reducing
power of an element is related to the ease of the atom to lose the
outermost shell electron. Since barium has larger atomic sizes, its
outermost shell electrons are less firmly held by the nucleus.
Therefore, barium has a higher tendency to lose its outermost shell
electrons than both calcium and strontium.
40.2 Variation in Properties of the s-Block Elements (SB p.58)
Back
171
The value of Hsoln of a solid does not indicate whether the solid is soluble in water or not. So how
can we predict the solubility of a solid in water?Answer
Back
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
Generally speaking, for a solid to be soluble in water, its enthalpy
change of solution has to be a negative or a small positive value.
172
(a) Give balanced chemical equations for the following reactions:
(i) Thermal decomposition of barium carbonate
(ii) Reaction between sodium peroxide and water
(iii) Reaction between calcium oxide and dilute hydrochloric acid
Answer
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
(a) (i) BaCO3(s) BaO(s) + CO2(g)
(ii) Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(aq)
(iii) CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)
173
(b) Suggest a reason why barium sulphate(VI) is insoluble in water, while potassium sulphate(VI) is soluble inwater although they have cations of similar sizes and the same anion.
(The ionic radii of potassium ion and barium ion are 0.133 nm and 0.135 nm respectively.)
Answer
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
174
(b) When an ionic solid dissolves in water, two processes are taking
place. They are the breakdown of the ionic lattice and the
subsequent stabilization of the ions by water molecules. The
enthalpy change involved in the whole dissolution process is known
as the enthalpy change of solution, Hsoln, which is equal to Hsoln =
Hhyd – Hlattice. For an ionic compound to be soluble in water, the
enthalpy change of solution has to be a negative or a small positive
value. The reason why barium sulphate(VI) is insoluble in water
while potassium sulphate(VI) is soluble in water is that potassium
ion has a smaller charge than barium ion. The Hlattice of potassium
sulphate(VI) is smaller in magnitude (less negative) than that of
barium sulphate(VI). As a result, the enthalpy change of solution of
potassium sulphate(VI) is more negative, and hence it is soluble in
water while barium sulphate(VI) is not.
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
175
(c) Compare the solubility of calcium sulphate(VI) and barium sulphate(VI) in water. Explain your answer. Answer
(c) Calcium sulphate(VI) is expected to be more soluble than barium
sulphate(VI). It is because calcium ion has a smaller size than
barium ion. This causes the Hhyd of calcium sulphate(VI) to be more
negative than that of barium sulphate(VI). As a result, the Hsoln of
calcium sulphate(VI) becomes more negative than that of barium
sulphate(VI), and hence calcium sulphate(VI) is more soluble in
water than barium sulphate(VI).
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
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