Chapter 9
The sp-block elements (II)
CHEM 462
Wednesday, Nov. 10
T. Hughbanks
Alkali Metals - Group 1
Chemistry is dominated by the low
ionization energies, extreme electropositive
character
Atomic configurations ns1
Bonding in the elements is weak, bonding
in most compounds is ionic
Li is the only element that shows any slight
covalent chemistry.
The symptoms of having only one loosely
bound valence electron . . .
I.E.
(kJ•mol-1)
m.p. b.p. Density Hatomization
(˚C) (˚C) (g•cm3)
(kJ•mol-1)
Li
520
180 1347
0.53
162
Na
496
98
881
0.97
108
K
420
64
766
0.86
90
Rb
403
39
688
1.53
82
Cs
376
29
705
1.87
78
More Physical Properties . . .
Hhyd
E˚
ionic rad
(pm)
kJ/mol
Shyd
J/mol•K
Metallic
radius
(pm)
M+/M (V)
Li
76
-519
-140
152
-3.04
Na
102
-404
-110
186
-2.71
K
138
-321
-70
230
-2.93
Rb
149
-296
-70
247
-2.98
Cs
170
-271
-60
267
-3.03
Alkaline-Earth Metals - Group 2
Electropositive character of elements still very
important
Atomic configurations ns2
Bonding in the elements is stronger than group
1, but weaker than other metals. Bonding in
most compounds is ionic, but some covalent
character is seen for Be, slightly so for Mg.
2 valence electrons are better than one . . .
I.E.1
I.E.2
(kJ•mol-1) (kJ•mol-1)
melting Density
pts. ˚C (g•cm3)
Hvap
(kJ•mol-1)
Be
899
1758
1287
1.848
309
Mg
738
1450
649
1.738
127
Ca
590
1146
839
1.55
155
Sr
549
1064
768
2.63
158
Ba
503
964
727
3.62
136
More Physical Properties . . .
metallic
ionic
radius, pm radius, pm
Hhyd
Shyd
(kJ/mol)
(J/mol•K)
Be
112
27
-2500
-300
Mg
160
72
-1931
-320
Ca
197
100
-1586
-230
Sr
215
126
-1456
-220
Ba
224
142
-1316
-200
Sources/uses of alkali metals
Li and Na from Down’s cell electrolysis
K, Rb, Cs from reaction of MCl with Na. e.g,:
Na(l) + KCl(l)
K(g) + NaCl(l)
~ 850 ˚C
(What drives this reaction? For the reaction,
Na(s) + KCl(s) K(s) + NaCl(s), G˚ = +25 kJ)
Na is used heavily in industrial processes,
including its use to make other metals.
Na2CO3 is used heavily in glass manufacture.
Li2CO3 is used to treat manic-depressives.
Sources/uses of alkaline-earth metals
Be comes from the mineral beryl, Be3Al2Si6O18.
(with 2% AlIII/CrIII ‘doping’ this is emerald!)
MgO (from CaMg(CO3 )2 , dolomite) is a main
source. Mg metal is obtained by electrolysis of
MgCl2. Over half of the worldwide production (4
105 tons) goes to lightweight Al/Mg alloys.
Thermite reaction, Al reduction
Ca, Sr, Ba are
produced by Al
reduction at high T:
3 MO(s) + Al(s) Al2O3(s) + M(s)
The reaction
shown here is for
Al reduction of
iron.
Chemistry of alkali metals
All alkali metals react with water:
2 M(s) + 2 H2O 2MOH + H2(g)
with hydrogen: M(s) + H2(g) 2MH
With halogens: M(s) + X2(g) 2MX
with oxygen…
2Li(s) + O2(g) 2 Li2O(s)
Chemistry of alkali metals
For the other alkali metals the reaction with
O2 doesn’t go all the way to M2O:
2 Na(s) + O2(g) 2 Na2O2(s)
M + O2(g) 2 MO2(s), M = K, Rb, Cs
(What is the nature of oxygen in these products?)
Li is one of the few metals that reacts directly
with nitrogen at room temperature:
3 Li(s) + 1/2 N2(g) Li3N(s)
Other MI metals make no nitrides, MII all do.
Crown Ethers and Cryptands
These are macrocyclic ligands that can be designed
to selectively bind different alkali metal cations
(‘87 Nobel prize: Cram, Lehn, Pederson).
Dye used these to prepare alkalides and electrides.
Stability Constants for Alkali Metals Formed with
Cryptands of Various Cavity sizes in H2O
O
O z
O
O
Oy
O
N
x
N
Li
x y z
Na
K
Rb
Cs
(86 pm) (112 pm) (144 pm) (158 pm) (184 pm)
0 0 1
4.30
2.80
2.0
2.0
2.0
1 1 0
2.50
5.40
3.95
2.55
2.0
1 1 1
2.0
3.90
5.40
4.35
2.0
1 1 2
2.0
2.0
2.2
2.05
2.20
2 2 1
2.0
2.0
2.0
0.7
2.0
3 3 3
2.0
2.0
2.0
0.5
2.0
Liquid Ammonia solutions!
All the alkali metals and the three last three
alkaline earths form amazing solutions in
liquid ammonia. E.g.,
Na(s)
Na (soln)
Na+ + e– (solvated)
Electrons in dilute solutions behave like
“particles-in-boxes” formed by NH3
molecules. All these solutions have the same
deep blue color. Concentrated solutions have
a golden color and are electronic conductors.
Reaction
Summary
(C&W:
p. 301)
Na Reaction
Summary
(C&W:
p. 301)
K, Rb, Cs - Reaction Summary
(C&W: p. 302)