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C3-Group1-Alkali-Metals


Group 1 elements Alkali metals (IA)
Assigned reading materials Rayner-DIC p245-267; p156-158 Optional reading materials Rayner-DIC p253 lithium batteries; p265 11.15 biological aspects

ns1 Li Na K Rb Cs Fr

Generalization
alkali metals (ⅠA): ns1
电 离 能 、 电 负 性 减 小 金 属 性 、 还 原 性 增 强 原 子 半 径 增 大

alkaline earth metals (ⅡA): ns2

锂 钠 钾 铷 铯 钫

lithium sodium potassium rubidium cesium francium
金属性、还原性减弱


镁 钙 锶 钡 镭
电离能、电负性增大

beryllium
magnesium calcium strontium barium radium

原子半径减小

Properties of simple substances

Metallic luster, low density, low
hardness, low mp., good electric and thermal conductivity. Weaker metallic bond.

Occurrence
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Na: rock salt (NaCl) seawater sylvinite (KCl/NaCl) borax (Na2[B4O5(OH)4]· 8H2O) 硼砂 Chile saltpetre (NaNO3) 硝石 K: sylvite (KCl) seawater sylvinite (KCl/NaCl) carnallite (KCl· MgCl2· 6H2O) 光卤石 Li, Rb, Cs: various silicate minerals, e.g. spodumene (LiAlSi2O6) 锂辉石

Extraction of sodium – Downs process

Extraction of potassium
Na(l) + KCl(l) ? K(g) + NaCl(l)
? ? ?

At 850 ? C K b.p. 766 ? C Na b.p. 890 ? C

The industrial synthesis of NaOH

Typical reactions for alkali metals

Species of dioxygen
species Bond order
Dioxygenyl cation

ν(O-O)/cm-1

d(O-O)/?

radical

O 2+

2.5

1905

1.12

radical

O2
O 2O22-

oxygen
Superoxide dioxide(1-) Peroxide Dioxide(2-)

2
1.5 1

1580
1097 802

1.21
1.33 1.49

biradical
radical no

Molecular orbital energy level diagram of O22-

Why alkali metals form different oxides? – A thermodynamic viewpoint

M 2O 2 ? M 2O ? 1 O 2 2 M ? Li ,Na ? ,K ? ,Rb ? ,Cs ?
?

?rG ? ? ?r H ?r H
?

?

? T? r S ?
?

Assume ? r S ?(Li ) ? ? r S ?(Na ) ? ? r S ?(K ) ? ? r S ?(Rb ) ? ? r S ?(Cs ) ? ?lat H ?(M 2O 2 ) ? ?lat H ?(M 2O ) ? ?d H
?

?lat H ?

?

? 287.2?zc z A 0.345 (1 ? )kcal / mol rc ? rA rc ? rA

? 287.2?zc z A 1 ? rc ? rA rc ? rA 1 1 ? ) rc ? rA rc ? rA

?lat H ?(M 2O 2 ) ? ?lat H ?(M 2O ) ? f ( ? f( 1 1 ? ? rO 2 ? rM ? ? r
2

rM

)? f

rO

2?

? rO 2 ?
2

?

O 2?

(rM ? ? rO 2 ? )(rM ? ? r
2

O

) 2?

? 0

? ?lat H ?(Li2O 2 ) ? ?lat H ?(Li2O ) ? ?lat H ?(Cs 2O 2 ) ? ?lat H ?(Cs 2O )

MO 2 ?

1 2

M 2O 2 ? 1 O 2 2
?

M ? Na ? ,K ? ,Rb ? ,Cs ?
?rG ? ? ?r H ?r H
?

? T? r S ?
1 2

Assume ? r S ?(Na ) ? ? r S ?(K ) ? ? r S ?(Rb ) ? ? r S ?(Cs ) ? ?lat H ?(MO 2 ) ?
?

?lat H ?(M 2O ) ? ?d H

?

?lat H

?

? 287.2?zc z A ?zc z A ? rc ? rA rc ? rA
1 2

?lat H ?(MO 2 ) ?

?lat H ?(M 2O 2 ) ?

1 ?3?1? 2 2?1?1 ? f( ? 2 ) rM ? ? rO ? rM ? ? rO 2 ?
2 2

? rO ? ? rO
2

2

2?

?lat H ?(MO 2 ) ?

1 2

?lat H ?(M 2O 2 ) ?
1 2

?f ? 0 (rM ? ? rO ? )
2

? ?lat H ?(NaO 2 ) ?

?lat H ?(Na2O 2 ) ? ?lat H ?(CsO 2 ) ? ?lat H ?(Cs 2O 2 )

Why do alkali metals form different oxides? Li2O (LiO0.5) Na2O2 (NaO) KO2
?

?

?

The large alkali metal cations stabilize large low-charge anions. The less polarizing cations (those with low charge density) stabilize larger polarizable anions. Vice versa.

The atypical property of lithium
Li Na 0.9 496 -2.71 K 0.8 419 -2.93 Rb 0.8 403 -2.98 Cs 0.8 376 -3.03

χp
EI1 (kJ/mol)

1.0 520

E? (M+/M) -3.04 (V)

?

?

Why Li is the most powerful reducing metal among the alkali metals? Which one, Li or Na, reacts with water more vigorously? Why?

The atypical property of lithium
Smaller radius results in higher hydration energy Kinetic factors determine the chemical rates

Each Li+ has 25.3 hydrated water molecules; △Hhydration = -519 kJ· mol-1

M.p. (Li) = 180.54 ? C M.p. (Na) = 97.81 ? C (why) The melt metal has larger surface. ? LiOH is less soluble in water than NaOH. (why) (At 288K, 5.3 vs. 26.4 mol/L) The metal covered with hydroxide has smaller reaction surface.
?

Flame reaction 焰色反应





Li

Na

K

Rb

Cs

Ca

Sr

Ba





深红
670.8


589.2


766.5

红紫
780.0


455.5

橙红
714.9

深红
687.8

绿
553.5

波 长 / nm

See also: Rayner-DIC p248 Flame color

Interaction of alkali metals with ammonia

Solubility of alkali metal in ammonia (-35℃) 碱金属元素 M 溶解度/ (mol ·L-1) Li 15.7 Na 10.8 K 11.8 Rb 12.5 Cs 13.0

Solvated electrons
1. Has better electric conductivity than ionic compounds in water. 2. Less dense than liquid ammonia 3. Dilute solution is paramagnetic; at higher concentration, less paramagnetic. 2 e- = e224. Blue color (broad 1500 nm) is attributed to solvated e. M+ is colorless. 5. Excellent reducing agents. NH4+ + e- = NH3 + ? H2 Fe(CO)5 + 2e- = [Fe(CO)4]2- + CO

Salt solubility rule and thermodynamic explanation (Large-large and small-small)
? ? ?

?

ΔGdissol. =ΔHdissol. –TΔSdissol. ΔSdissol is of less significance for dissolution of a salt. ΔHdissol is the main factor to affect the dissolution. ΔHdissol = Ucrystal +ΔHhydration

?H dissol. ? f1 (

rM ?

1 1 1 ) ? f2 ( ) ? f3 ( ) ? rX ? rM ? rX ? 1 1 1 )?( )?( )] ? rX ? rM ? rX ?

assume f 2 ? f 3 ; f1 ? 5 f 2 ?H dissol. ? f 2 [5( rM ?

?H dissol. ? f 2 [5( 1. rM ? ? rX ? ? 2

rM ?

1 1 1 )?( ) ? ( )] ? rX ? rM ? rX ?

1 1 1 ?H dissol. ? f 2 [5 ? ? ? ] ? 0.25 f 2 2?2 2 2 2. rM ? ? rX ? ? 10 1 1 1 ?H dissol. ? f 2 [5 ? ? ? ] ? 0.05 f 2 10 ? 10 10 10 3. rM ? ? 2; rX ? ? 10 ?H dissol. 1 1 1 ? f 2 [5 ? ? ? ] ? ?0.18 f 2 2 ? 10 2 10

Conclusion: 1. rM+ ≈ rXU prevails 2. rM+ >> rXrM+ << rXΔHdissol dominates

离子型盐溶解性规律
相差易溶 ? 大对大,小对小,不易溶 ? 大对小,小对大,较易溶 ? Explain why LiOH is less souble than NaOH?
?

Macrocyclic complexes -- 1987 Noble Prize in Chemistry

C.Pedersen 美国化学家 首次报道“冠醚” (crown ether)

J.M.Lehn 法国生物化学家 首次报道“穴醚” (cryptant)

D.Cram 美国有机化学家

提出“主-客体化学”
(host-guest chemistry)

Crown ether

?

?

?

12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, and diaza-18-C-6 Great affinity to alkali metals Kassoc. Depends on the match between the radius of crown ether and the size of M+

In the presence of 18-crown-6 potassium permanganate dissolves in benzene – “purple benzene”

[18-C-6,K]+

Cryptand:
3-dimensional interior cavity; offer much better selectivity and strength of binding for alkali metals
Structure of [2.2.2]cryptand encapsulating a K+

Host-guest chemistry

[2,2,1]

[2,2,2]

Hard-Soft Acid-Base Concept (HSAB)
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?

?

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(1) Hardness (a) Species with large charge to radius ratios (b) Species hard to be polarized (2) Softness (a) Species with small charge to radius ratios (b) Species easy to be polarized (3) Hard acids (metal ions) tend to bind to hard bases (ligands) -- electrostatic. (4) Soft acids (metal ions) tend to bind to soft bases (ligands) -- covalent.

What factors contribute to stabilize [18-C-6,K]+?
?

Enthalpy consideration: hard acid (K+) prefers hard base (O) → favorable ΔH Entropy consideration: ΔS > 0 18C6 + [K(H2O)n]+ → [18C6,K]+ + nH2O Size matching

?

?

Exercises:
?

Umland-GC p881-888: 21.83, 21.147.

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Rayner-DIC p267: 11.1, 11.2, 11.6, 11.9, 11.11, 11.12, 11.16, 11.18, 11.24, 11.26, 11.30, 11.32


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