al chemistry c. y. yeung p. 01 periodic relationship among the oxides, chlorides & hydrides of...
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AL Chemistry
C. Y. Yeung p. 01
Periodic Relationship amongPeriodic Relationship amongthe Oxides, Chlorides & Hydridesthe Oxides, Chlorides & Hydrides
of the elements Li to Clof the elements Li to Cl
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C. Y. Yeung p. 02
An Overview …….
Li
Na
Be
Mg
B
Al Si
Gp I Gp II Gp III Gp IV
same no. of outermost e-, similar chemical properties
ions have similar charge density, similar chemical properties
DiagonalRelationship
Diagonal relationship does not apply to non-metals!
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(A) Periodicity in Chemical Properties of Oxides
BASIC OXIDESBASIC OXIDES[ionic][ionic]
ref. p.21 Behaviour of Oxides in Water.
Li
Na
Be
Mg
B
Al
C
Si
N
P
O
S
F
ClMAINLYMAINLY
ACIDIC OXIDESACIDIC OXIDES[covalent][covalent]AMPHOTERIC OXIDES
[ionic with covalent character]
C. Y. Yeung p. 03
react with both acids and bases
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BeO, Al2O3
AMPHOTERIC OXIDES[ionic with covalent character]
(insoluble in water)(no reaction with water)
dissolves in acid,to give Be2+ and Al3+
dissolves in base,to give [Be(OH)4]2-
and [Al(OH)4]-
reacts withlimiting amount
of acidAl(OH)3
reacts withOH-
C. Y. Yeung p. 04
acts as a base acts as an acid
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C. Y. Yeung p. 05
1993 P1 Q.2
Al reacts with excess NaOH with effervescence,forming solution which gives a white precipitationon addition of dilute HCl ……1. A redox reaction between Al and H2O!
Al is oxidized to Al(OH)4-, H2O is reduced to H2.
Al + 4OH- Al(OH)4- + 3e-
2H2O + 2e- H2 + 2OH-+ ( )3( )2
2Al + 2OH- + 6H2O 2Al(OH)4- + 3H2
2. Partial neutralization of Al(OH)4-
Al(OH)4- + H+ Al(OH)3 + H2O
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C. Y. Yeung p. 06
Li
Na
Be
Mg
B
Al
C
Si
N
P
O
S
F
Cl
Non-metal Oxides
MAINLYMAINLYACIDIC OXIDESACIDIC OXIDES
[covalent][covalent]
except CO, N2O, NO and O2
[neutral]
SiOSiO22
acidic
giant covalent structure
insoluble in water
soluble in strong base!(NaOH)
SiO2(s) + 2NaOH(aq) Na2SiO3(aq) + H2O(l)[sodium silicate (IV)]
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C. Y. Yeung p. 07
Non-metal Oxides
Li
Na
Be
Mg
B
Al
C
Si
N
P
O
S
F
Cl
PP44OO1010acidic
simple molecular structure
absorb water vigorously!
P4O10(s) + 6H2O(l) 4H3PO4(aq)
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Group IGroup I Group VIIGroup VIIionic covalent
neutralchlorides
acidicchlorides
slightly acidicchlorides
Acidity
related to the extent of hydrolysis …
More hydrolysis, more acidic
depends on …
small cation with high +ve charges, OR
molecules with polar bond(s)
(B) Periodicity in Chemical Properties of Chlorides
C. Y. Yeung p. 08
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Example 1Example 1BeCl2 Be2+ + 2Cl-
Be2+
H2O
H2O
OH2
OH2
small size withhigh +ve charges
Be2+
O
H
HH2O
H2O OH2
OH2
O
H
HBe+
H2O
H2O OH2
+
Finally, [Be(H2O)3OH]+(aq) + H3O+
(aq)
[Be(H2O)4]2+(aq)
C. Y. Yeung p. 09
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SimilarlySimilarly
[Mg(H2O)4]2+(aq) [Mg(H2O)3OH]+
(aq) + H3O+ (aq)
MgCl2(s) + 4H2O(l) [Mg(H2O)4]2+(aq) + 2Cl-
(aq)
[Al(H2O)6]3+(aq) [Al(H2O)5OH]2+
(aq) + H3O+ (aq)
AlCl3(s) + 6H2O(l) [Al(H2O)6]3+(aq) + 3Cl-
(aq)
C. Y. Yeung p. 10
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Example 2Example 2
BCl3 + 3H2O B(OH)3 + 3HCl
B
Cl
Cl Cl
+
O
H
H-
electron- deficient !
O
H
H
B
Cl
Cl
Cl
O
HH
+
-B
Cl
Cl OH
+ Cl-+ H3O+
BOH
OH
HO
+ 3 HCl
[H3BO3]
C. Y. Yeung p. 11
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Example 3Example 3
PCl3 + 3H2O P(OH)3 + 3HCl
P
Cl
Cl Cl
+
O
H
H-
O
H
H
P
Cl
Cl
Cl
O
HH
+
-P
Cl
Cl OH
+ Cl-+ H3O+
POH
OH
HO
+ 3 HCl
[H3PO3]
Wrong !!
C. Y. Yeung p. 12
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O
H
H
O
H
H
+ Cl-+ H3O+
+ 3 HCl
PCl3 + 3H2O P(OH)3 + 3HCl
[H3PO3]
PCl
Cl
+Cl
O
HH
+
-PCl
Cl
ClP
ClCl
OH
PO
O
OH
H
H
PO
O
O
H
H
H
extended octet!
[H3PO3]
C. Y. Yeung p. 13
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Try to explain ….Try to explain ….
PCl5 + 4H2O H3PO4 + 5HCl
OHH
PClCl
ClCl
Cl
PCl
ClO
Cl
Cl
H
PCl
ClO
Cl
Cl
H
+
PO
O
O
H
H
HO
[H3PO4]
Cl
H
+
C. Y. Yeung p. 14
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Try to explain ….Try to explain ….
NCl3 + 3H2O NH3 + 3HOCl
NCl
Cl
Cl
similarelectronegativity !
OH
H
NCl
Cl
H+ HO—Cl
NH
H
H+ 3 HO—Cl
C. Y. Yeung p. 15
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Rate of Chloride (XClRate of Chloride (XClnn) Hydrolysis …?) Hydrolysis …?
► if low lying vacant d-orbitals of X is available,
► X forms more bonds with incoming H2O molecules
► lower Activation Energy
► higher reaction rate !
hydrolytic rate: 3rd period XCln > 2nd period XCln
C. Y. Yeung p. 16
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Hydrides (XHn)
► ionic hydrides (Gp I – III) : H- (hydride anion)
It is a reducing agent ! 2H- H2 + 2 e-
reducing power
reducing
morereducing
Explained by“difference in electronegativities” bet
ween X and H
LiH HFNaH HCl
Periodic table
C. Y. Yeung p. 17
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Example 1: NaH is a stronger R.A. than LiH.
Reason: The electronegativity difference between Na and H is larger than that between Li and H. more ionic character H- anions are formed more readily more reducing
Example 2: NaH is a stronger R.A. than MgH2.
Reason: (electronegativity) between Mg and H is smaller more covalent character less H- anions are formed less reducing
C. Y. Yeung p. 18
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Acid-base Properties of XHn
Gp V hydrides -- basic ~ due to the lone pair of e-
LiH and BeH2 are basic !H- + H+ H2 H- + H2O H2 + OH-
LiH HFH2O
Periodic table
NH3CH4B2H6BeH2
basic
neutral
basic
neutral
acidic
basicity
basic
morebasic
acidic
moreacidic
Gp VI, VII hydrides -- acidic ~ due to the nucleophilic attacked of OH- or H2O on the + H .
C. Y. Yeung p. 19
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Hydrolytic Reactions of XHn
Example 1: Hydrolysis of Gp I & II hydrides
NaH + H2O NaOH + H2
MgH2 + 2H2O Mg(OH)2 + 2H2
Example 2: Hydrolysis of Gp IV hydrides (**)
CH4 + H2O no reaction !
SiH4 + 2H2O SiO22H2O + 2H2
WHY ???C. Y. Yeung p. 20
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Explain the difference in reactivity with water between CH4 and SiH4.
(1995 P1, Q.2)
C. Y. Yeung p. 21
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C. Y. Yeung p. 22
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Explain the difference in reactivity with water between SiH4 and H2S.
In H2S, the polarity is H+—S-.Therefore nucleophilic attack of H2O on H2SGives H3O+ and HS-.
Whereas SiH4 gives an alkaline solution since the polarity is Si+—H-.
C. Y. Yeung p. 23
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Compare the basicity of NH3 and PH3.Explain your answer.
NH3 is more basic.
The lone pair e- of N is a sp3 hybrid orbital of 2s and 2p orbitals.
The lone pair e- of P is a sp3 hybrid orbital of 3s and 3p orbitals.
The former is less diffused than the latter one.
The lone pair of NH3 is a better electron-donor than that of PH3.
N
H
H
H
P
H
H
H
In fact, NH3 hydrolysed in water, but PH3 is insoluble and has no reaction with water at all!
C. Y. Yeung p. 24
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Final encounter …..
Due to the strong H—F bond, which does not favour dissociation of the bond.
Exceptionally low acidity of HF … ?
Due to the formation of strong H-bond between HF and H3O+. This lowers the free [H+] in the solution and thus lowers the acidity.
OH
H
H+H F
hydrogen bond
C. Y. Yeung p. 25