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AL Chemistry
Periodic Relationship among
the Oxides, Chlorides & Hydrides
of the elements Li to Cl
C. Y. Yeung p. 01
An Overview …….
Diagonal
Relationship
Gp I
Gp II
Gp III
Li
Be
B
Na
Mg
Al
Gp IV
Si
same no. of outermost e-,
similar chemical properties
ions have similar charge density,
similar chemical properties
Diagonal relationship does not apply to non-metals!
C. Y. Yeung p. 02
(A) Periodicity in Chemical Properties of Oxides
Li Be B C N O F
Na Mg Al Si P S Cl
BASIC OXIDES
[ionic]
AMPHOTERIC OXIDES
[ionic with covalent character]
MAINLY
ACIDIC OXIDES
[covalent]
react with both acids and bases
ref. p.21 Behaviour of Oxides in Water.
C. Y. Yeung p. 03
AMPHOTERIC OXIDES
[ionic with covalent character]
BeO, Al2O3
(insoluble in water)
(no reaction with water)
acts as a base
acts as an acid
dissolves in acid,
to give Be2+ and Al3+
dissolves in base,
to give [Be(OH)4]2and [Al(OH)4]-
reacts with
OH-
Al(OH)3
reacts with
limiting amount
of acid
C. Y. Yeung p. 04
1993 P1 Q.2
Al reacts with excess NaOH with effervescence,
forming solution which gives a white precipitation
on 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- )2
+ ( 2H2O + 2e- H2 + 2OH)3
2Al + 2OH- + 6H2O 2Al(OH)4- + 3H2
2. Partial neutralization of Al(OH)4Al(OH)4- + H+ Al(OH)3 + H2O
C. Y. Yeung p. 05
Non-metal Oxides
Liacidic
Be B C N O F
Na Mg Al Si P S Cl
SiO
2
giant covalent
structure
MAINLY
ACIDIC OXIDES
[covalent]
except CO, N2O, NO and O2
[neutral]
insoluble in water
soluble in strong base!
(NaOH)
SiO2(s) + 2NaOH(aq) Na2SiO3(aq) + H2O(l)
[sodium silicate (IV)]
C. Y. Yeung p. 06
Non-metal Oxides
Li Be B C N O F
Na Mg Al Si P S Cl
simple
molecular
structure
P4O10
acidic
absorb water vigorously!
P4O10(s) + 6H2O(l) 4H3PO4(aq)
C. Y. Yeung p. 07
(B) Periodicity in Chemical Properties of Chlorides
Group I
Group VII
ionic
covalent
neutral
chlorides
slightly acidic
chlorides
acidic
chlorides
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)
C. Y. Yeung p. 08
Example 1
Be2+ +
BeCl
2Cl-
2
OH2
H 2O
Be2+
H 2O
OH2
small size with
high +ve charges
[Be(H2O)4]2+(aq)
H
H
H 2O
O
Be2+
H 2O
Finally,
OH2
H
H 2O
H 2O
Be+
OH2
O+ H
OH2
[Be(H2O)3OH]+(aq) + H3O+ (aq)
C. Y. Yeung p. 09
Similarly
MgCl2(s) + 4H2O(l) [Mg(H2O)4]2+(aq) + 2Cl- (aq)
[Mg(H2O)4]2+(aq) [Mg(H2O)3OH]+(aq) + H3O+ (aq)
AlCl3(s) + 6H2O(l) [Al(H2O)6]3+(aq) + 3Cl- (aq)
[Al(H2O)6]3+(aq) [Al(H2O)5OH]2+(aq) + H3O+ (aq)
C. Y. Yeung p. 10
Example 2
[H3BO3]
B(OH)3 +
BCl3 + 3H2O
electron
- deficient !
Cl
Cl
d+
B
B
Cl
Cl
Cl
Cl
B
Cl
Cl
+
d-
3HCl
OH
+ H3O+ + Cl-
OH
B
HO
OH
+ 3 HCl
C. Y. Yeung p. 11
Example 3
[H3PO3]
P(OH)3
PCl3 + 3H2O
Cl
d+
Cl
P
Cl
Cl
P
Cl
Cl
+
d-
3HCl
Cl
Cl
P
+
OH
+ H3O+ + Cl-
OH
P
HO
OH
+ 3 HCl
C. Y. Yeung p. 12
[H3PO3]
P(OH)3
PCl3 + 3H2O
+
3HCl
extended octet!
P
Cl
d+
Cl
Cl
Cl
P
P
Cl
Cl
Cl
+
Cl
OH
+ H3O + Cl+
H
H
[H3PO3]
O
P
H
O
O H
P
H
O
O
O H
+ 3 HCl
C. Y. Yeung p. 13
Try to explain ….
H3PO4
Cl
Cl
P
5HCl
Cl
Cl
O
P
Cl
O
P
Cl
Cl
+
Cl +
Cl
Cl
Cl
H
Cl
H
Cl
+
H
PCl5 + 4H2O
Cl
H
[H3PO4]
O
O
P
H
O
O H
C. Y. Yeung p. 14
Try to explain ….
NCl3 + 3H2O
Cl
+
Cl
3HOCl
H
N
Cl
N
Cl
NH3
+ HO—Cl
Cl
similar
electronegativity !
H
N
H
+ 3 HO—Cl
H
C. Y. Yeung p. 15
Rate of Chloride (XCln) 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
Hydrides (XHn)
► ionic hydrides (Gp I – III) :
H- (hydride anion)
It is a reducing agent !
2H- H2 + 2 e-
reducing power
reducing
LiH
NaH
more
reducing
HF
HCl
Periodic table
Explained by
“difference in electronegativities”
between X and H
C. Y. Yeung p. 17
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
Acid-base Properties of XHn
basicity
neutral
neutral
acidic
basic
LiH BeH2 B2H6 CH4 NH3 H2O HF
basic
more
basic
LiH and BeH2
are basic !
H- + H+ H2
H- + H2O H2 + OH-
basic
acidic
Periodic table
more
acidic
Gp V hydrides -- basic
~ due to the lone pair of e-
Gp VI, VII hydrides -- acidic
~ due to the nucleophilic
attacked of OH- or H2O on
the d+ H .
C. Y. Yeung p. 19
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
Explain the difference in reactivity with
water between CH4 and SiH4.
(1995 P1, Q.2)
C. Y. Yeung p. 21
C. Y. Yeung p. 22
Explain the difference in reactivity with
water between SiH4 and H2S.
In H2S, the polarity is Hd+—Sd-.
Therefore nucleophilic attack of H2O on H2S
Gives H3O+ and HS-.
Whereas SiH4 gives an alkaline solution
since the polarity is Sid+—Hd-.
C. Y. Yeung p. 23
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.
N
H
H
H
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.
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
Final encounter …..
Exceptionally low acidity of HF … ?
Due to the strong H—F bond, which does
not favour dissociation of the bond.
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.
H
+
O
F
H
H
H
hydrogen bond
C. Y. Yeung p. 25