C. Y. Yeung p. 01

1
Periodic Relationship among the Oxides, Chlorides
Hydrides of the elements Li to Cl
C. Y. Yeung p. 01
2
Diagonal Relationship
? An Overview .
? Diagonal relationship does not apply to
non-metals!
C. Y. Yeung p. 02
3
(A) Periodicity in Chemical Properties of Oxides
MAINLY ACIDIC OXIDES covalent
BASIC OXIDES ionic
AMPHOTERIC OXIDES ionic with covalent character
? react with both acids and bases ?
? ref. p.21 Behaviour of Oxides in Water.
C. Y. Yeung p. 03
4
AMPHOTERIC OXIDES ionic with covalent character
BeO, Al2O3
(insoluble in water) (no reaction with water)
dissolves in acid, to give Be2 and Al3
dissolves in base, to give Be(OH)42- and
Al(OH)4-
Al(OH)3
C. Y. Yeung p. 04
5
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-
2H2O 2e- ? H2 2OH-
2Al 2OH- 6H2O ? 2Al(OH)4- 3H2
2. Partial neutralization of Al(OH)4-
Al(OH)4- H ? Al(OH)3 H2O
C. Y. Yeung p. 05
6
Non-metal Oxides
acidic
SiO2
giant covalent structure
except CO, N2O, NO and O2 neutral
SiO2(s) 2NaOH(aq) ? Na2SiO3(aq) H2O(l)
sodium silicate (IV)
C. Y. Yeung p. 06
7
Non-metal Oxides
simple molecular structure
P4O10
acidic
absorb water vigorously!
P4O10(s) 6H2O(l) ? 4H3PO4(aq)
C. Y. Yeung p. 07
8
(B) Periodicity in Chemical Properties of
Chlorides
Group I
Group VII
C. Y. Yeung p. 08
9
Example 1
Be(H2O)42(aq)
OH2
C. Y. Yeung p. 09
10
Similarly
MgCl2(s) 4H2O(l) ? Mg(H2O)42(aq)
2Cl- (aq)
Mg(H2O)42(aq) ? Mg(H2O)3OH(aq) H3O
(aq)
AlCl3(s) 6H2O(l) ? Al(H2O)63(aq)
3Cl- (aq)
Al(H2O)63(aq) ? Al(H2O)5OH2(aq) H3O
(aq)
C. Y. Yeung p. 10
11
Example 2
H3BO3
electron - deficient !
d
d-
C. Y. Yeung p. 11
12
Example 3
H3PO3
C. Y. Yeung p. 12
13
extended octet!
H3PO3
C. Y. Yeung p. 13
14
Try to explain .
H3PO4
C. Y. Yeung p. 14
15
Try to explain .
HOCl
3 HOCl
C. Y. Yeung p. 15
16
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 !
C. Y. Yeung p. 16
17
Hydrides (XHn)
reducing power ?
reducing
? ionic hydrides (Gp I III) H-
(hydride anion)
? It is a reducing agent ! 2H- ? H2 2 e-
more reducing
Explained by difference in electronegativities
between X and H
C. Y. Yeung p. 17
18
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
19
Acid-base Properties of XHn
basicity ?
neutral
neutral
acidic
basic
basic
basic
acidic
more acidic
more basic
? Gp V hydrides -- basic due to the
lone pair of e-
? LiH and BeH2 are basic ! H- H ? H2 H-
H2O ? H2 OH-
? Gp VI, VII hydrides -- acidic due to
the nucleophilic attacked of OH- or H2O
on the d H .
C. Y. Yeung p. 19
20
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 ? SiO2?2H2O 2H2
WHY ???
C. Y. Yeung p. 20
21
Explain the difference in reactivity with water
between CH4 and SiH4. (1995 P1, Q.2)
C. Y. Yeung p. 21
22
C. Y. Yeung p. 22
23
Explain the difference in reactivity with water
between SiH4 and H2S.
In H2S, the polarity is HdSd-. Therefore
nucleophilic attack of H2O on H2S Gives H3O and
HS-.
Whereas SiH4 gives an alkaline solution since the
polarity is SidHd-.
C. Y. Yeung p. 23
24
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.
In fact, NH3 hydrolysed in water, but PH3 is
insoluble and has no reaction with water at all!
C. Y. Yeung p. 24
25
Final encounter ..
Exceptionally low acidity of HF ?
Due to the strong HF 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.
C. Y. Yeung p. 25