1. Periodic Relationship among the Oxides, Chlorides & Hydrides
AL Chemistry
Periodic Relationship among
the Oxides, Chlorides & Hydrides
of the elements Li to Cl
C. Y. Yeung p. 01

2. Gp I Gp II Gp III Gp IV Li Na Be Mg B Al Si Diagonal  An Overview …….
Relationship
 An Overview …….
Gp I
Gp II
Gp III
Gp IV Li Na Be Mg B Al 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

3. [ionic with covalent character]  react with both acids and bases 
(A) Periodicity in Chemical Properties of Oxides Li Na Be Mg B Al C Si N P O S F Cl
BASIC OXIDES
[ionic]
MAINLY
ACIDIC OXIDES
[covalent]
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. [ionic with covalent character] (no reaction with water)
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]2-
and [Al(OH)4]-
reacts with
OH-
reacts with
limiting amount
of acid
Al(OH)3
C. Y. Yeung p. 04

5. Al(OH)4- + H+  Al(OH)3 + H2O
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.
+ ( )3
( )2
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. SiO2 Li Na Be Mg B Al C Si N P O S F Cl
Non-metal Oxides Li Na Be Mg B Al C Si N P O S F Cl
MAINLY
ACIDIC OXIDES
[covalent]
acidic
SiO2
giant covalent structure
except CO, N2O, NO and O2
[neutral]
soluble in strong base!
(NaOH)
insoluble in water
SiO2(s) + 2NaOH(aq)  Na2SiO3(aq) + H2O(l)
[sodium silicate (IV)]
C. Y. Yeung p. 06

7. simple molecular structure absorb water vigorously!
Non-metal Oxides Li Na Be Mg B Al C Si N P O S F Cl
simple molecular structure
P4O10
acidic
absorb water vigorously!
P4O10(s) + 6H2O(l)  4H3PO4(aq)
C. Y. Yeung p. 07

8. Group I Group VII (B) Periodicity in Chemical Properties of Chlorides
ionic
covalent
neutral
chlorides
acidic
slightly acidic
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

9. [Be(H2O)3OH]+(aq) + H3O+ (aq)
Example 1
BeCl2
Be2+ +
2Cl-
H2O
OH2
Be2+
small size with
high +ve charges
[Be(H2O)4]2+(aq)
Be2+ O H
H2O
OH2 O H
Be+
H2O
OH2 +
OH2
Finally,
[Be(H2O)3OH]+(aq) + H3O+ (aq)
C. Y. Yeung p. 09

10. 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

11. Example 2 [H3BO3] BCl3 + 3H2O B(OH)3 + 3HCl B Cl B Cl O H + - B Cl OH
electron
- deficient ! B Cl B Cl O H + - B Cl OH d+ O H +
Cl-
H3O+ d- O H B OH HO +
3 HCl
C. Y. Yeung p. 11

12. Wrong !! Example 3 [H3PO3] PCl3 + 3H2O P(OH)3 + 3HCl P Cl d+ O H d- +
C. Y. Yeung p. 12

13. PCl3 + 3H2O P(OH)3 + 3HCl [H3PO3] O H + - P Cl P Cl d+ P Cl OH O H +
extended octet! O H + - P Cl P Cl d+ P Cl OH O H +
Cl-
H3O+ O H P O H P O H
[H3PO3] +
3 HCl
C. Y. Yeung p. 13

14. Try to explain …. PCl5 + 4H2O H3PO4 + 5HCl P Cl P Cl O H P Cl O Cl H +
C. Y. Yeung p. 14

15. Try to explain …. NCl3 + 3H2O NH3 + 3HOCl O H N Cl H N Cl + HO—Cl N H
similar
electronegativity ! N H
+ 3 HO—Cl
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 !
 hydrolytic rate:
3rd period XCln > 2nd period XCln
C. Y. Yeung p. 16

17. “difference in electronegativities” between X and H
Hydrides (XHn)
reducing power 
LiH HF
NaH
HCl
Periodic table
reducing
► ionic hydrides (Gp I – III) :
H- (hydride anion)
 It is a reducing agent !
2H-  H e-
more
reducing
Explained by
“difference in electronegativities” between X and H
C. Y. Yeung p. 17

18.  H- anions are formed more readily  more reducing
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
basic
acidic
LiH HF
H2O
Periodic table
NH3
CH4
B2H6
BeH2
basic
basic
acidic
more
basic
more
acidic
 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. WHY ??? 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 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

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. Exceptionally low acidity of HF … ?
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. O H + F
hydrogen bond
C. Y. Yeung p. 25