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

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!

(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

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

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

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)]

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)

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

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

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

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

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

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

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

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

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

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

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

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

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 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

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

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