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

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

[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

[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

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

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

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

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

[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

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] 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

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

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

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

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

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

“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-  H2 + 2 e- more reducing Explained by “difference in electronegativities” between X and H C. Y. Yeung p. 17

 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

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

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

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. 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

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