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AL Chemistry

AL Chemistry. Periodic Relationship among the Oxides, Chlorides & Hydrides of the elements Li to Cl. C. Y. Yeung p. 01. Gp IV. Gp I. Gp III. Gp II. Be. B. Li. Na. Si. Mg. Al. same no. of outermost e - ,  similar chemical properties. ions have similar charge density,

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AL Chemistry

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

  2. Gp IV Gp I Gp III Gp II Be B Li Na Si Mg Al same no. of outermost e-,  similar chemical properties ions have similar charge density,  similar chemical properties Diagonal Relationship  An Overview …….  Diagonal relationship does not apply to non-metals! C. Y. Yeung p. 02

  3. Li Be C B N O F Na Si Cl Mg P S Al (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. acts as an acid acts as a base reacts with OH- reacts with limiting amount of acid 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)4]2- and [Al(OH)4]- Al(OH)3 C. Y. Yeung p. 04

  5. ( )2 + ( )3 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. MAINLY ACIDIC OXIDES [covalent] Li Be C B N O F Na Si Cl Mg P S Al insoluble in water soluble in strong base! (NaOH) 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. Li Be C B N O F Na Si Cl Mg P S Al Non-metal Oxides simple molecular structure P4O10 acidic absorb water vigorously! P4O10(s) + 6H2O(l) 4H3PO4(aq) C. Y. Yeung p. 07

  8. covalent ionic 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) (B) Periodicity in Chemical Properties of Chlorides Group I Group VII C. Y. Yeung p. 08

  9. Be2+ + 2Cl- BeCl2 small size with high +ve charges OH2 H2O H H OH2 H2O + O O Be2+ Be2+ H H H2O H2O Be+ OH2 OH2 H2O H2O Finally, [Be(H2O)3OH]+(aq) + H3O+ (aq) Example 1 [Be(H2O)4]2+(aq) OH2 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. B(OH)3 BCl3 + 3H2O + 3HCl Cl Cl Cl - Cl B B B H H Cl Cl OH Cl Cl O O + O H H H H + Cl- + H3O+ OH B OH HO + 3 HCl Example 2 [H3BO3] electron - deficient ! d+ d- C. Y. Yeung p. 11

  12. P(OH)3 PCl3 + 3H2O + 3HCl Cl Cl Cl - d+ Cl P P P H H Cl Cl OH Cl Cl O O + O H H H H + Cl- + H3O+ d- OH Wrong !! P OH HO + 3 HCl Example 3 [H3PO3] C. Y. Yeung p. 12

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

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

  15. NH3 NCl3 + 3H2O + 3HOCl Cl H H N N N Cl H Cl Cl Cl H O H H similar electronegativity ! Try to explain …. + HO—Cl + 3 HO—Cl C. Y. Yeung p. 15

  16.  hydrolytic rate: 3rd period XCln > 2nd period XCln 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. HF LiH NaH HCl Periodic table 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. HF BeH2 NH3 LiH B2H6 CH4 H2O Periodic table 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 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. F H + H O H H hydrogen bond 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. C. Y. Yeung p. 25

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