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Variation in Chemical Properties of s-block elements

Variation in Chemical Properties of s-block elements. Diagonal relationship. Diagonal relationship. Trends in Reactivity of s-block elements. Increases ∵ Ionization enthalpy decreases, the outer electrons are less strongly held. Decreases

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Variation in Chemical Properties of s-block elements

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  1. Variation in Chemical Properties of s-block elements

  2. Diagonal relationship

  3. Diagonal relationship

  4. Trends in Reactivity of s-block elements Increases ∵ Ionization enthalpy decreases, the outer electrons are less strongly held Decreases ∵ Sum of 1st and 2nd IE of Group II elements > 1st IE of Group I elements

  5. Reaction with hydrogen • All s-block elements (except Be) react directly with hydrogen, forming ionic hydrides (except MgH2 which is covalent). • For group I elements: 2M(s) + H2(g)  2MH(s) • For group II elements: M(s) + H2(g)  MH2(s)

  6. All hydrides of s-block elements are ionic (except BeH2 and MgH2, which are covalent) • H- ions exist in ionic hydrides. • When molten ionic hydrides are electrolyzed, hydrogen gas is liberated at the anode. • This differs from general electrolysis, at which hydrogen gas is liberated at the cathode.

  7. Reaction with oxygen • All s-block elements react directly with oxygen, forming ionic monoxides (O2-), peroxides(O22-), and even superoxides(O2-).

  8. What oxide(s) to form?

  9. Different types of oxides are formed. Why? • The thermal stabilities of monoxides, peroxides, or superoxides depend on the degree of polarization of the anion by the cation. • Polarizabilities: O2- > O22- > O2- • peroxides and superoxides can only be form with cations which are less polarizing.

  10. Main types of oxides of gp I and gp II metals:

  11. Equations • For Group I elements: 4M(s) + O2(g)  2M2O(s) (monoxide) 2M(s) + O2(g)  M2O2(s) (peroxide) M(s) + O2(g)  MO2(s) (superoxide) • For Group II elements: 2M(s) + O2(g)  MO(s) (monoxide) M(s) + O2(g)  MO2(s) (peroxide) (superoxides are unstable to form)

  12. Reaction with chlorine • All s-block elements react directly with chlorine, forming ionic chlorides. (except BeCl2 and MgCl2 which are covalent). • For group I elements: 2M(s) + Cl2(g)  2MCl(s) • For group II elements: M(s) + Cl2(g)  MCl2(s)

  13. Reaction with water Reactivity increases down the group Reactivity decreases from GpI to Gp II

  14. Reaction with water • s-block elements react with water to form hydroxide/oxide and hydrogen gas • For very reactive elements (Cs, Rb, K, Na, Ba, Sr, Ca, Li) they react with water to form hydroxide and hydrogen. M(s) + H2O(l)  MOH(aq) + ½H2(g) M(s) + 2H2O(l)  M(OH)2(aq) + H2(g)

  15. Reaction of Mg with water • Mg does not / reacts extremely slowly with cold water. • Mg reacts readily hot water, forming Mg(OH)2(s) and hydrogen. Mg(s) + 2H2O(l)  Mg(OH)2(s) + H2(g) • Mg reacts readily with steam, forming MgO(s) and hdyrogen. Mg(s) + H2O(g)  MgO(s) + H2(g)

  16. Magnesium in hot water

  17. Mg reacts with steam Why is here a burning flame?

  18. Reaction of hydrides with water • Hydrogen gas and OH- will be formed. • For hydrides of Group I elements: MH (s) + H2O (l)  MOH (aq) + H2 (g) • For hydrides of Group II elements: MH2 (s) + 2H2O (l)  M(OH)2(aq) +2H2(g) (Note: some M(OH)2 is insoluble)

  19. Reaction of oxides ofGroup I elements with water • React exothermically with water to form soluble hydroxides (alkalis) M2O(s) + H2O(l)  2MOH (aq) • They react even more exothermically with dilute acids (neutralization) M2O(s) + 2H+(aq)  2M+(aq) + H2O(l)

  20. Reaction of oxides ofGroup II elements with water

  21. Reaction of chlorides with water • All chlorides of s-block elements are ionic (except BeCl2 which is covalent). • All chlorides of s-block elements dissolve in water, without hydrolysis, giving a neutral solution (except BeCl2). MCl (s) + H2O (l)  M+(aq) + Cl-(aq) MCl2(s) + H2O (l)  M2+(aq) + 2Cl-(aq)

  22. Thermal Stabilities of salts • When a salt is thermally stable at a high temperature, it means it is stable even when it is strongly heated. • If a salt is thermally unstable at room temperature, it does not exit even at room temperature. • Some salts are thermally stable at room temperature, but decompose when heated.

  23. Thermal stabilities of metal oxides

  24. Thermal stabilities of ionic compounds are affected by 1) Charge of ions: The higher the charge of the ions, the stronger is the attraction between the ions, the more stable the ionic compound (for small anions only). 2) Size of the ions : The smaller the ions, the closer is the distance between the ions, the more stable is the compound.

  25. Thermal stabilities of ionic compounds are affected by • The extent of distortion of the electron cloud of the anion (for large polarizable anion) by the neighouring cation. • The greater the distortion, the less stable it is.

  26. Decomposition of CO32- , OH-and NO3- • CO32-(s)  O2-(s) + CO2(g) • 2OH-(s)  O2-(s) + H2O(g) • 2NO3-(s)  2NO2-(s) + O2(g) • or • 4NO3-(s)  2O2-(s) + 4NO2(g) + O2(g)

  27. Decomposition of CO32-

  28. Decomposition of CO32- Conclusion: Thermal stability of carbonate increases down the group (for both Gp I and II)

  29. Decomposition of OH- Conclusion: Thermal stability of hydroxides increases down the group (for both Gp I and II)

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