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40 The s-Block Elements 40.1 Characteristic Properties of the s-Block Elements 40.2 Variation in Properties of the s-Block Elements Variation in Properties of the Compounds of the s-Block Elements 40.3 1 New Way Chemistry for Hong Kong A-Level Book 4 1 The Syllabus • 8.1 Characteristic Properties • • • • • • 2 Metallic character Low electronegativity Formation of basic oxides and hydroxides Fixed Oxidation state in their compounds Weak tendency to form complexes Flame colours of salts – flame test New Way Chemistry for Hong Kong A-Level Book 4 2 The Syllabus • 8.2 Variation in properties of the s-block elements and their compounds Variations in atomic radii, ionisation enthalpies, hydration enthalpies and melting points. Interpretation of these variations in terms of structure and bonding. Reactions of the elements with oxygen and water. Reactions of the oxides with water, dilute acids and dilute alkalis. Relative thermal stability of the carbonates and hydroxides. Relative solubility of the sulphates(VI) and hydroxides 3 New Way Chemistry for Hong Kong A-Level Book 4 3 Notes p. 1 s-Block elements: • Consists of Group IA and Group IIA elements • Outermost electron shell: ns1 ns2 • Highly reactive metals • Good reducing agents • Fixed oxidation states +1 for Group I elements +2 for Group II elements 4 New Way Chemistry for Hong Kong A-Level Book 4 4 40.1 Characteristic Properties of the s-Block Elements 5 New Way Chemistry for Hong Kong A-Level Book 4 5 40.1 Characteristic Properties of the s-Block Elements (SB p.38) Metallic Character (not mentioned in notes) Group I elements: • Silvery in colour, tarnish rapidly in air ∴ keep immersed under paraffin oil or in vacuum sealed tubes • Soft, low boiling and melting points ∵ weak metallic bond due to only 1 e– is contributed to form bonds • Low density ∵ body-centred cubic structure -- have more spaces Cutting Rubidium 6 New Way Chemistry for Hong Kong A-Level Book 4 6 Group I elements: Lithium Rubidium 7 Sodium Potassium Caesium New Way Chemistry for Hong Kong A-Level Book 4 7 40.1 Characteristic Properties of the s-Block Elements (SB p.39) Some information about Group I elements Atomic Ionic Group radius radius I metal (nm) (nm) Li Na K Rb Cs Fr 0.152 0.186 0.231 0.244 0.262 0.270 0.060 0.095 0.133 0.148 0.169 0.176 Crystal structure b b b b b — Melting Boiling point point (C) (C) 180.5 97.8 63.7 39.1 28.4 27 1330 890 774 688 690 680 Density Abundance (g cm–3) on earth (%) 0.53 0.97 0.86 1.53 1.87 — 0.0020 2.36 2.09 0.009 0 0.000 10 Trace “b” denotes body-centred cubic structure 8 New Way Chemistry for Hong Kong A-Level Book 4 8 40.1 Characteristic Properties of the s-Block Elements (SB p.39) Group II elements: • silvery in colour • harder and higher boiling and melting points than Group I counterparts ∵ stronger metallic bond due to 2e– are contributed to form bond and smaller atomic sizes • show different crystal structures 9 New Way Chemistry for Hong Kong A-Level Book 4 9 Calcium Group II elements: Beryllium Strontium 10 Magnesium Barium New Way Chemistry for Hong Kong A-Level Book 4 Radium 10 40.1 Characteristic Properties of the s-Block Elements (SB p.39) Some information about Group II elements Atomic Ionic Melting Group Crystal radius radius point II metal structure (nm) (nm) (C) Be Mg Ca Sr Ba Ra 0.112 0.160 0.197 0.215 0.217 0.220 0.031 0.065 0.099 0.113 0.135 0.140 h h f f b — 1278 648.8 839 769 729 697 Boiling point (C) Density Abundance (g cm–3) on earth (%) 2477 1100 1480 1380 1640 1140 1.85 1.75 1.55 2.54 3.60 5.0 0.000 28 2.33 4.15 0.038 0.042 Trace “h”, “f” and “b” denote hexagonal close-packed, face-centred cubic and body-centred cubic structures respectively 11 New Way Chemistry for Hong Kong A-Level Book 4 11 Variation in Physical Properties Atomic Radius and Ionic Radius (notes p. 1) 12 New Way Chemistry for Hong Kong A-Level Book 4 12 41.3 Variation in Properties of the s-Block Elements (SB p.52) Question: The atomic and ionic radii increase down the Groups, why? ∵ outermost shell electrons become further away, and more inner shells shielding the outermost shell electrons attraction between the nucleus and the outermost shell electrons decreases atomic and ionic radii increase 13 New Way Chemistry for Hong Kong A-Level Book 4 13 Question: Atomic and ionic radii decrease when going from Group I to II in each period, why? ∵ Group II elements have 1 more proton and electron than Group I elements. Increase in nuclear charge outweighs the increase in shielding effect of additional electron of the same shell. 14 atomic and ionic radii decrease New Way Chemistry for Hong Kong A-Level Book 4 14 41.3 Variation in Properties of the s-Block Elements (SB p.52) Question: Ionic radius of any Group I or II element is smaller than the atomic radius, why? ∵ after losing the outermost shell electron(s), there is one electron shell less in the cation than in the atom. Increase in p/e ratio 15 New Way Chemistry for Hong Kong A-Level Book 4 15 41.3 Variation in Properties of the s-Block Elements (SB p.53) Ionization Enthalpy (notes p. 2) 16 New Way Chemistry for Hong Kong A-Level Book 4 16 41.3 Variation in Properties of the s-Block Elements (SB p.54) Variations in the 1st, 2nd and 3rd ionization enthalpies of Group II elements 17 New Way Chemistry for Hong Kong A-Level Book 4 17 1st I.E. is much smaller than 2nd I.E. for Gp. I elements For the 1st I.E., electron is further away from the nucleus and shielding effect of inner shell electrons small 1st I.E. For 2nd I.E., electron is removed from stable noble gas configuration and higher effective nuclear charge large 2nd I.E. 18 New Way Chemistry for Hong Kong A-Level Book 4 18 The ionization enthalpies decrease down the Groups Reason: • atomic sizes increase down the group the outermost shell electron(s) is/are further away from the nucleus, they will be better shielded by inner electron shells. less attractive force experienced less energy is required to remove the electrons Because of the high I.E., Li and Be forms a few covalent compounds instead of forming Li+ and Be2+ respectively. 19 New Way Chemistry for Hong Kong A-Level Book 4 19 40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 3)) Low Electronegativity • All have low electronegativity values ∵ the outermost electron shell is effectively shielded by inner electron shells. - Low effective nuclear charge. • Decrease when going down the group ∵ the outermost electron shell are further away from nucleus - increase in shielding effect. 20 New Way Chemistry for Hong Kong A-Level Book 4 20 40.1 Characteristic Properties of the s-Block Elements (SB p.41) Group II elements are relatively more electronegative than Group I counterparts ∵ 21 higher nuclear charge, stronger attraction to outermost shell electrons Group I element Electronegativity Group II element Electronegativity Li Na K Rb Cs Fr 1.0 0.9 0.8 0.8 0.7 — Be Mg Ca Sr Ba Ra 1.5 1.2 1.0 1.0 0.9 — New Way Chemistry for Hong Kong A-Level Book 4 21 40.1 Characteristic Properties of the s-Block Elements (SB p.43) Characteristic Flame Colours of Salts • The outermost shell electrons of Group I & II elements are weakly held The electrons can be excited to higher energy levels on heating When electrons return to ground state, radiations are emitted The radiations fall into the visible light region The flame colour is a characteristic property of the element 22 New Way Chemistry for Hong Kong A-Level Book 4 22 Flame Test 23 New Way Chemistry for Hong Kong A-Level Book 4 23 40.1 Characteristic Properties of the s-Block Elements (SB p.43) 24 New Way Chemistry for Hong Kong A-Level Book 4 24 Flame colours 25 New Way Chemistry for Hong Kong A-Level Book 4 25 40.1 Characteristic Properties of the s-Block Elements (SB p.43) Weak tendency to form complexes (not mentioned in notes) Complex: Polyatomic ion or neutral molecule formed when molecular or ionic gropups (called ligands) form dative covalent bonds with a central ion. Group I & II elements seldom form complex: - s-block ions do not have low energy vacant orbitals available for dative covalent bonds. - Low ionic charge 26 New Way Chemistry for Hong Kong A-Level Book 4 26 41.3 Variation in Properties of the s-Block Elements (SB p.55) Melting Point (notes p. 4) Variations in melting points of Groups I and II elements 27 New Way Chemistry for Hong Kong A-Level Book 4 27 Observations: • melting point decreases as going down Groups I and II Reason: • the ionic size of the elements increases attraction between ions and electrons becomes weaker metallic bond is weaker 28 New Way Chemistry for Hong Kong A-Level Book 4 28 Observations: • melting points of Group II elements are much higher than those of Group I elements Reason: • no. of valence electrons per mole contributed to the delocalized electron sea is greater. • Group II elements have higher ionic charge the attractive force between ions and electrons are stronger metallic bond is stronger 29 New Way Chemistry for Hong Kong A-Level Book 4 29 41.3 Variation in Properties of the s-Block Elements (SB p.56) Observations: • irregularity in the general decrease in melting point down Group II elements Reason: • different metallic crystal structures of the Group II elements 30 New Way Chemistry for Hong Kong A-Level Book 4 Group Crystal II metal structure Be Mg Ca Sr Ba Ra h h f f b — 30 Extraction of sodium (not in syllabus) Downs Cell 31 New Way Chemistry for Hong Kong A-Level Book 4 31 Manufacture of sodium hydroxide graphite anodes chlorine + used brine saturated brine mercury alloyed with sodium flow of mercury Water flowing mercury (as cathode) Mercury (recycle) Flowing mercury cell 32 New Way Chemistry for Hong Kong A-Level Book 4 32 During electrolysis, chlorine is liberated at the anode and sodium at the cathode. At anode (graphite): - 2Cl (aq) Cl2(g) + 2e- At cathode (mercury): Na+(aq) + e- Na(s); Na(s) + Hg(l) Na/Hg(l) sodium amalgam 33 New Way Chemistry for Hong Kong A-Level Book 4 33 Flowing mercury cell 34 Q. 1b; Q.8 New Way Chemistry for Hong Kong A-Level Book 4 34 40.3 Variation in Properties of the s-Block Elements (SB p.56, notes p. 8) Hydration Enthalpy Xn+(g) + aq Xn+(aq) Hydration enthalpy (Hhyd) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions. 35 • Hhyd must be negative value. • Hhyd depends on charge density charge/size Higher the charge, stronger the attraction, more energy released Smaller the size, stronger the attraction, more energy released New Way Chemistry for Hong Kong A-Level Book 4 35 M+ Variations in hydration enthalpy of Groups I and II elements 36 New Way Chemistry for Hong Kong A-Level Book 4 36 • magnitude of hydration enthalpies become smaller (less negative) as going down the Groups Reason: • the ionic size of the elements increases down the group, the charge density decreases the attractive force between water molecules and ions becomes weaker the hydration enthalpy becomes less negative Down the group, fewer molecules of water of crystallization Na2CO3.10H2O K2CO3.2H2O 37 MgSO4.7H2O MgCl2.6H2O CaSO4.2H2O CaCl2.6H2O SrSO4 BaCl2.2H2O New Way Chemistry for Hong Kong A-Level Book 4 37 Observations: • hydration enthalpies of Group II ions are more negative than those of Group I ions Reason: • Group II ions have higher charge and smaller size charge density is much higher that of Group I ions the attractive force would be much stronger 38 New Way Chemistry for Hong Kong A-Level Book 4 38 Lattice Enthalpies of Group I Halides (p.10) 39 New Way Chemistry for Hong Kong A-Level Book 4 39 Lattice Enthalpies of Group I Halides (p.10) • Good agreement between calculated and measured value. Why? • Lattice Enthalpies decrease down the group: Reasons: Size increase Internuclear distance increase Attractive force between opposite ions decrease 40 New Way Chemistry for Hong Kong A-Level Book 4 40 Lattice Enthalpies of Group II Halides (p.11) • Discrepancies occurred between calculated and measured values. Reason: Covalent characters occurred in small cations. • Group II Halides have a higher lattice enthalpies than Group I Halides. Reason: Higher charge; smaller size. 41 New Way Chemistry for Hong Kong A-Level Book 4 41 40.1 Characteristic Properties of the s-Block Elements (SB p.43, notes p. 13) Formation of Hydroxides – reactions with water • All Group I metals react with H2O to form metal hydroxides and H2 gas e.g. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) 2K(s) + 2H2O(l) 2KOH(aq) + H2(g) Li+H2O 42 Na +H2O New Way Chemistry for Hong Kong A-Level Book 4 42 Rb+H2O K+H2O Cs+H2O 43 New Way Chemistry for Hong Kong A-Level Book 4 43 40.1 Characteristic Properties of the s-Block Elements (SB p.43) • All Group II metals (except Be) react with H2O to form metal hydroxides and H2 gas (Mg reacts with hot water). Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g) e.g. Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g) • 44 Be does not react with H2O(l or g) New Way Chemistry for Hong Kong A-Level Book 4 44 Strontium + water 45 Barium + water New Way Chemistry for Hong Kong A-Level Book 4 45 40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 14) Formation of Basic Oxides Group I elements • Produce more than one type of oxides (except Li) • All are ionic • Three types of oxides: normal oxides (monoxides), peroxides, superoxides • Relationship between three oxides: O2– O2 monoxide 46 1 O2 2 2– peroxide O O O2 2O2– superoxide New Way Chemistry for Hong Kong A-Level Book 4 46 40.1 Characteristic Properties of the s-Block Elements (SB p.41) • Li forms the monoxide only 180C 4Li(s) + O2(g) 2Li2O(s) • Na forms the monoxide and peroxide when O2 is abundant 180C 4Na(s) + O2(g) 2Na2O(s) 300C 2Na2O(s) + O2(g) 2Na2O2(s) 47 New Way Chemistry for Hong Kong A-Level Book 4 47 40.1 Characteristic Properties of the s-Block Elements (SB p.41) • K forms the monoxide, peroxide and superoxide 180C 4K(s) + O2(g) 2K2O(s) 300C 2K2O(s) + O2(g) 2K2O2(s) 3000C K2O2(s) + O2(g) 2KO2(s) • Rb, Cs also forms superoxides 3000C Rb2O2(s) + O2(g) 2RbO2(s) Cs2O2(s) + O2(g) 3000C 2CsO2(s) 48 New Way Chemistry for Hong Kong A-Level Book 4 48 41.2 Characteristic Properties of the s-Block Elements (SB p.45) 49 Group I element Monoxide Peroxide Superoxide Li Na K Rb Cs Li2O Na2O K2O Rb2O Cs2O — Na2O2 K2O2 Rb2O2 Cs2O2 — — KO2 RbO2 CsO2 New Way Chemistry for Hong Kong A-Level Book 4 49 40.1 Characteristic Properties of the s-Block Elements (SB p.42 notes p. 14) • Li does not form peroxides or superoxides Reason: Li+ is small high polarizing power serious distortion on electron cloud of peroxide or superoxide (large polyatomic anions) more distortion , more unstable Li2O2 and LiO2 do not exist • K+, Rb+ and Cs+ ions are large Low polarizing power peroxides and superoxides are relatively stable 50 New Way Chemistry for Hong Kong A-Level Book 4 50 41.2 Characteristic Properties of the s-Block Elements (SB p.46, notes p. 14) Group II Elements • Form normal oxides only, except Sr, Ba which can form peroxides. • All are basic (except BeO which is amphoteric), why? 2Be(s) + O2(g) 2BeO(s) 2Mg(s) + O2(g) 2MgO(s) 2Ca(s) + O2(g) 2CaO(s) 2Ba(s) + O2(g) 2BaO(s) 2BaO(s) + O2(g) 51 2BaO2(s) New Way Chemistry for Hong Kong A-Level Book 4 51 Strontium + air 52 New Way Chemistry for Hong Kong A-Level Book 4 Barium + air 52 40.1 Characteristic Properties of the s-Block Elements (SB p.43) Group II element Normal oxide Peroxide Superoxide Be Mg Ca Sr Ba BeO MgO CaO SrO BaO — — — SrO2 BaO2 — — — — — Be, Mg, Ca peroxide do not exist, why? Reason: High charge density high polarizing power serious distortion on electron cloud of the peroxide ion 53 New Way Chemistry for Hong Kong A-Level Book 4 53 40.3 Variation in Properties of the compounds of the s-Block Elements ( p.59) notes p. 14 2(e) Reactions of Oxides of s-Block Elements Reaction with Water • Group I oxides react with H2O to form hydroxides • Normal oxides: e.g. Li2O(s) + H2O(l) 2LiOH(aq) • Peroxides: e.g. Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(aq) • 54 Dissolution of Na2O2 in H2O containing phenolphthalein Superoxides: e.g. 2KO2(s) + 2H2O(l) 2KOH(aq) + H2O2(aq) + O2(g) New Way Chemistry for Hong Kong A-Level Book 4 54 • Group II oxides (except BeO, MgO) react with H2O to form a weakly alkaline solution e.g. CaO(s) + H2O(l) Ca(OH)2(aq) (weakly alkaline) • The basicity of all Group II oxides increases down the group • BeO is amphoteric BeO(s) + 2H+(aq) Be2+(aq) + H2O(l) hot BeO(s) + 2OH–(aq) + H2O(l) [Be(OH)4]2–(aq) hot • MgO is slightly soluble in water, but dissolves in acids to form salts • 55 BaO2(s) + 2H2O(l) Ba(OH)2(aq) + H2O2(aq) New Way Chemistry for Hong Kong A-Level Book 4 55 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60, not mentioned in notes) Reaction with Acids 56 • All oxides of s-Block elements are basic except BeO which is amphoteric • Normal oxides: e.g. CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l) • Peroxides: e.g. Na2O2(s) + 2HCl(aq) 2NaCl(aq) + H2O2(aq) • Superoxides: e.g. 2KO2(s) + 2HCl(aq) 2KCl(aq) + H2O2(aq) + O2(g) New Way Chemistry for Hong Kong A-Level Book 4 56 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60) Reaction with Alkalis • No reaction between the oxides of s-block elements with alkalis except BeO • BeO is amphoteric, it reacts with NaOH to give Na2Be(OH)4 BeO(s) + 2NaOH(aq) + H2O(l) Na2Be(OH)4(aq) 57 New Way Chemistry for Hong Kong A-Level Book 4 57 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60) notes p. 15, 18 Relative Thermal Stability of the Carbonates and Hydroxides Thermal stability refers to the resistance of a compound to decomposition on heating • The higher the thermal stability of a compound, the higher is the temperature needed to decompose it • The thermal stability of ionic compounds depends on: (1) charges & (2) sizes of ions 58 New Way Chemistry for Hong Kong A-Level Book 4 58 40.3 Variation in Properties of the compounds of the s-Block Elements (p.61) notes p. 18 • Compound with large polarizable polyatomic anion (large electron cloud, as shown in notes), the thermal stability depends on the polarizing power (charge density) of cations The stronger the polarizing power, the electron cloud of anion will be distorted to greater extent The compound tends to be less thermal stable 59 New Way Chemistry for Hong Kong A-Level Book 4 59 40.3 Variation in Properties of the compounds of the s-Block Elements (p.61) Group II carbonates/hydroxides are less stable than Group I • Group II ions are smaller and have a higher charge than Group I ions in the same period Greater polarizing power The carbonates and hydroxides of Group II metals are less stable on heating e.g. K2CO3 is stable upon heating while CaCO3 decomposes on heating 60 New Way Chemistry for Hong Kong A-Level Book 4 60 40.3 Variation in Properties of the compounds of the s-Block Elements (p.61) • Most carbonates and hydroxides of Group II metals readily undergo decomposition on heating to give oxides (more stable) MgCO3(s) MgO(s) + CO2(g) e.g. Ca(OH)2(s) CaO(s) + H2O(g) - O H 2+ Mg MgO - + O H - Mg2+ 61 - O O C O MgO New Way Chemistry for Hong Kong A-Level Book 4 + H2O CO2 61 40.3 Variation in Properties of the compounds of the s-Block Elements (p.62) • Down the group, the size of cations increases polarizing power decreases compound with large anion become more stable ∴ thermal stability of carbonates & hydroxides of Groups I and II metals increases down the group Do Q. 2b on p. 73 Effect of sizes of cations on thermal stability of compounds 62 New Way Chemistry for Hong Kong A-Level Book 4 62 Q. Explain briefly why lithium hydrogencarbonate does not exist as a solid while other Group I hydrogencarbonates can be found in solid state. A. In solid form, the cation and anion are close to each other. Due to small size of Li+, it has a high polarizing power. This distorts the electron cloud of HCO3-, making the anion unstable. As the size of cations increases down the group, the polarizing power decreases, therefore, solid hydrogencarbonates can be formed. 63 New Way Chemistry for Hong Kong A-Level Book 4 63 Effect of Heat on s-block carbonates and hydroxides (p.19) i. Carbonates Group I: All are thermally stable except Lithium. Group II: All decompose on heating forming metal oxides and carbon dioxide. ii. Hydroxides (p.21) 64 Group I: All are thermally stable except Lithium. Group II: All decompose on heating forming metal oxides and water. New Way Chemistry for Hong Kong A-Level Book 4 64 40.3 Variation in Properties of the compounds of the s-Block Elements (p.63) notes p. 21 Relative Solubility of the Sulphates(VI) and Hydroxides Processes involved in Dissolution and their Energetics • 65 When an ionic solid is dissolved in water, two processes are taken place: 1. Breakdown of the ionic solid (-ve lattice enthalpy) 2. Stabilization of ions by water molecules (hydration enthalpy released) New Way Chemistry for Hong Kong A-Level Book 4 65 Dissolution of NaCl 66 New Way Chemistry for Hong Kong A-Level Book 4 66 Solubility of s-block Sulphates and Hydroxides (p.23) MX(s) Hs M+(aq) + X-(aq) -U Hhyd M+(g) + X-(g) A low modulus of lattice enthalpy and a high modulus of hydration enthalpy favour the dissolving process. 67 New Way Chemistry for Hong Kong A-Level Book 4 67 Effect of charge and size of ions on Hhyd and Hlattice + - Z Z H lattice + r +r 1 1 H hyd + + r r 68 New Way Chemistry for Hong Kong A-Level Book 4 68 Solubility of s-block Sulphates and Hydroxides i. For large anions, like sulphates When moving down the group, the decrease in size of the cation does not cause a significant change of U. However, Hhyd become less negative and has a significant change the solubility of sulphates decreases down the group. 69 SO42- SO42- MgSO4 SrSO4 New Way Chemistry for Hong Kong A-Level Book 4 69 ii. For smaller anions, like hydroxides When moving down the group, the increase in size of the cation causes a significant change of U but Hhyd change a little because of the great hydration energy of the anion. Therefore the solubility of hydroxide increases down the group. Mg(OH)2 iii. 70 Sr(OH)2 Group I sulphates and hydroxides are more soluble than that of Group II. Why? New Way Chemistry for Hong Kong A-Level Book 4 70 40.3 Variation in Properties of the compounds of the s-Block Elements (p.23) Relative Solubility of the Sulphates(VI) and Hydroxides –Trend and Interpretation • The sulphates(VI) and hydroxides of Group I metals are more soluble in water than those of Group II metals ∵ Group I metals has a smaller charge and larger size than Group II metals in the same period The lattice enthalpies of Group I compounds are smaller in magnitude than those of Group II compounds The enthalpy changes of solution are more –ve 71 New Way Chemistry for Hong Kong A-Level Book 4 71 Do Q. 6, 10 and Q. 7 on p. 74 The END 72 New Way Chemistry for Hong Kong A-Level Book 4 72