New Way Chemistry for Hong Kong A-Level Book 41 1 The s-Block Elements 40.1Characteristic Properties of the s-Block Elements 40.2Variation in Properties of the s-Block Elements 40.3Variation in Properties of the Compounds of the s-Block Elements 40

New Way Chemistry for Hong Kong A-Level Book 42 2 The Syllabus 8.1 Characteristic Properties 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 43 3 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

New Way Chemistry for Hong Kong A-Level Book 44 4 s-Block elements: Consists of Group IA and Group IIA elements Outermost electron shell: ns 1 ns 2 Highly reactive metals Good reducing agents Fixed oxidation states +1 for Group I elements +2 for Group II elements Notes p. 1

New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the s-Block Elements

New Way Chemistry for Hong Kong A-Level Book 46 6 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 Metallic Character (not mentioned in notes) 40.1 Characteristic Properties of the s-Block Elements (SB p.38) Cutting Rubidium

New Way Chemistry for Hong Kong A-Level Book 47 7 Group I elements: LithiumSodiumPotassium Rubidium Caesium

New Way Chemistry for Hong Kong A-Level Book 48 8 Group I metal Atomic radius (nm) Ionic radius (nm) Crystal structure Melting point (  C) Boiling point (  C) Density (g cm –3 ) Abundance on earth (%) Li Na K Rb Cs Fr bbbbb—bbbbb— — Trace “b” denotes body-centred cubic structure Some information about Group I elements 40.1 Characteristic Properties of the s-Block Elements (SB p.39)

New Way Chemistry for Hong Kong A-Level Book 49 9 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 40.1 Characteristic Properties of the s-Block Elements (SB p.39)

New Way Chemistry for Hong Kong A-Level Book 410 Group II elements: Beryllium Magnesium Calcium Strontium Barium Radium

New Way Chemistry for Hong Kong A-Level Book 411 Group II metal Atomic radius (nm) Ionic radius (nm) Crystal structure Melting point (  C) Boiling point (  C) Density (g cm –3 ) Abundance on earth (%) Be Mg Ca Sr Ba Ra hhffb—hhffb— Trace “h”, “f” and “b” denote hexagonal close-packed, face-centred cubic and body-centred cubic structures respectively Some information about Group II elements 40.1 Characteristic Properties of the s-Block Elements (SB p.39)

New Way Chemistry for Hong Kong A-Level Book 412 Atomic Radius and Ionic Radius (notes p. 1) Variation in Physical Properties

New Way Chemistry for Hong Kong A-Level Book 413 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 41.3 Variation in Properties of the s-Block Elements (SB p.52)

New Way Chemistry for Hong Kong A-Level Book 414 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.  atomic and ionic radii decrease

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the s-Block Elements (SB p.53) Ionization Enthalpy (notes p. 2)

New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the s-Block Elements (SB p.54) Variations in the 1st, 2nd and 3rd ionization enthalpies of Group II elements

New Way Chemistry for Hong Kong A-Level Book st I.E. is much smaller than 2 nd I.E. for Gp. I elements For the 1 st I.E., electron is further away from the nucleus and shielding effect of inner shell electrons  small 1 st I.E. For 2 nd I.E., electron is removed from stable noble gas configuration and higher effective nuclear charge  large 2 nd I.E.

New Way Chemistry for Hong Kong A-Level Book 419 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 Be 2+ respectively.

New Way Chemistry for Hong Kong A-Level Book 420 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 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 3))

New Way Chemistry for Hong Kong A-Level Book 421 Group II elements are relatively more electronegative than Group I counterparts ∵ higher nuclear charge, stronger attraction to outermost shell electrons Group I element Electro- negativity Group II element Electro- negativity Li Na K Rb Cs Fr — Be Mg Ca Sr Ba Ra — 40.1 Characteristic Properties of the s-Block Elements (SB p.41)

New Way Chemistry for Hong Kong A-Level Book 422 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 40.1 Characteristic Properties of the s-Block Elements (SB p.43)

New Way Chemistry for Hong Kong A-Level Book 423 Flame Test

New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the s-Block Elements (SB p.43)

New Way Chemistry for Hong Kong A-Level Book 425 Flame colours

New Way Chemistry for Hong Kong A-Level Book 426 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 Characteristic Properties of the s-Block Elements (SB p.43) 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

New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the s-Block Elements (SB p.55) Variations in melting points of Groups I and II elements Melting Point (notes p. 4)

New Way Chemistry for Hong Kong A-Level Book 428 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

New Way Chemistry for Hong Kong A-Level Book 429 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

New Way Chemistry for Hong Kong A-Level Book 430 Observations: irregularity in the general decrease in melting point down Group II elements Reason: different metallic crystal structures of the Group II elements 41.3 Variation in Properties of the s-Block Elements (SB p.56) Group II metal Crystal structure Be Mg Ca Sr Ba Ra hhffb—hhffb—

New Way Chemistry for Hong Kong A-Level Book 431 Extraction of sodium (not in syllabus) Downs Cell

New Way Chemistry for Hong Kong A-Level Book 432 Manufacture of sodium hydroxide graphite anodes chlorine used brine mercury alloyed with sodium flow of mercury flowing mercury (as cathode) saturated brine   Flowing mercury cell Water Mercury (recycle)

New Way Chemistry for Hong Kong A-Level Book 433 During electrolysis, chlorine is liberated at the anode and sodium at the cathode. At anode (graphite):2Cl  (aq)  Cl 2 (g) + 2e  At cathode (mercury):Na + (aq) + e   Na(s); Na(s) + Hg(l)  Na/Hg(l) sodium amalgam

New Way Chemistry for Hong Kong A-Level Book 434 Flowing mercury cell Q. 1b; Q.8

New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the s-Block Elements (SB p.56, notes p. 8) Hydration Enthalpy Hydration enthalpy (  H hyd ) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions.  H hyd must be negative value.  H hyd depends on charge density charge/size  Higher the charge, stronger the attraction, more energy released  Smaller the size, stronger the attraction, more energy released X n+ (g) + aq  X n+ (aq)

New Way Chemistry for Hong Kong A-Level Book 436 Variations in hydration enthalpy of Groups I and II elements M+M+

New Way Chemistry for Hong Kong A-Level Book 437 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  Na 2 CO 3.10H 2 OMgSO 4.7H 2 OMgCl 2.6H 2 O  K 2 CO 3.2H 2 OCaSO 4.2H 2 OCaCl 2.6H 2 O  SrSO 4 BaCl 2.2H 2 O

New Way Chemistry for Hong Kong A-Level Book 438 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

New Way Chemistry for Hong Kong A-Level Book 439 Lattice Enthalpies of Group I Halides (p.10)

New Way Chemistry for Hong Kong A-Level Book 440 Lattice Enthalpies of Group I Halides (p.10) Lattice Enthalpies decrease down the group: Reasons: Size increase Internuclear distance increase Attractive force between opposite ions decrease Good agreement between calculated and measured value. Why?

New Way Chemistry for Hong Kong A-Level Book 441 Lattice Enthalpies of Group II Halides (p.11) Discrepancies occurred between calculated and measured values. Reason: Covalent characters Covalent characters occurred in small cations. Group II Halides have a higher lattice enthalpies than Group I Halides. Reason: Higher charge; smaller size.

New Way Chemistry for Hong Kong A-Level Book 442 Formation of Hydroxides – reactions with water All Group I metals react with H 2 O to form metal hydroxides and H 2 gas e.g.2Na(s) + 2H 2 O(l)  2NaOH(aq) + H 2 (g) 2K(s) + 2H 2 O(l)  2KOH(aq) + H 2 (g) 40.1 Characteristic Properties of the s-Block Elements (SB p.43, notes p. 13) Li+H 2 O Na +H 2 O

New Way Chemistry for Hong Kong A-Level Book 443 K+H 2 O Rb+H 2 O Cs+H 2 O

New Way Chemistry for Hong Kong A-Level Book 444 All Group II metals (except Be) react with H 2 O to form metal hydroxides and H 2 gas (Mg reacts with hot water). e.g.Ca(s) + 2H 2 O(l)  Ca(OH) 2 (aq) + H 2 (g) Sr(s) + 2H 2 O(l)  Sr(OH) 2 (aq) + H 2 (g) Be does not react with H 2 O(l or g) 40.1 Characteristic Properties of the s-Block Elements (SB p.43)

New Way Chemistry for Hong Kong A-Level Book 445 Strontium + water Barium + water

New Way Chemistry for Hong Kong A-Level Book 446 Formation of Basic Oxides Group I elements (except LiProduce more than one type of oxides (except Li) All are ionic Three types of oxides: normal oxides (monoxides), peroxides, superoxides Relationship between three oxides: O 2–  O 2 2–  2O 2 – monoxideperoxidesuperoxide 40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 14)

New Way Chemistry for Hong Kong A-Level Book 447 Li forms the monoxide onlyLi forms the monoxide only 4Li(s) + O 2 (g)  2Li 2 O(s) 180  C Na forms the monoxide and peroxide when O 2 is abundant 4Na(s) + O 2 (g)  2Na 2 O(s) 2Na 2 O(s) + O 2 (g)  2Na 2 O 2 (s) 180  C 300  C 40.1 Characteristic Properties of the s-Block Elements (SB p.41)

New Way Chemistry for Hong Kong A-Level Book 448 K forms the monoxide, peroxide and superoxide 4K(s) + O 2 (g)  2K 2 O(s) 2K 2 O(s) + O 2 (g)  2K 2 O 2 (s) K 2 O 2 (s) + O 2 (g)  2KO 2 (s) 180  C 300  C 3000  C Rb, Cs also forms superoxides Rb 2 O 2 (s) + O 2 (g)  2RbO 2 (s) Cs 2 O 2 (s) + O 2 (g)  2CsO 2 (s) 3000  C 40.1 Characteristic Properties of the s-Block Elements (SB p.41)

New Way Chemistry for Hong Kong A-Level Book Characteristic Properties of the s-Block Elements (SB p.45) Group I element MonoxidePeroxideSuperoxide Li Na K Rb Cs Li 2 O Na 2 O K 2 O Rb 2 O Cs 2 O — Na 2 O 2 K 2 O 2 Rb 2 O 2 Cs 2 O 2 — KO 2 RbO 2 CsO 2

New Way Chemistry for Hong Kong A-Level Book 450 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  Li 2 O 2 and LiO 2 do not exist K +, Rb + and Cs + ions are large  Low polarizing power  peroxides and superoxides are relatively stable 40.1 Characteristic Properties of the s-Block Elements (SB p.42 notes p. 14)

New Way Chemistry for Hong Kong A-Level Book 451 Form normal oxides only, except Sr, Ba which can form peroxides. All are basic (except BeO which is amphoteric), why? 41.2 Characteristic Properties of the s-Block Elements (SB p.46, notes p. 14) Group II Elements 2Be(s) + O 2 (g)  2BeO(s) 2Mg(s) + O 2 (g)  2MgO(s) 2Ca(s) + O 2 (g)  2CaO(s) 2Ba(s) + O 2 (g)  2BaO(s) 2BaO(s) + O 2 (g)2BaO 2 (s)

New Way Chemistry for Hong Kong A-Level Book 452 Strontium + airBarium + air

New Way Chemistry for Hong Kong A-Level Book 453 Group II element Normal oxide PeroxideSuperoxide Be Mg Ca Sr Ba BeO MgO CaO SrO BaO — SrO 2 BaO 2 —————————— Be, Mg, Ca peroxide do not exist, why? Reason:  High charge density  high polarizing power  serious distortion on electron cloud of the peroxide ion 40.1 Characteristic Properties of the s-Block Elements (SB p.43)

New Way Chemistry for Hong Kong A-Level Book Variation in Properties of the compounds of the s-Block Elements ( p.59) notes p. 14 2(e) Reaction with Water Reactions of Oxides of s-Block Elements Group I oxides react with H 2 O to form hydroxides Normal oxides: e.g. Li 2 O(s) + H 2 O(l)  2LiOH(aq) Peroxides: e.g. Na 2 O 2 (s) + 2H 2 O(l)  2NaOH(aq) + H 2 O 2 (aq) Superoxides: e.g. 2KO 2 (s) + 2H 2 O(l)  2KOH(aq) + H 2 O 2 (aq) + O 2 (g) Dissolution of Na 2 O 2 in H 2 O containing phenolphthalein

New Way Chemistry for Hong Kong A-Level Book 455 Group II oxides (except BeO, MgO) react with H 2 O to form a weakly alkaline solution e.g. CaO(s) + H 2 O(l)  Ca(OH) 2 (aq) (weakly alkaline) The basicity of all Group II oxides increases down the group MgO is slightly soluble in water, but dissolves in acids to form salts BeO is amphoteric BeO(s) + 2H + (aq)  Be 2+ (aq) + H 2 O(l) BeO(s) + 2OH – (aq) + H 2 O(l)  [Be(OH) 4 ] 2– (aq) hot BaO 2 (s) + 2H 2 O(l)  Ba(OH) 2 (aq) + H 2 O 2 (aq)

New Way Chemistry for Hong Kong A-Level Book 456 Reaction with Acids All oxides of s-Block elements are basic except BeO which is amphoteric Normal oxides: e.g. CaO(s) + 2HCl(aq)  CaCl 2 (aq) + H 2 O(l) Peroxides: e.g. Na 2 O 2 (s) + 2HCl(aq)  2NaCl(aq) + H 2 O 2 (aq) Superoxides: e.g. 2KO 2 (s) + 2HCl(aq)  2KCl(aq) + H 2 O 2 (aq) + O 2 (g) 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60, not mentioned in notes)

New Way Chemistry for Hong Kong A-Level Book 457 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 Na 2 Be(OH) 4 BeO(s) + 2NaOH(aq) + H 2 O(l)  Na 2 Be(OH) 4 (aq) 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60)

New Way Chemistry for Hong Kong A-Level Book 458 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 40.3 Variation in Properties of the compounds of the s-Block Elements (p.60) notes p. 15, 18

New Way Chemistry for Hong Kong A-Level Book 459 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 40.3 Variation in Properties of the compounds of the s-Block Elements (p.61) notes p. 18

New Way Chemistry for Hong Kong A-Level Book 460 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. K 2 CO 3 is stable upon heating while CaCO 3 decomposes on heating 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

New Way Chemistry for Hong Kong A-Level Book 461 Most carbonates and hydroxides of Group II metals readily undergo decomposition on heating to give oxides (more stable) e.g.MgCO 3 (s)  MgO(s) + CO 2 (g) Ca(OH) 2 (s)  CaO(s) + H 2 O(g) 40.3 Variation in Properties of the compounds of the s-Block Elements (p.61)

New Way Chemistry for Hong Kong A-Level Book 462 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 Effect of sizes of cations on thermal stability of compounds 40.3 Variation in Properties of the compounds of the s-Block Elements (p.62) Do Q. 2b on p. 73

New Way Chemistry for Hong Kong A-Level Book 463 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 HCO 3 -, making the anion unstable. As the size of cations increases down the group, the polarizing power decreases, therefore, solid hydrogencarbonates can be formed.

New Way Chemistry for Hong Kong A-Level Book 464 Effect of Heat on s-block carbonates and hydroxides (p.19) Carbonates i. Carbonates All Group I: All are thermally stable except Lithium. All Group II: All decompose on heating forming metal oxides and carbon dioxide. Hydroxides ii. Hydroxides (p.21) All Group I: All are thermally stable except Lithium. All Group II: All decompose on heating forming metal oxides and water.

New Way Chemistry for Hong Kong A-Level Book 465 Relative Solubility of the Sulphates(VI) and Hydroxides 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) Processes involved in Dissolution and their Energetics 40.3 Variation in Properties of the compounds of the s-Block Elements (p.63) notes p. 21

New Way Chemistry for Hong Kong A-Level Book 466 Dissolution of NaCl

New Way Chemistry for Hong Kong A-Level Book 467 Solubility of s-block Sulphates and Hydroxides (p.23) MX(s)M + (aq) + X - (aq) M + (g) + X - (g)  Hs  U  H hyd A low modulus of lattice enthalpy and a high modulus of hydration enthalpy favour the dissolving process.

New Way Chemistry for Hong Kong A-Level Book 468 Effect of charge and size of ions on  H hyd and  H lattice

New Way Chemistry for Hong Kong A-Level Book 469 Solubility of s-block Sulphates and Hydroxides For large anions, like sulphates i. For large anions, like sulphates the solubility of sulphates decreases down the group. When moving down the group, the decrease in size of the cation does not cause a significant change of  U. However,  H hyd become less negative and has a significant change  the solubility of sulphates decreases down the group. SO 4 2- MgSO 4 SO 4 2- SrSO 4

New Way Chemistry for Hong Kong A-Level Book 470 For smaller anions, like hydroxides ii. For smaller anions, like hydroxides the solubility of hydroxide increases down the group. When moving down the group, the increase in size of the cation causes a significant change of  U but  H hyd change a little because of the great hydration energy of the anion. Therefore the solubility of hydroxide increases down the group. Group I sulphates and hydroxides are more soluble than that of Group II. Why? iii. Group I sulphates and hydroxides are more soluble than that of Group II. Why? Mg(OH) 2 Sr(OH) 2

New Way Chemistry for Hong Kong A-Level Book 471 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 Relative Solubility of the Sulphates(VI) and Hydroxides –Trend and Interpretation 40.3 Variation in Properties of the compounds of the s-Block Elements (p.23)

New Way Chemistry for Hong Kong A-Level Book 472 The END Do Q. 6, 10 and Q. 7 on p. 74