1 The s-Block Elements

2 Elements of Groups IA* (the alkali metals) and IIA* (the alkaline earth metals)  constitute the s-block elements  their outermost shell electrons are in the s orbital *Note: In the following, Groups IA and IIA are abbreviated as Groups I and II respectively.

3 The s-block elements

4 The s-Block Elements Similarities 1.highly reactive metals 2.strong reducing agents 3.form ionic compounds with fixed oxidation states of +1 for Group I elements and +2 for Group II elements

5 [Rn] 7s 2 [Xe] 6s 2 [Kr] 5s 2 [Ar] 4s 2 [Ne] 3s 2 [He] 2s 2 Electronic configuration *Ra Radium Ba Barium Sr Strontium Ca Calcium Mg Magnesium Be Beryllium Group II [Rn] 7s 1 [Xe] 6s 1 [Kr] 5s 1 [Ar] 4s 1 [Ne] 3s 1 [He] 2s 1 Electronic configuration *Fr Francium Cs Caesium Rb Rubidium K Potassium Na Sodium Li Lithium Group I Q.1

6 Group I elements Lithium

7 Group I elements Sodium

8 Group I elements Potassium

9 Group I elements Rubidium

10 Group I elements Francium - radioactive

11 Group I elements Beryllium

12 Group I elements Magnesium

13 Group I elements Calcium

14 Group I elements Strontium

15 Group I elements Barium

16 Group I elements Radium - radioactive

17 Characteristic Properties of the s-Block Elements

18 Group I element Electronegativity value Group II element Electronegativity value Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – All have low electronegativity.  electropositive

19 Group I element Electronegativity value Group II element Electronegativity value Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – EN  down the group EN : Group II > Group I ( ∵ greater ENC)

20 Group I m.p.(  C)b.p.(  C) Group II m.p.(  C)b.p.(  C) Li Be Na98883Mg K63760Ca Rb39688Sr Cs29690Ba Fr--Ra-- Bonding Strength of metallic bond : Group II > Group I m.p./b.p. : Group II > Group I

21 Group I m.p.(  C)b.p.(  C) Group II m.p.(  C)b.p.(  C) Li Be Na98883Mg K63760Ca Rb39688Sr Cs29690Ba Fr--Ra-- Hardness : - Group I < Group II Na/K…can be easily cut with a knife

22 Structure Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba Group IStructure Density (g cm  3 ) Group IIStructure Density (g cm  3 ) Lib.c.c.0.53Beh.c.p.1.86 Nab.c.c.0.97Mgh.c.p.1.74 Kb.c.c.0.86Caf.c.c.1.55 Rbb.c.c.1.53Srf.c.c.2.54 Csb.c.c.1.90Bab.c.c.3.59 Fr--Ra-- Density : Group II > Group I

23 Structure Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba Group IStructure Density (g cm  3 ) Group IIStructure Density (g cm  3 ) Lib.c.c.0.53Beh.c.p.1.86 Nab.c.c.0.97Mgh.c.p.1.74 Kb.c.c.0.86Caf.c.c.1.55 Rbb.c.c.1.53Srf.c.c.2.54 Csb.c.c.1.90Bab.c.c.3.59 Fr--Ra-- Density also depends on size and mass of the atoms

24 Metallic charater (Reactivity) : - M n+ (aq) + ne   M(s) High tendency to lose electrons as shown by –ve E  Group I (V)Group II (V) Li Be Na Mg K Ca Rb Sr Cs Ba -2.90

25 Metallic charater (Reactivity) : - Group I > Group II Group I (V)Group II (V) Li Be Na Mg K Ca Rb Sr Cs Ba  down the groups

26 Sodium is stored under paraffin oil sodium

27 Caesium and rubidium are stored in vacuum-sealed ampoules caesium rubidium

28 Formation of Basic Oxides All alkali metals form more than one type of oxide on burning in air (except lithium) 1. Group I Elements

29 Three types of oxides:  normal oxides  peroxides  superoxides 1. Group I Elements O 2– oxide ion O 2 2– peroxide ion 2O 2 – superoxide ion Abundant supply

30 1. Group I Elements Type of oxide formed depends on 1.supply of oxygen 2.reaction temperature 3.charge density of M +

31 Lithium  when it is burnt in air, it forms normal oxide only 1. Group I Elements 4Li(s) + O 2 (g) 2Li 2 O(s) lithium oxide

32 Sodium  when it is burnt in an abundant supply of oxygen  forms both the normal oxide and the peroxide 1. Group I Elements 4Na(s) + O 2 (g) 2Na 2 O(s) sodium oxide 2Na 2 O(s) + O 2 (g) 2Na 2 O 2 (s) sodium peroxide excess

33 Potassium, rubidium and caesium  form All three types of oxides when burnt in sufficient supply of oxygen 1. Group I Elements

34 Potassium: 4K(s) + O 2 (g)  2K 2 O(s) potassium oxide 2K 2 O(s) + O 2 (g)  2K 2 O 2 (s) potassium peroxide K 2 O 2 (s) + O 2 (g)  2KO 2 (s) potassium superoxide 1. Group I Elements

35 Rubidium: 4Rb(s) + O 2 (g)  2Rb 2 O(s) 2Rb 2 O(s) + O 2 (g)  2Rb 2 O 2 (s) Rb 2 O 2 (s) + O 2 (g)  2RbO 2 (s) 1. Group I Elements

36 Caesium: 4Cs(s) + O 2 (g)  2Cs 2 O(s) 2Cs 2 O(s) + O 2 (g)  2Cs 2 O 2 (s) Cs 2 O 2 (s) + O 2 (g)  2CsO2(s) 1. Group I Elements

37 Group I element Normal oxidePeroxideSuperoxide 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 Oxides formed by Group I elements Cations with high charge densities (Li + or Na + ) tend to polarize the large electron clouds of peroxide ions and/or superoxide ions  Making them decompose to give oxide ions

38 1. Group I Elements The electron cloud of the superoxide ion is greatly distorted by the small lithium ion

39 Group I element Normal oxidePeroxideSuperoxide 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 Oxides formed by Group I elements White solids Slightly coloured solids Highly coloured solids

40 KO 2 used as oxygen generators and CO 2 scrubbers in spacecrafts 4KO 2 + 2H 2 O  4KOH + 3O 2 2KOH + CO 2  K 2 CO 3 + H 2 O

41 Beryllium, magnesium and calcium  form normal oxides only on burning in air 2Be(s) + O 2 (g)  2BeO(s) 2Mg(s) + O 2 (g)  2MgO(s) 2Ca(s) + O 2 (g)  2CaO(s) 2. Group II Elements

42 Q.2(a) Be 2+, Mg 2+ and Ba 2+ have higher charge densities  more polarizing  distort the electron cloud of O 2 2   O 2 2  decomposes to give O 2 

43 Q.2(b) 2SrO(s) + O 2 (g) 2SrO 2 (s) strontium peroxide Sr(s) + O 2 (g) SrO 2 (s) 2Sr(s) + O 2 (g)  2SrO(s) strontium oxide

44 2Ba(s) + O 2 (g)  2BaO(s) barium oxide 2BaO(s) + O 2 (g)2BaO 2 (s) barium peroxide 500  C 700  C Q.2(b) Ba(s) + O 2 (g) BaO 2 (s)

45 Group II element Normal oxidePeroxideSuperoxide Be Mg Ca Sr Ba BeO MgO CaO SrO BaO – SrO 2 BaO 2 –––––––––– Oxides formed by Group II elements KO 2 superoxide

46 Group II element Normal oxidePeroxideSuperoxide Be Mg Ca Sr Ba BeO MgO CaO SrO BaO – SrO 2 BaO 2 –––––––––– Oxides formed by Group II elements All these oxides are basic in nature (except beryllium oxide which is amphoteric)

47 2Li(s) + 2H 2 O(l)  2LiOH(aq) + H 2 (g) 2Na(s) + 2H 2 O(l)  2NaOH(aq) + H 2 (g) 2K(s) + 2H 2 O(l)  2KOH(aq) + H 2 (g) 2Rb(s) + 2H 2 O(l)  2RbOH(aq) + H 2 (g) 2Cs(s) + 2H 2 O(l)  2CsOH(aq) + H 2 (g) 1. Group I hydroxides Formation of hydroxides

48 For normal oxides, M 2 O(s) + H 2 O(l)  2MOH(aq) 1. Group I hydroxides Formation of hydroxides For peroxides, M 2 O 2 (s) + 2H 2 O(l)  2MOH(aq) + H 2 O 2 (aq) For superoxides, 2MO 2 (s) + 2H 2 O(l)  2MOH(aq) + H 2 O 2 (aq) + O 2 (g)

49 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) Ba(s) + 2H 2 O(l)  Ba(OH) 2 (aq) + H 2 (g) Mg reacts with steam but not water. Be does not react with water and steam. Mg(s) + H 2 O(g)  MgO(s) + H 2 (g) 2. Group II hydroxides Formation of hydroxides

50 2. Group II hydroxides Formation of hydroxides CaO(s) + H 2 O(l)  Ca(OH) 2 (aq) SrO(s) + H 2 O(l)  Sr(OH) 2 (aq) BaO(s) + H 2 O(l)  Ba(OH) 2 (aq) MgO(s) + H 2 O(l) Mg(OH) 2 (aq) slightly soluble BeO(s) + H 2 O(l)  No reaction

51 Ionic Bonding with Fixed Oxidation State in their Compounds Group I : +1 Group II : +2 ∵ Low 1 st I.E. but very high 2 nd I.E. ∵ Low 1 st and 2 nd I.E. but very high 3 rd I.E. Predominantly ionic

52 Group I element Oxide HydrideChloride Oxidation state of Group I element in the compound Li Na K Rb Cs Li 2 O Na 2 O 2 KO 2 RbO 2 CsO 2 LiH NaH KH RbH CsH LiCl NaCl KCl RbCl CsCl +1 Chemical formulae of some Group I compounds and the oxidation states of Group I elements in the compounds

53 Group II element Oxide HydrideChloride Oxidation state of Group II element in the compound Be Mg Ca Sr Ba BeO MgO CaO SrO BaO BeH 2 MgH 2 CaH 2 SrH 2 BaH 2 BeCl 2 MgCl 2 CaCl 2 SrCl 2 BaCl 2 +2 Chemical formulae of some Group II compounds and the oxidation states of Group II elements in the compounds

54 Weak Tendency to Form Complexes A complex is formed when a central metal atom or ion is surrounded by other molecules or ions (called ligands) which form dative covalent bonds with the central metal atom or ion using their lone pair.

55 Weak Tendency to Form Complexes Unlike transition metals, all s-block metals (except Be) show little tendency to form complexes

56 Weak Tendency to Form Complexes Reasons : - 1. Absence of low-lying vacant d-orbtals to accept lone pairs from ligands. For Na +, 1s 2, 2s 2, 2p 6, 3s, 3p, 3d High-lying relative to 2p For Fe 2+, 1s 2, 2s 2, 2p 6, 3s 2, 3p 3, 3d 6 Low-lying relative to 3p

57 Weak Tendency to Form Complexes Reasons : - 2. s-block cations (M +, M 2+ ) have relatively low charge densities  less polarizing and less able to accept lone pairs from ligands.

58 All six bonds are strong dative covalent bonds A complex ion, [Co(NH 3 ) 6 ] 3+ A hydrated ion, Na + (aq) Dipole-ion attraction Weaker than dative bond

59 Weak Tendency to Form Complexes Owing to its high charge density, Be 2+ can form complexes

60 [Be(H 2 O) 4 ] 2+ (aq) + H 2 O(l) [Be(H 2 O) 3 (OH)] + (aq) + H 3 O + (aq) [Be(H 2 O) 3 (OH)] + (aq) + H 2 O(l) [Be(H 2 O) 2 (OH) 2 ](s) + H 3 O + (aq) [Be(H 2 O) 2 (OH) 2 ](s) + H 2 O(l) [Be(H 2 O)(OH) 3 ]  (aq) + H 3 O + (aq) [Be(H 2 O)(OH) 3 ]  (aq) + H 2 O(l) [Be(OH) 4 ] 2  (aq) + H 3 O + (aq) [Be(H 2 O) 4 ] 2+ (aq) + 4H 2 O(l) [Be(OH) 4 ] 2  (aq) + 4H 3 O + (aq) Overall reaction : (1) + (2) + (3) + (4)

61 [Be(H 2 O) 4 ] 2+ (aq) + H 2 O(l) [Be(H 2 O) 3 (OH)] + (aq) + H 3 O + (aq) (1) [Be(H 2 O) 3 (OH)] + (aq) + H 2 O(l) [Be(H 2 O) 2 (OH) 2 ](s) + H 3 O + (aq) (2) [Be(H 2 O) 2 (OH) 2 ](s) + H 2 O(l) [Be(H 2 O)(OH) 3 ]  (aq) + H 3 O + (aq) (3) [Be(H 2 O)(OH) 3 ]  (aq) + H 2 O(l) [Be(OH) 4 ] 2  (aq) + H 3 O + (aq) (4) Overall reaction : (1) + (2) + (3) + (4) [Be(H 2 O) 4 ] 2+ (aq) + 4H 2 O(l) [Be(OH) 4 ] 2  (aq) + 4H 3 O + (aq) pH   equilibrium positions shifts to the right

62 [Be(H 2 O) 4 ] 2+ (aq) + H 2 O(l) [Be(H 2 O) 3 (OH)] + (aq) + H 3 O + (aq) (1) [Be(H 2 O) 3 (OH)] + (aq) + H 2 O(l) [Be(H 2 O) 2 (OH) 2 ](s) + H 3 O + (aq) (2) [Be(H 2 O) 2 (OH) 2 ](s) + H 2 O(l) [Be(H 2 O)(OH) 3 ]  (aq) + H 3 O + (aq) (3) [Be(H 2 O)(OH) 3 ]  (aq) + H 2 O(l) [Be(OH) 4 ] 2  (aq) + H 3 O + (aq) (4) (1) + (2) Be 2+ (aq) + 2OH  (aq) Be(OH) 2 (s) [Be(H 2 O) 4 ] 2+ (aq) + 2H 2 O(l) [Be(H 2 O) 2 (OH) 2 ](s) + 2H 3 O + (aq) + 2OH  (aq) [Be(H 2 O) 4 ] 2+ (aq) + 2OH  (aq) [Be(H 2 O) 2 (OH) 2 ](s) + 2H2O Or simply,

63 Characteristic Flame Colours of Salts Most s-block elements and their compounds give a characteristic flame colour in the flame test Group I element Flame colour Group II element Flame colour LiCrimsonBe- NaGolden yellowMgBright white KLilacCaBrick red RbBluish redSrBlood red CsBlueBaApple green

64 Mechanism : - 1.In the hotter part of the flame, 2.In the cooler part of the flame, Na(g) Na(g) * heat Na(g) * Na(g) cool [Ne] 3p 1 [Ne] 3s 1 Ground state [Ne] 3s 1 [Ne] 3p 1 + golden yellow light Visible region

65 Mechanism : - For salts of s-block elements, the metal ions of the salts are first converted to metal atoms Na + Cl  Na(g) + Cl(g) heat Na(g) Na(g) * heat Na(g) * Na(g) cool + golden yellow light Na 2 CO 3 (s) Na + Cl  (more volatile) Conc. HCl

66 Q.3 Na + (g) Na + (g) * heat Na + (g) * Na + (g) cool + uv light [He] 2s 2 2p 6 [He] 2s 2 2p 5 3s 1 [He] 2s 2 2p 6 2p 3s 3p visible uv

67 Li Na K Ca Pt or nichrome(an alloy of Ni and Cr) is suitable for making the wire because 1.They have no reaction with conc. HCl 2.They do not impart visible light when heated

68 Variation in Physical Properties of s-block Elements 1. Atomic Radius and Ionic Radius 2. Ionization Enthalpies 3. Hydration Enthalpies 4. Melting Points

69 1. Atomic Radius and Ionic Radius Group I element Atomic radius (nm) Group II element Atomic radius (nm) Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra  down the groups ∵ the outermost electrons are further away from the nuclei

70 1. Atomic Radius and Ionic Radius Group I element Atomic radius (nm) Group II element Atomic radius (nm) Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra Group II < Group I ∵ ENC  from left to right across the periods

71 On moving down the groups, first  sharply (e.g. from Li to K) then slowly (e.g. from K to Fr)

72 1. There is a sharp  in NC from 19 K to 37 Rb Outermost e  is drawn closer to the nucleus

73 2. The inner d-electrons (of Rb, Cs, Sr, Ba) have poor shielding effect on the outermost electrons  transition contraction

74 2. Ionization Enthalpy Group I element 1 st IE2 nd IE Group II element 1 st IE2 nd IE3 rd IE Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – Both atomic radius and ENC  down the groups Atomic radius is more important IE  down the groups

75 2. Ionization Enthalpy Group I element 1 st IE2 nd IE Group II element 1 st IE2 nd IE3 rd IE Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – For Group I elements, 2 nd IE >> 1 st IE because 1.the outer s-electron is well shielded by inner shell electrons

76 2. Ionization Enthalpy Group I element 1 st IE2 nd IE Group II element 1 st IE2 nd IE3 rd IE Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – For Group I elements, 2 nd IE >> 1 st IE because 2.the 2 nd electron is closer to the nucleus and is poorly shielded by other electrons in the same shell which is completely filled.

77 2. Ionization Enthalpy Group I element 1 st IE2 nd IE Group II element 1 st IE2 nd IE3 rd IE Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – For Group II elements, 3 rd IE >> 2 nd IE Similar reasons can be applied

78 Variations in the first and second ionization enthalpies of Group I elements

79 Variations in the first, second and third ionization enthalpies of Group II elements

80 2. Ionization Enthalpy Group I element 1 st IE2 nd IE Group II element 1 st IE2 nd IE3 rd IE Li Na K Rb Cs Fr – Be Mg Ca Sr Ba Ra – Group II > Group I ∵ The outer s-electrons of Group II atoms are closer to the nucleus and experience higher ENC

81 3. Hydration enthalpy Hydration enthalpy (  H hyd ) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions. M + (g) + aq  M + (aq)  H =  H hyd M 2+ (g) + aq  M 2+ (aq)  H =  H hyd  always has a negative value

82 Group I ion Hydration enthalpy (kJ mol –1 ) Group II ion Hydration enthalpy (kJ mol –1 ) Li + Na + K + Rb + Cs + Fr + –519 –406 –322 –301 –276 – Be 2+ Mg 2+ Ca 2+ Sr 2+ Ba 2+ Ra 2+ –2 450 –1 920 –1 650 –1 480 –1 360 –  down the groups ∵ charge density of metal ions  down the groups  attraction between ions and water molecules  +

83 Group I ion Hydration enthalpy (kJ mol –1 ) Group II ion Hydration enthalpy (kJ mol –1 ) Li + Na + K + Rb + Cs + Fr + –519 –406 –322 –301 –276 – Be 2+ Mg 2+ Ca 2+ Sr 2+ Ba 2+ Ra 2+ –2 450 –1 920 –1 650 –1 480 –1 360 – Group II > Group I ∵ Group II ions have higher charge and small size  higher charge density  stronger ion-dipole interaction

84 Variations in hydration enthalpy of the ions of Groups I and II elements

85 The melting points of s-block elements depend on the metallic bond strength which in turn depends on 1.charge density of cations 2.number of valence electrons participating in the sea of electrons 3.packing efficiency of the crystal lattices 4. Melting Point

86 Group I element Melting Point (  C) Group II element Melting Point (  C) Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra  down the groups ∵ ionic radii  down the groups  charge density   interaction between ions and electron sea 

87 Group I element Melting Point (  C) Group II element Melting Point (  C) Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra Group II > Group I ∵ (a)Group II cations have higher charge density (b)More valence electrons are involved in the sea of electrons (c)Packing efficiency : Group II > Group I

88 Reason not known !!

89 Variation in Chemical Properties s-Block elements have strong reducing power ∵ low ionization enthalpies low atomization enthalpies

90 Hydration enthalpy Atomization enthalpy M(g) Ionization enthalpy M + (g) ~E a M(s) M + (aq) M(s)  M + (aq) + e   H < 0

91 Hydration enthalpy Atomization enthalpy M(g) Ionization enthalpy M + (g) ~E a M(s) M + (aq) Reactivity : Na > Ca (depends on E a ) Position in e.c.s. : Ca > Na (depends on  H o or E o

92 Variation in Chemical Properties The reactivity of s-block elements  down the groups ∵ both I.E. and A.E.  down the groups  E a  down the groups  Reaction rate  down the groups

93 Variation in Chemical Properties Reactivity : Group I > Group II ∵ both I.E. and A.E.  across the periods  E a  across the periods  Reaction rate  across the periods

94 1. Reactions with hydrogen Group I 2M(s) + H 2 (g) 2MH(s) 300  C – 500  C Group II M(s) + H 2 (g) MH 2 (s) 600  C – 700  C

95 1. Reactions with hydrogen 4LiH + AlCl 3 LiAlH 4 + 3LiCl Dry ether Reducing agent in organic syntheses

96 Most s-block elements  show a silvery white lustre when they are freshly cut  they tarnish rapidly upon exposure to the atmosphere  they react with oxygen in the air to form an oxide layer 2. Reactions with Oxygen

97 Sodium shows a silvery white lustre when freshly cut

98 Group I (p.2) Group II 2M(s) + O 2 (g) 2MO(s) heat M(s) + O 2 (g) MO 2 (s) heat

99 3. Reactions with Chlorine Group I 2M(s) + Cl 2 (g) 2MCl(s) heat Group II M(s) + Cl 2 (g) MCl 2 (s) heat

Reactions with water or steam Group I 2M(s) + H 2 O(l) 2MOH(aq) + H 2 (g) heat Group II M(s) + 2H 2 O(l) M(OH) 2 (aq) + H 2 (g) heat Mg reacts with steam but not water Be has no reaction with either water or steam Mg(s) + H 2 O(g) MgO(s) + H 2 (g) heat

101 Variation in chemical properties of the compounds of s-block elements Reactions of oxides Reactions of hydrides Reactions of chlorides

102 Reactions of oxides M 2 O(s) + H 2 O(l)  2MOH(aq) M 2 O 2 (s) + 2H 2 O(l)  2MOH(aq) + H 2 O 2 (aq) 2MO 2 (s) + 2H 2 O(l)  2MOH(aq) + H 2 O 2 (aq) + O 2 (g) Group I 1. Reactions with water

103 Na 2 O 2 is used in qualitative analysis of Cr 3+ 2Cr(OH) 3 (s) + 3Na 2 O 2 (s)  2Na 2 CrO 4 (aq) + 2NaOH(aq) + 2H 2 O(l) green yellow

104 Reactions of oxides CaO(s) + H 2 O(l)  Ca(OH) 2 (aq) SrO(s) + H 2 O(l)  Sr(OH) 2 (aq) BaO(s) + H 2 O(l)  Ba(OH) 2 (aq) MgO(s) + H 2 O(l) Mg(OH) 2 (aq) slightly soluble BeO(s) + H 2 O(l)  No reaction Group II increasing basicity

105 Reactions of oxides M 2 O(s) + 2HCl(aq)  2MCl(aq) + H 2 O(l) M 2 O 2 (s) + 2HCl(aq)  2MCl(aq) + H 2 O 2 (aq) 2MO 2 (s) + 2HCl(aq)  2MCl(aq) + H 2 O 2 (aq) + O 2 (g) Group I 2. Reactions with acids Group II MO(s) + 2HCl(aq)  MCl 2 (aq) + H 2 O(l) More vigorous than those with water

106 Reactions of oxides 3. Reactions with alkalis Reaction with water instead except BeO BeO(s) + 2OH  (aq) + H 2 O(l)  Be(OH) 4 2  (aq) amphoteric

107 Reactions of hydrides MH(s) H 2 O or NaOH(aq) MOH(aq) + H 2 (g) MCl(aq) + H 2 (g) HCl(aq) H  (a strong base) tends to react with protonic reagents to release H 2 Reactivity  down the groups More vigorous

108 Reactions of chlorides No significant reactions with water, acids or alkalis Group I Group II Do not undergo significant hydrolysis except BeCl 2 and MgCl 2 BeCl 2 (aq) + 2H 2 O(l)  Be(OH) 2 (aq) + 2HCl(aq) MgCl 2 (aq) + H 2 O(l)  Mg(OH)Cl(aq) + HCl(aq) Basic salt More favoured in alkaline solutions

109 Relative Thermal Stability of the Carbonates and Hydroxides of s-Block Elements Thermal stability refers to the resistance of a compound to undergo decomposition on heating.

110 Thermal decomposition reactions Metal carbonates M 2 CO 3 (s) M 2 O(s) + CO 2 heat MCO 3 (s) MO(s) + CO 2 heat Metal hydroxides 2MOH(s) M 2 O(s) + H 2 O(g) heat M(OH) 2 (s) MO(s) + H 2 O heat

111 Relative thermal stability can be measured in two ways 1. By comparing the decomposition temperatures A higher decomposition temperature  a greater thermal stability

112 Metal carbonate BeCO 3 MgCO 3 CaCO 3 SrCO 3 BaCO 3 Decomposition temperature /  C ~ Decomposition temperature is the temperature at which the pressure of CO 2 in equilibrium with the solid carbonate reaches 1 atm in a closed system. Below the DT, some CO 2 can still be detected but the pressure is less than 1 atm

113 Example: 1. The Carbonates BeCO 3 (s) BeO(s) + CO 2 (g) MgCO 3 (s) MgO(s) + CO 2 (g) CaCO 3 (s) CaO(s) + CO 2 (g)SrCO 3 (s) SrO(s) + CO 2 (g)BaCO 3 (s) BaO(s) + CO 2 (g)

114 Relative thermal stability can be measured in two ways 2. By comparing the standard enthalpy changes of thermal decomposition reactions A more positive  H value  a thermally more stable compound M(OH) 2 (s)  MO(s) + H 2 O(g)  H > 0

115 Metal hydroxide Be(OH) 2 Mg(OH) 2 Ca(OH) 2 Sr(OH) 2 Ba(OH) 2  H o / kJ mol  Trends : - 1.  down the groups 2.Group I > Group II 3.Li resembles Mg more than the other group 1 elements (diagonal relationship, pp.14-15)

The Hydroxides Be(OH) 2 (s)BeO(s) + H 2 O(g)  H = +54 kJ mol –1 Mg(OH) 2 (s)MgO(s) + H 2 O(g)  H = +81 kJ mol –1 Ca(OH) 2 (s)CaO(s) + H 2 O(g)  H = +109 kJ mol –1 Sr(OH) 2 (s)SrO(s) + H 2 O(g)  H = +127 kJ mol –1 Ba(OH) 2 (s)BaO(s) + H 2 O(g)  H = +146 kJ mol –1

117 Factors affecting thermal stability of carbonates and hydroxides 1.Polarizing power of cation 2.Polarizability of polyatomic anion 3.Lattice enthalpy of metal oxide produced

118 Interpretation of trends in thermal stability of carbonates and hydroxides 1.Group I > Group II (a)M 2+ ions have higher charge densities than M + ions  M 2+ ions are more polarizing than M + ions  Can polarize more the electron cloud of polyatomic anions

119 M 2+ MO + CO 2 M 2+ heat MO + H 2 O polarization

120 Polarizability  as the size of anion 

121 Polyatomic ion Thermal decomposition

122 When a compound with large anions undergoes thermal decomposition, a compound with small anions will be formed since small anions are less easily polarized

123 Simple ion more stable compound with stronger bond

124 M 2+ S2S2 M S polarization Stronger ionic bond with covalent character Simple ion

125 Interpretation of trends in thermal stability of carbonates and hydroxides 1.Group I > Group II (b)M 2+ ions have higher charge densities than M + ions  Lattice enthalpy : MO > M 2 O  Energetic stability : MO > M 2 O

126 CaCO 3 (s) CaO(s) + CO 2 (g) heat Na 2 CO 3 (s) Na 2 O(s) + CO 2 (g) more favourable less favourable heat more stable less stable Thermal stability of carbonates : - Group I > Group II

127 Interpretation of trends in thermal stability of carbonates and hydroxides 2.Thermal stability  down the groups ∵ s ize of cations  down the groups ∴ (a)charge density/polarizing power of cation  down the groups (b)lattice enthalpies of MO/M 2 O  down the groups

128 MgCO 3 (s) MgO(s) + CO 2 (g) heat more favourable more stable BaCO 3 (s) BaO(s) + CO 2 (g) heat less favourable less stable more polarized less polarized Thermal stability of carbonates : -  down the groups

129 Effect of sizes of the cations on thermal stability of the carbonates and hydroxides of both Groups I and II metals

130 Interpretation of trends in thermal stability of carbonates and hydroxides 3.Li compounds resemble Mg compounds (diagonal relationship) Charge density/polarizing power : - Li +  Mg 2+

131 Interpretation of trends in thermal stability of carbonates and hydroxides 4.Thermal stability of nitrates follows similar patterns (Optional) 2MNO 3 (s) 2MNO 2 + O 2 heat 2M(NO 3 ) 2 (s) 2MO + 4NO 2 + O 2 heat

132 Relative Solubility of the Sulphates(VI) and Hydroxides of s-Block Elements In general, Group I >> Group II

133 Compounds Solubility / mol per 100 of water Mg(OH)  10  3 Ca(OH)  10  3 Sr(OH)  10  3 Ba(OH) 2 15  10  3 Compounds Solubility / mol per 100 of water MgSO  10  4 CaSO 4 11  10  4 SrSO  10  4 BaSO  10  4 Q.4 Size and/or charge of the anion   Polarizability of anion   Covalent character   Solubility in water  In general,

134 Compounds Solubility / mol per 100 of water Mg(OH)  10  3 Ca(OH)  10  3 Sr(OH)  10  3 Ba(OH) 2 15  10  3  down the group Compounds Solubility / mol per 100 of water MgSO  10  4 CaSO 4 11  10  4 SrSO  10  4 BaSO  10  4  down the group

135 Two processes are 1.the breakdown of the ionic lattice 2.the subsequent stabilization of the ions by water molecules (this process is called hydration) 1. Processes involved in Dissolution and their Energetics

136 1.the breakdown of the ionic lattice 2.the subsequent stabilization of the ions by water molecules (this process is called hydration) NaCl(s)  Na + (g) + Cl  (g) Na + (g) + Cl  (g) + aq  Na + (aq) + Cl  (aq)  H 2 = (hydration enthalpy) < 0  H 1 = (  lattice enthalpy) > 0

137  H solution -  H L = +776 kJ mol  1  H hydration = -772 kJ mol  1 = ( ) kJ mol  1 = +4 kJ mol  1

138 If, we expect the solids to dissolve in water Solubility  as becomes more –ve (less +ve) Solids (e.g. NaCl) with small +ve values are also soluble in water if the dissolution involves an increase in the entropy of the system.

139  Spontaneous dissolution is always positive Dissolution with slightly positive can be spontaneous

140 Trends and Interpretations 1. The solubility of Group(II) sulphate decreases down the group On moving down the group, cationic radius(r + )  both and become less -veHowever,  less rapidly than

141 Trends and Interpretations  constant less –ve down the group  +ve constant less –ve down the group  Solubility  down the group

142 Trends and Interpretations  constant  more rapidly down the group  less rapidly down the group less –ve down the group  Solubility  down the group (-ve) (+ve)

143 Trends and Interpretations 2. The solubility of Group(II) hydroxides increases down the group On moving down the group, cationic radius(r + )  both and become less -veHowever,  more rapidly than

144 Trends and Interpretations  less rapidly down the group  more rapidly down the group more –ve down the group  Solubility  down the group (-ve) (+ve) less +ve down the group

145 For s-block compounds with small anions (e.g. OH , F  ), solubility in water  down the group For s-block compounds with large anions (e.g. SO 4 2 , CO 3 2- ), solubility in water  down the group For s-block compounds with medium size anions (e.g. Br  ), solubility in water exhibits irregular pattern down the group

146 Compounds Solubility / mol per 100 of water MgBr  10  1 CaBr  10  1 SrBr  10  1 BaBr  10  1 Irregular Solublily : First  and then  (-ve) (+ve) First  more rapidly Then  more rapidly First less +ve down the group Then less -ve down the group

147 Group II compounds with doubly-charged anions (MX) are less soluble than those with singly-charged anions (MY 2 ) Reasons : 1.  H L of MX >  H L of MY 2 2.  H L is the major factor affecting solubility   H solution of MX is more positive  Solubility : MX < MY 2

148 Solubility : Group I > Group II Reasons : For a given anions, both  H L and  H hydration become more –ve from Group I to Group II However,  H L is the major factor affecting solubility   H solution : Group I is less positve than Group II  Solubility : Group I > Group II

149 Diagonal relationship

150 Reaction Other Group I elements LithiumMagnesium Combination with O 2 Peroxides and superoxides Li 2 O (normal oxide)MgO (normal oxide) Combination with N 2 No reactionLi 3 NMg 3 N 2 Action of heat on carbonate No reaction (thermally stable) Decomposes to give Li 2 O and CO 2 Decomposes to give MgO and CO 2 Action of heat on hydroxide No reaction (thermally stable) Decomposes to give Li 2 O and H 2 O Decomposes to give MgO and H 2 O Action of heat on nitrate Decomposes to give MNO 2 and O 2 Decomposes to give Li 2 O, NO 2 and O 2 Decomposes to give MgO, NO 2 and O 2 Hydrogen carbonatesExist as solidsOnly exist in solution Solubility of salts in water Most salts are more soluble than those of Li, Mg. Fluoride, hydroxide, carbonate, phosphate, ethanedioate are sparingly soluble. Solubility of salts in organic solvents. Halides only slightly soluble in organic solvents Halides (with covalent character) dissolve in organic solvents

151 The END

152 Metals are sometimes referred to as electropositive elements. Why? Answer They have low electronegativity values. Back 40.1 Characteristic Properties of the s-Block Elements (SB p.40)

153 s-Block compounds give a characteristic flame colour in the flame test. Based on this, can you give one use of s-block compounds? Answer s-Block compounds can be used in fireworks. Back 40.1 Characteristic Properties of the s-Block Elements (SB p.46)

154 (a)Which ion has a greater ionic radius, potassium ion or calcium ion? Explain your answer. Answer (a)Potassium ion (0.133 nm) has a greater ionic radius than calcium ion (0.099 nm). In fact, potassium ion and calcium ion are isoelectronic and have the same number of electron shells. However, calcium ion has one more proton than potassium ion, the electron cloud of calcium ion will experience greater attractive forces from the nucleus. This leads to a smaller ionic radius of calcium ion Characteristic Properties of the s-Block Elements (SB p.48)

155 (b)Explain why Group I elements show a fixed oxidation state of +1 in their compounds in terms of ionization enthalpies. Answer 40.1 Characteristic Properties of the s-Block Elements (SB p.48)

156 (b)Group I elements form ions with an oxidation state of +1 only. It is because they have only one outermost shell electron. Once this outermost shell electron is removed, a stable fully-filled electronic configuration is obtained. Therefore, the first ionization enthalpies of Group I elements are low. The second ionization involves the removal of an electron from an inner electron shell. Once this electron is removed, the stable electronic configuration will be disrupted. Therefore, their second ionization enthalpies are very high. As a result, Group I elements form predominantly ionic compounds with non-metals by losing their single outermost shell electron, and they form ions having a fixed oxidation state of Characteristic Properties of the s-Block Elements (SB p.48)

157 (c)Ions of Group I and Group II elements have a very low tendency to form complexes. Give one reason to explain your answer. Answer (c)As ions of Group I and Group II elements do not have low-lying vacant orbitals available for forming dative covalent bonds with the lone pair electrons of surrounding ligands, they rarely form complexes Characteristic Properties of the s-Block Elements (SB p.48)

158 (d)Give one test which would enable you to distinguish a sodium compound from a potassium compound. Answer (d)Sodium compounds and potassium compounds can be distinguished by conducting a flame test. In the flame test, sodium compounds give a golden yellow flame, while potassium compounds give a lilac flame. Back 40.1 Characteristic Properties of the s-Block Elements (SB p.48)

159 What is a dative covalent bond? How is it formed? Answer A dative covalent bond is a covalent bond in which the shared pair of electrons is supplied by only one of the bonded atoms. A dative covalent bond is formed by the overlapping of an empty orbital of an atom with an orbital occupied by a lone pair of electrons of another atom. Back 40.1 Characteristic Properties of the s-Block Elements (SB p.48)

160 (a)(i)List the factors that affect the value of the ionization enthalpy of an atom. Answer (a)(i)There are four main factors affecting the magnitude of the ionization enthalpy of an atom. They are the electronic configuration of an atom, the nuclear charge, the screening effect, and the atomic radius Variation in Properties of the s-Block Elements (SB p.56)

161 (a) (ii)Why is ionization enthalpy of an atom always positive? Answer (a)(ii)Ionization enthalpy of an atom always has a positive value because energy is required to overcome the attractive forces between the nucleus and the electron to be removed Variation in Properties of the s-Block Elements (SB p.56)

162 (a) (iii)Describe the general trend of the first and second ionization enthalpies down Group I of the Periodic Table. Answer 40.2 Variation in Properties of the s-Block Elements (SB p.56)

Variation in Properties of the s-Block Elements (SB p.56) (a)(iii)The first ionization enthalpies of Group I elements are relatively low. The outermost s electron is located in a new electron shell. The attractive force between this s electron and the nucleus is relatively weak. Also, this s electron is effectively shielded from the attraction of the nucleus by the fully-filled inner electron shells. Once this electron is removed, a stable octet or duplet electronic configuration is obtained. Consequently, this s electron is relatively easy to be removed, and hence the first ionization enthalpies of Group I elements are relatively low. However, the second ionization of Group I elements involves the loss of an inner shell electron which is closer to the nucleus. The removal of this electron disrupts the stable electronic configuration. Therefore, the second ionization enthalpies of Group I elements are extremely high.

164 (b)(i)List the factors that affect the value of the hydration enthalpy of an ion. Answer (b)(i)The value of the hydration enthalpy of an ion depends on the size and the charge of the ion Variation in Properties of the s-Block Elements (SB p.56)

165 (b) (ii)Why does hydration enthalpy of an ion always have a negative value? Answer 40.2 Variation in Properties of the s-Block Elements (SB p.56) (b)(ii)Hydration enthalpy of an ion always has a negative value because it is the amount of energy released resulting from the attraction between the ion and water molecules.

166 (b) (iii)Describe the general trend of the hydration enthalpy down Group II of the Periodic Table. Answer 40.2 Variation in Properties of the s-Block Elements (SB p.56) (b)(iii)Going down Group II, the hydration enthalpy of the ions decreases (becomes less negative). Since the ions get larger in size on moving down the group, the charge density of the ions falls. As a result, the electrostatic attraction between the ions and water molecules becomes weaker, and the hydration enthalpy becomes less negative down the group. Back

167 The burning of lithium, sodium and potassium in oxygen gives different types of oxides. Why do the metals behave differently? Answer 40.2 Variation in Properties of the s-Block Elements (SB p.57)

168 On burning in air, lithium forms only lithium oxide, and it does not form the peroxide or superoxide. This is because the size of lithium ion is very small, leading to its high polarizing power. When a peroxide ion or superoxide ion approaches a lithium ion, the electron cloud of the peroxide ion or superoxide ion (large in size) would be greatly distorted by the lithium ion. The greater the distortion of the electron cloud, the lower the stability of the compound. That is why lithium peroxide and lithium superoxide do not exist. Sodium ion has a larger size than lithium ion. Its lower polarizing power allows it to form the peroxide when sodium is burnt in air. Potassium ion has a much larger size, so it has relatively low polarizing power. The electron cloud of the peroxide ion or superoxide ion would not be seriously distorted by potassium ion. This allows the peroxide ions or superoxide ions to pack around potassium ion with a higher stability. As a result, potassium is able to form stable peroxide or superoxide on burning in air. Back 40.2 Variation in Properties of the s-Block Elements (SB p.57)

169 (a)Suggest a reason why the reaction of lithium with water is less vigorous than those of sodium and potassium. Answer (a)The reactivity of Group I metals with water is related to the relative ease of the metal atoms to lose the outermost shell electron. Going down the group, as the atomic size increases, the outermost shell electron becomes easier to be removed. Therefore, the reactivity of Group I metals towards water increases down the group. Lithium reacts with water vigorously. Sodium reacts with water violently and moves on the water surface with a hissing sound Variation in Properties of the s-Block Elements (SB p.58)

170 (b)Which element is the strongest reducing agent, calcium, strontium or barium? Answer (b)Barium is the strongest reducing agent. It is because the reducing power of an element is related to the ease of the atom to lose the outermost shell electron. Since barium has larger atomic sizes, its outermost shell electrons are less firmly held by the nucleus. Therefore, barium has a higher tendency to lose its outermost shell electrons than both calcium and strontium Variation in Properties of the s-Block Elements (SB p.58) Back

171 The value of  H soln of a solid does not indicate whether the solid is soluble in water or not. So how can we predict the solubility of a solid in water? Answer Back 40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64) Generally speaking, for a solid to be soluble in water, its enthalpy change of solution has to be a negative or a small positive value.

172 (a)Give balanced chemical equations for the following reactions: (i)Thermal decomposition of barium carbonate (ii)Reaction between sodium peroxide and water (iii)Reaction between calcium oxide and dilute hydrochloric acid Answer 40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65) (a)(i)BaCO 3 (s)  BaO(s) + CO 2 (g) (ii)Na 2 O 2 (s) + 2H 2 O(l)  2NaOH(aq) + H 2 O 2 (aq) (iii)CaO(s) + 2HCl(aq)  CaCl 2 (aq) + H 2 O(l) 

173 (b)Suggest a reason why barium sulphate(VI) is insoluble in water, while potassium sulphate(VI) is soluble in water although they have cations of similar sizes and the same anion. (The ionic radii of potassium ion and barium ion are nm and nm respectively.) Answer 40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)

174 (b)When an ionic solid dissolves in water, two processes are taking place. They are the breakdown of the ionic lattice and the subsequent stabilization of the ions by water molecules. The enthalpy change involved in the whole dissolution process is known as the enthalpy change of solution,  H soln, which is equal to  H soln =  H hyd –  H lattice. For an ionic compound to be soluble in water, the enthalpy change of solution has to be a negative or a small positive value. The reason why barium sulphate(VI) is insoluble in water while potassium sulphate(VI) is soluble in water is that potassium ion has a smaller charge than barium ion. The  H lattice of potassium sulphate(VI) is smaller in magnitude (less negative) than that of barium sulphate(VI). As a result, the enthalpy change of solution of potassium sulphate(VI) is more negative, and hence it is soluble in water while barium sulphate(VI) is not Variation in Properties of the Compounds of the s-Block Elements (SB p.65)

175 (c)Compare the solubility of calcium sulphate(VI) and barium sulphate(VI) in water. Explain your answer. Answer (c)Calcium sulphate(VI) is expected to be more soluble than barium sulphate(VI). It is because calcium ion has a smaller size than barium ion. This causes the  H hyd of calcium sulphate(VI) to be more negative than that of barium sulphate(VI). As a result, the  H soln of calcium sulphate(VI) becomes more negative than that of barium sulphate(VI), and hence calcium sulphate(VI) is more soluble in water than barium sulphate(VI) Variation in Properties of the Compounds of the s-Block Elements (SB p.65) Back