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Chapter 41 The s-Block Elements 41.1 Introduction 41.2 Characteristic Properties of the s-Block Elements 41.3 Variation in Properties of the s-Block Elements 41.4 Variation in Properties of the Compounds of the s-Block Elements 41.5 Uses of the Compounds of the s-Block Elements 1 New Way Chemistry for Hong Kong A-Level Book 4 1 41.1 Introduction (SB p.40) s-Block elements: • Consists of Group IA and Group IIA elements • Outermost shell electrons in s orbitals • Highly reactive metals • Good reducing agents • Fixed oxidation states +1 for Group I elements +2 for Group II elements 2 New Way Chemistry for Hong Kong A-Level Book 4 2 41.1 Introduction (SB p.41) Group I elements: Lithium Rubidium 3 Sodium Potassium Caesium New Way Chemistry for Hong Kong A-Level Book 4 3 41.1 Introduction (SB p.41) Calcium Group II elements: Beryllium Strontium 4 Magnesium Barium New Way Chemistry for Hong Kong A-Level Book 4 Radium 4 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Some characteristic properties of Group I metals Group I Electronic Electrometal configuration negativity Oxidation no. in compound Oxide formed Hydroxide formed Flame colour deep red yellow lilac bluish red blue — Li Na K Rb [He]2s1 [Ne]3s1 [Ar]4s1 [Kr]5s1 1.0 0.9 0.8 0.8 +1 +1 +1 +1 Li2O Na2O2 K2O2, KO2 Rb2O2,RBO2 LiOH NaOH KOH RbOH Cs Fr [Xe]6s1 [Rn]7s1 0.7 — +1 — Cs2O2,CsO2 — CsOH — 5 New Way Chemistry for Hong Kong A-Level Book 4 5 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Some characteristic properties of Group II elements Group II Electronic Electrometal configuration negativity Oxidation no. in compound Be Mg Ca Sr [He]2s2 [Ne]3s2 [Ar]4s2 [Kr]5s2 1.5 1.2 1.0 1.0 +2 +2 +2 +2 BeO MgO CaO SrO, SrO2 Ba Ra [Xe]6s2 [Rn]7s2 0.9 — +2 — BaO, BaO2 Ba(OH)2 — — 6 Oxide formed New Way Chemistry for Hong Kong A-Level Book 4 Hydroxide formed Be(OH)2 Mg(OH)2 Ca(OH)2 Sr(OH)2 Flame colour none none brick red blood red or crimson green — 6 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Metallic Character Group I elements: • Silvery in colour, tarnish rapidly in air ∴ keep immersed under paraffin oil or in vacuum sealed ampoules • 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 7 New Way Chemistry for Hong Kong A-Level Book 4 7 41.2 Characteristic Properties of the s-Block Elements (SB p.43) 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 41.2 Characteristic Properties of the s-Block Elements (SB p.43) Group II elements: • greyish in colour • harder and higher boiling and melting points (compared to 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 41.2 Characteristic Properties of the s-Block Elements (SB p.44) 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 10 New Way Chemistry for Hong Kong A-Level Book 4 10 41.2 Characteristic Properties of the s-Block Elements (SB p.44) Low Electronegativity • All have low electronegativity values ∵ the outermost s electrons are effectively shielded by inner electron shells. The tendency of losing electrons is relatively high. • Electronegativity values decrease when going down the group ∵ the outermost shell electrons are further and further away from nucleus 11 New Way Chemistry for Hong Kong A-Level Book 4 11 41.2 Characteristic Properties of the s-Block Elements (SB p.44) Group II elements are more electronegative than Group I counterparts ∵ higher nuclear charge, stronger attraction to outermost shell electrons i.e. more difficult to remove the electrons 12 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 12 41.2 Characteristic Properties of the s-Block Elements (SB p.45) 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 13 1 O2 2 2– peroxide O2 2O2– superoxide New Way Chemistry for Hong Kong A-Level Book 4 13 41.2 Characteristic Properties of the s-Block Elements (SB p.45) • 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) • 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) 14 New Way Chemistry for Hong Kong A-Level Book 4 14 41.2 Characteristic Properties of the s-Block Elements (SB p.45) • Rb, Cs also forms superoxides 3000C Rb2O2(s) + O2(g) 2RbO2(s) Cs2O2(s) + O2(g) 3000C 2CsO2(s) 15 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 15 41.2 Characteristic Properties of the s-Block Elements (SB p.46) • Li does not form peroxides or superoxides Reason: Li+ is small high polarizing power serious distortion on electron cloud of peroxide or superoxide more distortion , more unstable Li2O2 and LiO2 do not exist • K+, Rb+ and Cs+ ions are large Low polarizing power peroxides and superoxides are stable 16 New Way Chemistry for Hong Kong A-Level Book 4 16 41.2 Characteristic Properties of the s-Block Elements (SB p.46) Group II Elements • Form normal oxides only, except Sr, Ba which can form peroxides • All are basic (except BeO which is amphoteric) 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) 17 2BaO2(s) New Way Chemistry for Hong Kong A-Level Book 4 17 41.2 Characteristic Properties of the s-Block Elements (SB p.46) Group II element Normal oxide Peroxide Superoxide Be Mg Ca Sr Ba BeO MgO CaO SrO BaO — — — SrO2 BaO2 — — — — — Beryllium peroxide (BeO2) does not exist Reason: Be2+ is small high polarizing power serious distortion on electron cloud of the peroxide ion polarizing power of Be2+ > Li+, due to smaller size, higher charge BeO2 does not exist 18 New Way Chemistry for Hong Kong A-Level Book 4 18 41.2 Characteristic Properties of the s-Block Elements (SB p.47) Ba cannot form superoxide while K can Reason: polarizing power of Ba2+ > K+ high polarizing power more serious distortion on electron cloud of the superoxide ion Ba(O2)2 does not exist 19 New Way Chemistry for Hong Kong A-Level Book 4 19 41.2 Characteristic Properties of the s-Block Elements (SB p.47) Formation of Hydroxides • All Group I metals (except Li) 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) All Group I oxides react with H2O to form metal hydroxides General equation: M2O(s) + H2O(l) 2MOH(aq) M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) 2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g) 20 New Way Chemistry for Hong Kong A-Level Book 4 20 41.2 Characteristic Properties of the s-Block Elements (SB p.47) • All Group II metals (except Be & Mg) react with H2O to form metal hydroxides and H2 gas e.g. Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g) Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g) • Be does not react with H2O(l or g) • Mg does not react with H2O(l) but with H2O(g) Mg(s) + H2O(g) MgO(s) + H2(g) 21 Ca reacts with H2O readily at room temperature New Way Chemistry for Hong Kong A-Level Book 4 21 41.2 Characteristic Properties of the s-Block Elements (SB p.48) • The reactivity increases down the group. • The oxides of Ca, Sr, Ba react with H2O(l) to give hydroxides CaO(s) + H2O(l) Ca(OH)2(aq) SrO(s) + H2O(l) Sr(OH)2(aq) BaO(s) + H2O(l) Ba(OH)2(aq) 22 • MgO dissolves in acids to form salts but is slightly soluble in water • BeO is insoluble in both acids and water New Way Chemistry for Hong Kong A-Level Book 4 22 41.2 Characteristic Properties of the s-Block Elements (SB p.48) Bonding and Oxidation State in Compounds • s-Block elements form compounds that are predominantly ionic in nature • Oxidation state of Group I metals must be +1 Reason: • only 1 electron in the outermost s orbitals once this e- is removed, a stable electronic configuration is obtained 1st I.E. is low • The 2nd e- is removed from the stable octet 2nd I.E. is very high ∴ Group I metals predominantly form ions with a fixed oxidation number +1 23 New Way Chemistry for Hong Kong A-Level Book 4 23 41.2 Characteristic Properties of the s-Block Elements (SB p.48) Chemical formulae of some Group I compounds Oxidation number of Group I element in the compounds Group I element Oxide Li Li2O LiH LiCl +1 Na Na2O2 NaH NaCl +1 K KO2 KH KCl +1 Rb RbO2 RbH RbCl +1 Cs CsO2 CsH CsCl +1 24 Hydride Chloride New Way Chemistry for Hong Kong A-Level Book 4 24 41.2 Characteristic Properties of the s-Block Elements (SB p.49) • Oxidation state of Group II metals must be +2 Reason: • only 2 electrons in the outermost s orbitals once these 2eare removed, a stable electronic configuration is obtained sum of 1st I.E. and 2nd I.E. is low • the 3rd e- is removed from the stable octet 3rd I.E. is very high ∴ Group II metals predominantly form ions with a fixed oxidation number +2 25 New Way Chemistry for Hong Kong A-Level Book 4 25 41.2 Characteristic Properties of the s-Block Elements (SB p.49) Chemical formulae of some Group II compounds Group II Oxide Hydride Chloride element 26 Oxidation number of Group II element in the compounds Be BeO BeH2 BeCl2 +2 Mg MgO MgH2 MgCl2 +2 Ca CaO CaH2 CaCl2 +2 Sr SrO SrH2 SrCl2 +2 Ba BaO BaH2 BaCl2 +2 New Way Chemistry for Hong Kong A-Level Book 4 26 41.2 Characteristic Properties of the s-Block Elements (SB p.49) Weak Tendency to Form Complexes A complex is a polyatomic ion or neutral molecule formed when molecular or ionic groups (called ligands) form dative covalent bonds with a central metal atom or cation. e.g. 27 New Way Chemistry for Hong Kong A-Level Book 4 27 41.2 Characteristic Properties of the s-Block Elements (SB p.50) • Complex formation is common in d-block elements ∵ d-block metal ions utilize their low-lying vacant d-orbitals to accept the lone pair electrons from the surrounding ligands a complex is formed 28 • Since s-block metal ions do not possess low-lying vacant d-orbitals, they do not form complexes • When s-block metal ions are surrounded by polar molecules, there is only electrostatic attraction between the positive ion and the negative ends of the dipoles New Way Chemistry for Hong Kong A-Level Book 4 28 41.2 Characteristic Properties of the s-Block Elements (SB p.50) Characteristic Flame Colours of Salts • Many s-block elements can give characteristic flame colours in the flame test • 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 As the energy is quantized, the flame colour is a characteristic property of the element 29 New Way Chemistry for Hong Kong A-Level Book 4 29 41.2 Characteristic Properties of the s-Block Elements (SB p.50) Group I element Li Na K Rb Cs (a) (b) Flame colour Deep red Golden yellow Lilac Bluish red Blue (c) Group II element Ca Sr Ba Flame colour Brick red Blood red or crimson Green (d) (a) lithium (b) sodium (c) potassium (d) calcium 30 New Way Chemistry for Hong Kong A-Level Book 4 30 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (a) Which ion has a greater ionic radius, potassium ion or calcium ion? Give reasons for your choice. Answer (a) K+ ion (0.133 nm) has a greater ionic radius than Ca2+ ion (0.099 nm). In fact, K+ and Ca2+ ions are isoelectronic and have the same number of electron shells. However, because Ca2+ ion has one more proton than K+ ion, the electrons of Ca2+ ion will experience a greater attractive force from the nucleus. This leads to a smaller ionic radius of Ca2+ ion. 31 New Way Chemistry for Hong Kong A-Level Book 4 31 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (b) Explain why s-block elements are highly electropositive. Answer (b) All s-block elements are highly electropositive. This means that they lose their electrons easily. The reason is that the outermost shell electrons of the s-block elements are effectively shielded from the nucleus by the fullyfilled inner electron shells. So the electrons are less firmly held by the nucleus, and hence easily to be removed. 32 New Way Chemistry for Hong Kong A-Level Book 4 32 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (c) Explain why alkali metals show a fixed oxidation state of +1 in(c) their compounds in terms of ionization enthalpies. Alkali metals have the [ ] ns1 electronic configuration. The last s electron enters a new electron shell which is much Answer further away from the nucleus. Hence, the attractive force from the nucleus holding this electron is relatively weak. Also, this s electron is shielded from the attraction of the nucleus by the fully-filled inner electron shells. Moreover, once this electron is removed, a stable electronic configuration (octet) is attained. Consequently, the first ionization enthalpies of alkali metals are relatively low. Besides, as their second ionization enthalpies involve the removal of an electron from a fully-filled electron shell, their second ionization enthalpies are relatively large. As a result, alkali metals show a fixed oxidation state of +1 in all their compounds. 33 New Way Chemistry for Hong Kong A-Level Book 4 33 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (d) Give one test which would enable you to distinguish a sodium compound from a potassium compound. Answer (d) By conducting the flame test, sodium compounds will give a golden yellow flame colour, whereas potassium compounds will give a lilac flame colour. 34 New Way Chemistry for Hong Kong A-Level Book 4 34 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (e) Ions of alkali metals and alkaline earth metals have very low tendency to form complexes. Give one reason to account for this. Answer (e) Since ions of alkali metals and alkaline earth metals do not have low-lying vacant orbitals for forming dative covalent bonds with the lone pair electrons of surrounding ligands, they rarely form complexes. 35 New Way Chemistry for Hong Kong A-Level Book 4 35 41.3 Variation in Properties of the s-Block Elements (SB p.51) Variation in Physical Properties Atomic Radius and Ionic Radius Group I element Atomic radius (nm) Ionic radius (nm) Li 0.152 0.060 Be 0.112 0.031 Na 0.186 0.095 Mg 0.160 0.065 K 0.231 0.133 Ca 0.197 0.099 Rb 0.244 0.148 Sr 0.215 0.113 Cs 0.262 0.169 Ba 0.217 0.135 36 Atomic Group II radius element (nm) New Way Chemistry for Hong Kong A-Level Book 4 Ionic radius (nm) 36 41.3 Variation in Properties of the s-Block Elements (SB p.52) Variations in atomic radius and ionic radius of Groups I and II elements 37 New Way Chemistry for Hong Kong A-Level Book 4 37 41.3 Variation in Properties of the s-Block Elements (SB p.52) Observations: • ionic radius of any Group I or II element is smaller than the atomic radius ∵ after losing the outermost shell electron(s), there is one electron shell less in the cation than in the atom, electrons are hold more strongly by nucleus electron cloud contracts 38 New Way Chemistry for Hong Kong A-Level Book 4 38 41.3 Variation in Properties of the s-Block Elements (SB p.52) • the atomic and ionic radii increase down the Groups ∵ more and more electron shells occupied, 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 39 New Way Chemistry for Hong Kong A-Level Book 4 39 41.3 Variation in Properties of the s-Block Elements (SB p.52) • atomic and ionic radii decrease when going from Group I to II in each period ∵ Group II elements have 1 more proton and electron than Group I elements, increase in the number of protons leads to greater attractive force to hold electrons more strongly, but increase in the number of electrons does not lead to the increase in repulsive force between electrons and shielding effect atomic and ionic radii decrease 40 New Way Chemistry for Hong Kong A-Level Book 4 40 41.3 Variation in Properties of the s-Block Elements (SB p.53) Ionization Enthalpy 1st 2nd Group I ionization ionization element enthalpy enthalpy (kJ mol–1) (kJ mol–1) Li Na K Rb Cs Fr 41 519 494 418 402 376 381 7 300 4 560 3 070 2 370 2 420 — 1st 2nd Group II ionization ionization element enthalpy enthalpy (kJ mol–1) (kJ mol–1) Be Mg Ca Sr Ba Ra 900 736 590 548 502 510 New Way Chemistry for Hong Kong A-Level Book 4 1 760 1 450 1 150 1 060 966 979 3rd ionization enthalpy (kJ mol–1) 14 800 7 740 4 940 4 120 3 390 — 41 41.3 Variation in Properties of the s-Block Elements (SB p.54) Variations in the 1st and 2nd ionization enthalpies of Group I elements 42 New Way Chemistry for Hong Kong A-Level Book 4 42 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 43 New Way Chemistry for Hong Kong A-Level Book 4 43 41.3 Variation in Properties of the s-Block Elements (SB p.55) Observations: • 1st I.E. of Group I elements are low and the 2nd I.E. are extremely high Reason: • The outermost s electron enters a new electron shell which is further away from nucleus, and is shielded by inner electron shells the attractive force is weak, easy to be removed • The second electron to be removed is from the inner electron shell removal of 2nd electron will disrupt the stable electronic configuration 2nd I.E. are extremely high 44 New Way Chemistry for Hong Kong A-Level Book 4 44 41.3 Variation in Properties of the s-Block Elements (SB p.55) • 1st and 2nd I.E. of Group II elements are low but the 3rd I.E. are much higher Reason: • The outermost s electrons are further away from the nucleus and effectively shielded by inner electron shells attractive force is weak, easy to be removed • The 3rd electron to be removed is from inner electron shells removal of 3rd electron will disrupt the stable electronic configuration 3rd I.E. are much higher 45 New Way Chemistry for Hong Kong A-Level Book 4 45 41.3 Variation in Properties of the s-Block Elements (SB p.55) • the ionization enthalpies decrease down the Groups Reason: • atomic sizes increase down the group the outermost shell electron(s) will be better shielded by inner electron shells less attractive force experienced less energy is required to remove the electrons 46 New Way Chemistry for Hong Kong A-Level Book 4 46 41.3 Variation in Properties of the s-Block Elements (SB p.55) Melting Point • Melting points of Groups I and II elements depend on strength of metallic bonds • The stronger the bond, the higher is the melting point • Metallic bond strength depends on: (1) ionic radius, (2) no. of valence electrons 47 New Way Chemistry for Hong Kong A-Level Book 4 47 41.3 Variation in Properties of the s-Block Elements (SB p.55) Melting points of Groups I and II elements 48 Group I element Melting point (C) Group II element Melting point (C) Li 180 Be 1 280 Na 97.8 Mg 350 K 63.7 Ca 850 Rb 38.9 Sr 768 Cs 28.7 Ba 714 Fr 27 Ra 697 New Way Chemistry for Hong Kong A-Level Book 4 48 41.3 Variation in Properties of the s-Block Elements (SB p.55) Variations in melting points of Groups I and II elements 49 New Way Chemistry for Hong Kong A-Level Book 4 49 41.3 Variation in Properties of the s-Block Elements (SB p.56) Observations: • melting point decreases as going down Groups I and II Reason: • the ionic size of the elements increases • the no. of electrons in the delocalized electron sea remains unchanged charge density decreases • attraction between ions and electrons becomes weaker • metallic bond is weaker 50 New Way Chemistry for Hong Kong A-Level Book 4 50 41.3 Variation in Properties of the s-Block Elements (SB p.56) • melting points of Group II elements are much higher than those of Group I elements Reason: • no. of electrons per mole contributed to the delocalized electron sea by Group II elements is greater than those by Group I elements • Group II elements have shorter ionic radii • charge density of Group II elements is higher • the attractive force between ions and electrons are stronger • metallic bond is stronger 51 New Way Chemistry for Hong Kong A-Level Book 4 51 41.3 Variation in Properties of the s-Block Elements (SB p.56) • irregularity in the general decrease in melting point down Group II elements • melting point of Mg is lower than that of Ca Reason: • different metallic crystal structures of the Group II elements 52 New Way Chemistry for Hong Kong A-Level Book 4 52 41.3 Variation in Properties of the s-Block Elements (SB p.56) Hydration Enthalpy Hydration enthalpy (Hhyd) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions. 53 • Hhyd must be negative value as it is the energy released resulting from the attraction between water molecules and ions • Hhyd depends on charge density 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 53 41.3 Variation in Properties of the s-Block Elements (SB p.57) Hydration enthalpies of Groups I and II metal ions 54 Group I metal ion Hydration enthalpy (kJ mol–1) Group II metal ion Hydration enthalpy (kJ mol–1) Li+ – 519 Be2+ – 2 450 Na+ – 406 Mg2+ – 1 920 K+ – 322 Ca2+ – 1 650 Rb+ – 301 Sr2+ – 1 480 Cs+ – 276 Ba2+ – 1 360 Fr+ — Ra2+ — New Way Chemistry for Hong Kong A-Level Book 4 54 41.3 Variation in Properties of the s-Block Elements (SB p.57) Variations in hydration enthalpy of Groups I and II elements 55 New Way Chemistry for Hong Kong A-Level Book 4 55 41.3 Variation in Properties of the s-Block Elements (SB p.58) Observations: • hydration enthalpies become smaller 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 and less negative 56 New Way Chemistry for Hong Kong A-Level Book 4 56 41.3 Variation in Properties of the s-Block Elements (SB p.58) • 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 • the hydration enthalpies of Group II ions are more negative than those of Group I ions 57 New Way Chemistry for Hong Kong A-Level Book 4 57 41.3 Variation in Properties of the s-Block Elements (SB p.58) Check Point 41-2 (a) Why is hydration enthalpy always negative? Answer (a) Hydration enthalpy is a negative value as it is the amount of energy released resulting from the attraction between ions and water molecules. 58 New Way Chemistry for Hong Kong A-Level Book 4 58 41.3 Variation in Properties of the s-Block Elements (SB p.58) Check Point 41-2 (cont’d) (b) List the factors that affect the value of the hydration enthalpy of an ion. Answer (b) The value of hydration enthalpy of an ion depends on the charge and size of the ion. 59 New Way Chemistry for Hong Kong A-Level Book 4 59 41.3 Variation in Properties of the s-Block Elements (SB p.58) Check Point 41-2 (cont’d) (c) How does hydration enthalpy affect the solubility of an ionic compound? Answer (c) Ionic compounds with a large negative value of hydration enthalpy are expected to be more soluble in water. It is because the more negative the value of hydration enthalpy, the stronger is the attraction between the ions and water molecules. This means that the ions have a high solubility in water. 60 New Way Chemistry for Hong Kong A-Level Book 4 60 41.3 Variation in Properties of the s-Block Elements (SB p.58) Variation in Chemical Properties • s-Block elements are good reducing agents ∵ they have low I.E. The lower the I.E., the stronger is the reducing power • The reducing power increases down the group ∵ atomic size increases down the group, ionization enthalpy decreases down the group • Group I metals react readily with water, dilute acids and most non-metals • Group II metals are less reactive than Group I metals because of their greater I.E. 61 New Way Chemistry for Hong Kong A-Level Book 4 61 41.3 Variation in Properties of the s-Block Elements (SB p.58) Reaction with Hydrogen • All Group I metals react with H2(g) between 300C and 500C to form white crystalline compounds, called metal hydrides e.g. 2Na(s) + H2(g) 2NaH(s) • All Group II metals (except Be) react with H2(g) between 600C and 700C to form metal hydrides e.g. Ca(s) + H2(g) CaH2(s) 62 New Way Chemistry for Hong Kong A-Level Book 4 62 41.3 Variation in Properties of the s-Block Elements (SB p.58) • Reactivity of Groups I and II metals increases down the group • Most of s-block metal hydrides are ionic except BeH2 and MgH2 are predominantly covalent 63 New Way Chemistry for Hong Kong A-Level Book 4 63 41.3 Variation in Properties of the s-Block Elements (SB p.59) Reaction with Oxygen 64 • s-Block elements show a silvery white lustre when they are freshly cut, but tarnish upon exposure to air • All s-block elements burn in air to form one or more of the following oxides: • Li forms Li2O ; Na forms Na2O, Na2O2 ; K, Rb and Cs can form normal oxides, peroxides and superoxides New Way Chemistry for Hong Kong A-Level Book 4 64 41.3 Variation in Properties of the s-Block Elements (SB p.59) • Group II metals usually form normal oxides • Be forms BeO; Mg forms MgO; Sr and Ba can form normal oxides and peroxides Burning magnesium 65 Magnesium oxide New Way Chemistry for Hong Kong A-Level Book 4 65 41.3 Variation in Properties of the s-Block Elements (SB p.60) Oxides of the s-block elements 66 Metals that form oxides Type of oxide Formula Normal oxide O2– All Groups I and II elements Peroxide O22– Na, K, Rb, Cs, Sr, Ba Superoxide O2– K, Rb, Cs when exposed or burnt in air New Way Chemistry for Hong Kong A-Level Book 4 66 41.3 Variation in Properties of the s-Block Elements (SB p.60) Check Point 41-3 The products of burning lithium, sodium and potassium in oxygen are Li2O, Na2O2 and KO2 respectively. How do you down Group I, the sizes of of the increase. As the explain Going the different behaviour thecations metals? charge to radius ratios of the cations decrease, they become less Answer polarizing. Among the lithium, sodium and potassium ions, the polarizing power of lithium ion is the highest as it has the highest charge to radius ratio. Besides, among the three types of oxide ions (i.e. normal oxide, peroxide and superoxide ions), the polarizability of superoxide ion is the highest as it has the largest size. When a cation of high polarizing power approaches an anion of high polarizability, the electron cloud of the anion will be greatly distorted by the cation, and thus the compound formed will be unstable. As a result, Li tends to form the normal oxide Li2O, Na tends to form the peroxide Na2O2, and K tends to form the superoxide KO2. 67 New Way Chemistry for Hong Kong A-Level Book 4 67 41.3 Variation in Properties of the s-Block Elements (SB p.60) Reaction with Chlorine • All Group I metals react with Cl2 to form ionic solids with high melting point e.g. 2Na(s) + Cl2(g) 2NaCl(s) • All Group II metals (except Be) directly combine with Cl2 to form ionic chlorides e.g. Mg(s) + Cl2(g) MgCl2(s) • Be react with Cl2 when heated to form covalent chloride due to high charge density 68 Reaction between Na and Cl New Way Chemistry for Hong Kong A-Level Book 4 68 41.3 Variation in Properties of the s-Block Elements (SB p.60) Reaction with Water • All Group I metals react with cold water, forming hydroxides and hydrogen e.g. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) • The reduction of water by Group I metals involves the following half reaction: 2H2O(l) + 2e– 2OH– (aq) + H2(g) 69 New Way Chemistry for Hong Kong A-Level Book 4 69 41.3 Variation in Properties of the s-Block Elements (SB p.61) • The reactivity of Group I metals with water depends on the relative ease of the metals to donate the outermost shell electrons • Down the group atomic size increases easier to lose the outermost shell electron 70 New Way Chemistry for Hong Kong A-Level Book 4 70 41.3 Variation in Properties of the s-Block Elements (SB p.61) Li reacts with H2O vigorously 71 Na reacts with H2O violently K reacts with H2O almost explosively New Way Chemistry for Hong Kong A-Level Book 4 71 41.3 Variation in Properties of the s-Block Elements (SB p.61) 72 • Group II elements are less reactive than Group I elements • e.g. Be does not react with water Mg(s) + H2O(g) MgO(s) + H2(g) (vigorous) Mg(s) + 2H2O(l) Mg(OH)2(aq) + H2(g) (very slow) Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g) (moderate) Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g) (vigorous) New Way Chemistry for Hong Kong A-Level Book 4 72 41.3 Variation in Properties of the s-Block Elements (SB p.61) Check Point 41-4 (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 corresponds to the relative ease of the metals to donate the outermost shell electrons. Among all Group I metals, Li has the smallest atomic size, so its outermost shell electron is the most firmly held by the nucleus. It has a relatively low tendency to donate its outermost shell electron. Hence, Li reacts gently with water. 73 New Way Chemistry for Hong Kong A-Level Book 4 73 41.3 Variation in Properties of the s-Block Elements (SB p.61) Check Point 41-4 (cont’d) (b) Which element is the best reducing agent, Ca, Sr or Ba? Answer (b) Ba is the best reducing agent among the three elements. It is because the reducing power of an element depends on the ease of donating its outermost shell electrons. As Ba has the largest size among the three elements, its outermost shell electrons are less firmly held by the nucleus. Therefore, Ba has a higher tendency to donate its outermost shell electrons than the other two, and it is the best reducing agent among the three elements. 74 New Way Chemistry for Hong Kong A-Level Book 4 74 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.62) 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) Dissolution of Na2O2 in H2O containing phenolphthalein • Superoxides: e.g. 2KO2(s) + 2H2O(l) 2KOH(aq) + H2O2(aq) + O2(g) • The basicity of all Group I oxides increases down the group 75 New Way Chemistry for Hong Kong A-Level Book 4 75 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.62) • 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 and is insoluble in water or acids BeO(s) + 2H+(aq) Be2+(aq) + H2O(l) hot BeO(s) + 2OH–(aq) + H2O(l) [Be(OH)4]2–(aq) • MgO is slightly soluble in water, but dissolves in acids to form salts 76 New Way Chemistry for Hong Kong A-Level Book 4 76 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.63) • The reactivity of Group II peroxides increases down the group e.g. BaO2(s) + 2H2O(l) Ba(OH)2(aq) + H2O2(aq) • Metal peroxides and metal superoxides are powerful oxidizing agents because they give hydrogen peroxide as product • Metal peroxides and metal superoxides are useful for qualitative and quantitative analysis e.g. 2Cr(OH)3(s) + 3Na2O2(s) 2Na2CrO4(aq) + 2NaOH(aq) + 2H2O(l) 77 New Way Chemistry for Hong Kong A-Level Book 4 77 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.63) Reaction with Acids 78 • 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 78 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.63) 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) 79 New Way Chemistry for Hong Kong A-Level Book 4 79 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.63) Reactions of Hydrides of s-Block Elements • All Groups I and II metal hydrides react with H2O to form metal hydroxides and hydrogen gas • Their reactivity increases down the group e.g. NaH(s) + H2O(l) NaOH(aq) + H2(g) CaH2(s) + 2H2O(l) Ca(OH)2(aq) + 2H2(g) • The reaction is due to highly reactive hydride ion (H–) CaH2 reacts with H2O to give H2 and OH- H–(aq) + H2O(l) OH–(aq) + H2(g) 80 New Way Chemistry for Hong Kong A-Level Book 4 80 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.64) Reactions of Chlorides of s-Block Elements • The chlorides of Group I metals are ionic, soluble in water, no hydrolysis occur e.g. • LiCl(s) Li+(aq) + Cl–(aq) LiCl is the only deliquescent chloride, while other Group I chlorides are anhydrous • Group II metal chlorides exhibit some covalent character ∵ the electronegativity values of Group II metals are higher than those of Group I metals The degree of ionic character increases down the group 81 New Way Chemistry for Hong Kong A-Level Book 4 81 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.64) • BeCl2 is covalent and hydrolyzed by water readily to form the oxide BeCl2(s) + H2O(l) BeO(s) + 2HCl(g) • MgCl2 is slightly hydrolyzed in water MgCl2(s) + H2O(l) Mg(OH)Cl(s) + HCl(aq) • Heating the hydrated crystal will give off HCl and result in a basic salt MgCl2 • 6H2O(s) Mg(OH)Cl(s) + HCl(g) + 5H2O(l) 82 New Way Chemistry for Hong Kong A-Level Book 4 82 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.64) 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 & • 83 (2) sizes of ions The greater the charge to size ratios of ions, the higher is the thermal stability of the compound New Way Chemistry for Hong Kong A-Level Book 4 83 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.65) • Compound with large polarizable anion, the thermal stability depends on the polarizing power 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 84 New Way Chemistry for Hong Kong A-Level Book 4 84 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.65) • Most carbonates and hydroxides of Group I and II metals readily undergo decomposition on heating to give oxides e.g. MgCO3(s) MgO(s) + CO2(g) Ca(OH)2(s) CaO(s) + H2O(g) 85 New Way Chemistry for Hong Kong A-Level Book 4 85 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.65) • Oxide ion is smaller than CO32– and OH–, the charge density is higher Oxide ion is less polarizable The attraction between cation and anion is stronger The compound is more stable Groups I and II metal carbonates and hydroxides decompose to form metal oxides 86 New Way Chemistry for Hong Kong A-Level Book 4 86 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.65) • 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 87 New Way Chemistry for Hong Kong A-Level Book 4 87 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.65) • 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 88 New Way Chemistry for Hong Kong A-Level Book 4 88 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.66) The Carbonates • All Group I carbonates (except Li2CO3) can withstand a temperature around 800°C 700°C Li2CO3(s) Li2O(s) + CO2(g) • Li2CO3 is less stable than other Group I carbonates ∵ Li is the smallest and charge density is the highest Electron cloud of the carbonate ion is distorted to greater extent ∴ decompose more easily 89 New Way Chemistry for Hong Kong A-Level Book 4 89 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.66) • Group II metal ions has higher polarizing power their carbonates are less stable e.g. 100°C BeCO3(s) BeO(s) + CO2(g) 900°C CaCO3(s) CaO(s) + CO2(g) 1 360°C BaCO3(s) BaO(s) + CO2(g) • Going down the group, size of the cation increases polarizing power decreases compounds with large cations are more stable 90 New Way Chemistry for Hong Kong A-Level Book 4 90 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.66) The Hydroxides • All Group I hydroxides (except LiOH) are thermal stable 2LiOH(s) Li2O(s) + H2O(g) • The Group II metal hydroxides are less stable than those of Group I metals Be(OH)2(s) BeO(s) + H2O(g) H = +54 kJ mol–1 • 91 Ca(OH)2(s) CaO(s) + H2O(g) H = +109 kJ mol–1 Ba(OH)2(s) BaO(s) + H2O(g) H = +146 kJ mol–1 The thermal stability of Group II metal hydroxides increases down the group New Way Chemistry for Hong Kong A-Level Book 4 91 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.67) Relative Solubility of the Sulphates(VI) and Hydroxides Processes involved in Dissolution and their Energetics • When an ionic solid is dissolved in water, two processes are taken place: 1. Breakdown of the ionic solid 2. Stabilization of ions by water molecules (called hydration) • 1st process involves an input of energy to breakdown the lattice • 2nd process involves a release of energy when ions are hydrated (i.e. new bonds are formed between water molecules and ions) 92 New Way Chemistry for Hong Kong A-Level Book 4 92 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.67) • Take NaCl as an example: NaCl(s) Na+(aq) + Cl–(aq) • Hsoln = +4 kJ mol–1 This change involves 2 processes: 1st : NaCl(s) Na+(g) + Cl–(g) H = +776 kJ mol–1 The enthalpy change involved in this process is the reverse of lattice enthalpy. The lattice enthalpy is –776 kJ mol–1. 2nd : Na+(g) + Cl–(g) Na+(aq) + Cl–(aq) Hhyd = –772 kJ mol–1 This is the enthalpy change resulted from hydration of one mole of both gaseous ions 93 New Way Chemistry for Hong Kong A-Level Book 4 93 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.68) According to Hess’s law, The enthalpy change of solution is : Hsoln = Hhyd – Hlattice For a salt to be soluble in water, the Hsoln has to be a –ve or small positive value 94 New Way Chemistry for Hong Kong A-Level Book 4 94 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.68) 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 larger size and smaller charge 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 95 New Way Chemistry for Hong Kong A-Level Book 4 95 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) • For Group II metal sulphates(VI), cations are much smaller than anions • The Hlattice is determined by the reciprocal of the sum of cationic and anionic radii (i.e. 1 r r ) Large anionic radius makes the relatively small cationic radius insignificant w.r.t the sum of r+ and r– Down the group, increase in cationic size does not make a significant change in the Hlattice 96 New Way Chemistry for Hong Kong A-Level Book 4 96 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) • However, an increase in cationic size causes Hhyd to become less and less negative Hsoln becomes less and less exothermic The solubility of Group II metal sulphates(VI) decreases down the group 97 New Way Chemistry for Hong Kong A-Level Book 4 97 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) For Group II metal hydroxides, the sizes of cation and anion are in the same order of magnitude As H lattice 1 r r Going down the group, cationic size increases Less energy is required to break the lattice change in Hhyd is comparatively small Hsoln becomes more and more negative The solubility of Group II metal hydroxides increases down the group 98 New Way Chemistry for Hong Kong A-Level Book 4 98 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) Check Point 41-5 (a) Give balanced chemical equations for the following reactions: (i) decomposition of barium carbonate; (ii) reaction between sodium hydride and water; (iii) reaction between sodium peroxide and water. Answer (a) (i) BaCO3(s) BaO(s) + CO2(g) (ii) NaH(s) + H2O(l) NaOH(aq) + H2(g) (iii) Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(aq) 99 New Way Chemistry for Hong Kong A-Level Book 4 99 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) Check Point 41-5 (cont’d) (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 0.133 nm and 0.135 nm respectively.) Answer 100 New Way Chemistry for Hong Kong A-Level Book 4 100 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) (b) When an ionic solid is dissolved in water, two processes are taking place. They are the break down of the ionic solid (this is related to the lattice enthalpy, Hlattice), and subsequent stabilization of the ions by water molecules (this is related to the hydration enthalpy, Hhyd). The enthalpy change involved in the whole dissolution process is known as the enthalpy change of solution, Hsoln , which is equal to Hsoln = Hhyd – Hlattice. 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 whereas potassium sulphate(VI) is soluble in water is that potassium ion has a smaller charge than barium ion, and thus the Hlattice 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 whereas barium sulphate(VI) is not. 101 New Way Chemistry for Hong Kong A-Level Book 4 101 41.4 Variation in Properties of the compounds of the s-Block Elements (SB p.69) Check Point 41-5 (c) Compare the solubility of CaSO4 and BaSO4. Explain your answer. Answer (c) CaSO4 is expected to be more soluble than BaSO4. It is because Ca2+ has a smaller size than Ba2+, this causes the Hhyd of CaSO4 to be more negative than that of BaSO4. As a result, the Hsoln of CaSO4 becomes more negative than that of BaSO4 and hence CaSO4 is more soluble in water than BaSO4. 102 New Way Chemistry for Hong Kong A-Level Book 4 102 41.5 Uses of the Compounds of the s-Block Elements (SB p.69) Sodium Carbonate • Fusing sodium carbonate with calcium carbonate and silica to form silicates at 1500°C • Soda glass is a mixture of sodium silicate and calcium silicate Na2CO3(s) + SiO2(s) Na2SiO3(s) + CO2(g) CaCO3(s) + SiO2(s) CaSiO3(s) + CO2(g) 103 Soft glass is made by fusing SiO2 with Na2CO3 and CaCO3 New Way Chemistry for Hong Kong A-Level Book 4 103 41.5 Uses of the Compounds of the s-Block Elements (SB p.70) • Na2CO3 is used in water softening ∵ CO32– can precipitate Mg2+(aq) and Ca2+(aq) in hard water Na2CO3(aq) + Mg2+(aq) MgCO3(s) + 2Na+(aq) Na2CO3(aq) + Ca2+(aq) CaCO3(s) + 2Na+(aq) • A large amount of Na2CO3 is used in sewage treatment • Na2CO3 is also used in paper industry and in making soap and caustic alkalis 104 Washing powder contains Na2CO3 New Way Chemistry for Hong Kong A-Level Book 4 104 41.5 Uses of the Compounds of the s-Block Elements (SB p.70) Sodium Hydrogencarbonate • NaHCO3 is found in baking powder which also contains a solid acid • On adding water, the acid reacts with NaHCO3 to give CO2(g) • NaHCO3 decomposes at high temperatures to give off CO2(g) • The CO2(g) makes the cake rise and spongy HCO3–(s) + H+(aq) H2O(l) + CO2(g) 2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(l) • 105 Soft drinks are also made from NaHCO3 New Way Chemistry for Hong Kong A-Level Book 4 105 41.5 Uses of the Compounds of the s-Block Elements (SB p.70) Sodium Hydroxide • In saponification, fats and oils are hydrolyzed by NaOH to form soap 106 New Way Chemistry for Hong Kong A-Level Book 4 106 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) • NaOH is used to make soaps, detergents, dyes, paper and drugs Products manufactured from NaOH • 107 NaOH is also used in the manufacture of rayon and some very important organic compounds such as phenol and naphthol New Way Chemistry for Hong Kong A-Level Book 4 107 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) Magnesium Hydroxide • • Mg(OH)2 is a weak base and slightly soluble in water Mg(OH)2 is a good antacid (Milk of magnesia) to cure gastric pain by neutralizing excess HCl Mg(OH)2(s) + 2HCl(aq) MgCl2(aq) + 2H2O(l) Advantages: 1. Relatively insoluble, not absorbed by body but remain to act whatever acid is present 2. No burping due to no CO2 produced whereas hydrogencarbonate antacids do 108 108 New Way Chemistry for Hong Kong A-Level Book 4 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) Calcium Oxide and Calcium Carbonate • CaCO3 decomposes to CaO (quicklime) on heating • Addition of water to quicklime produces Ca(OH)2 (slaked lime) CaCO3(s) CaO(s) + CO2(g) CaO(s) + H2O(l) Ca(OH)2(s) • Slaked lime is used to neutralize the acids in industrial effluents Ca(OH)2(s) + 2H+(aq) Ca2+(aq) + 2H2O(l) 109 New Way Chemistry for Hong Kong A-Level Book 4 109 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) Strontium Compounds • The main use of strontium compounds is to make fireworks because they give a persistent and intense red flame on burning 110 New Way Chemistry for Hong Kong A-Level Book 4 110 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) Check Point 41-6 (a) How can sodium hydroxide be obtained on a large scale in industry? State two important industrial applications of sodium hydroxide. Answer (a) Sodium hydroxide can be obtained by electrolysis of saturated sodium chloride solution. Sodium hydroxide is used to manufacture soaps, detergents, dyes, rayon and some important organic compounds such as phenol and naphthol. 111 New Way Chemistry for Hong Kong A-Level Book 4 111 41.5 Uses of the Compounds of the s-Block Elements (SB p.71) Check Point 41-6 (cont’d) (b) Suggest compounds of cations to be used together with magnesium powder in fireworks to give crimson, brick red, yellow, green and lilac colours respectively. Answer (b) Fireworks should include lithium ion, calcium ion, sodium ion, barium ion and potassium ion. 112 New Way Chemistry for Hong Kong A-Level Book 4 112 The END 113 New Way Chemistry for Hong Kong A-Level Book 4 113