The s-Block Elements

صفحه 1:
The s-Block Elements Chapter 39 

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Members of the s-Block Elements ۲ IA Alkali metals IIA Alkaline Earth metals 

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Chapter summary * Characteristic properties of the s- block elements ¢ Variation in properties of the s-block elements * Variation in properties of the s-block compounds * Uses of compounds of the s-block elements 

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Characteristic properties of s-block elements * Metallic character * Low electronegativity * Basic oxides, hydroxides ¢ Ionic bond with fixed oxidation states * Characteristic flame colours * Weak tendency to from complex 

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Metallic character Tod ‏و و وه‎ ۵ 

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Electronegativity Group II Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Group I 1۸ ‏لا‎ ‎Na 0.9 1 0.8 Rb 0.8 Cs 0.7 Fr 0 * Low nuclear attraction for outer electrons ۰ Highly electropositive * Small electronegativi ty 

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Basic oxides, hydroxides Hydroxid es Be(OH), Mg(OH), Ca(OH), Sr(OH), Ba(OH), Oxide BeO MgO CaO SrO BaO, Ba,O, Hydroxid es LiOH NaOH KOH RbOH Oxide Li,O Na,O, Na,O, K,O,, KO, Rb,O,, RbO, 

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Oxides, Peroxide, Superoxide Reaction with water: Oxide: 0» + H,O > 20H: Peroxide: 0, + 2H,O > H,O, + 20H Superoxide: 20, + 2H,O > 20H: + H,O, + O, a :0:.0: Li does not form peroxide or super oxide Li,O, > Li,O + % O, Peroxide ion Super oxide 

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Hydroxides —$§$<$§ — All are soluble, base strength increase. Group II hydroxide Be Mg Ca Sr _ Ba Solubility increase, from Amphoteric to basic, base strength increase 

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Predominantly ionic with fixed oxidation state Group I: Most electropositive metals. Low first I.E. and extremely high second I.E. Form predominantly ionic compounds with non-metals by losing one electron. Fixed oxidation state of +1. Group II: Electropositive metals. Low first and second I.E. but very high third LE.. Have a fixed oxidation state of +2. Be and Mg compounds possess some degree of covalent character. 

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Characteristic flame colours Nat Cl (g) > Na(g) + Cl(g) Na(g) — Na* (g) [Ne]3s! [Ne]3p! Na‘(g) — Na(g) + hv (589nm, yellow) 

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Flame test Ca brick red Sr blood red Ba apple green <> HCl(aq) sample 

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Weak tendency to form complex Complex formation is a common feature of d-block element. e.g. Co(NH,),°* :NH, s-block metal ions | have nolowenergy H;Nx_ | _/:NH, vacant orbital available ‏ا‎ ‎for bonding with lone pairs :NH,; of surrounding ligands, they rarely farm cramniavac 

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Check point 39-1 

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Variation in properties of elements ¢ Atomic radii * Ionization enthalpies ¢ Hydration enthalpies ٠ Melting points ¢ Reactions with oxygen, water, hydrogen and chlorine 

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Ra Atomic radii (nm) Li ه ده واه ب-اه یاو نب دز ۳ ‎Ny NU bh‏ هم ‎By of af‏ إمم N N Be Mg Ca Sr Ba 

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Ionization Enthapy 3rd LE. 14800 7740 4940 4120 3390 2nd LE. 1760 1450 1150 1060 966 1st LE. 900 736 590 548 502 2nd 1.4 Group Be Mg Ca Sr Ba 7 4 3 2 2 1st LE. 519 494 418 402 376 

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Ionization Enthalpy 2000 ae 1500 nd IE 1000 Ca*—__ Bat 500 ‏تج‎ 1st LE. 

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Ionization Enthalpy Group I 1. Have generally low 1*' I.E. as it is well shielded from the nucleus by inner shells. 2. Removal of a 2" electron is much more difficult because it involves the removal of inner shell electron. 3. LE. decreases as the group is descended. As atomic radius increases, the outer e is further away from the well- 

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Ionization Enthalpy Group IT 1. Have low 1st and 2nd IE. 2. Removal of the 3™ electron is much more difficult as it involves the loss of an inner shell electron. 3. IE decrease as the group is descended. 4. IE of the group II is generally higher than group I. 

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Hydration Enthalpy M*(g) + aqueous > M*(aq) + heat -600 we 

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Hydration Enthalpy -2250 -600) -2000 -1750 -300} -1500) — 8 8 a ee Lit Na* K* Rb* Cs* Be** Mg? Ca®* Sr* Ba®* 

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Hydration Enthalpy sneral trends: On going down both groups, hydration enthalpy decreases. (As the ions get larger, the charge density of the ions decreases, the electrostatic attraction between ions and water molecules gets smaller.) . Group 2 ions have hydration enthalpies higher than group 1. ( Group 2 cations are doubly charged and have smaller sizes) 

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Variation in Melting Points 

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Variation in Melting Points Strength of metallic bond depends on: 1. Ionic radius 2. Number of e contributed to the electron sea per ator 3. Crystal lattice structure Note: The exceptionally high m.p. of calcium is due to contribution of d-orbital participation of metallic bonding. 

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Variation in Melting Points Group |Structu II re Be ‏سا‎ ‎Mg 1162 Ca CCP, Sr CCP. Ba B.C.C. 

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Reactions with oxygen S-block elements are strong reducing agents. Their reducing power increases down both groups. (As the atomic size increases, it becomes easier to remove the outermost electron) S-block elements reacts readily with oxygen. Except Be and Mg, they have to be stored under liquid paraffin to prevent contact with the atmosphere. 

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Reactions with oxygen Normal | Peroxide |Superoxi Oxide de Structur we De ‏موی به‎ ae e 3 :0-0: :0:.0: Formed |Li and Na and K, Rb, Cs by Group II |Ba 

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Check point 39-2 

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Reaction with water M(s) > M+(aq) + & H,O(1) + e — OH(aq) + % H,(g) Li -3.05 volt Na -2.71 1 3 Be -1.85 volt Rb -2.99 Mg -2.38 Cs -3.20 Ca -2.87 Sr -2.89 Ba -2.90 Energetic vs. Kinetic Factor 

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Reaction with hydrogen All the s-block elements except Be react directly with hydrogen. 2Na(s) + H,(g) - 2NaH(s) Ca(s) + H,(g) > CaH,(s) The reactivity increases down the group. Only BeH, and MgH, are covalent, others are ionic. 

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Reaction with chlorine All the s-block metals react directly with chlorine to produce chloride. All group I chlorides are ionic. BeCl, is essentially covalent, with comparatively low m.p. The lower members in group II form essentially ionic chlorides, with Mg having intermediate properties. 

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Check point 39-3 Although lithium has highly negative E®, it only reacts slowly with water. This illustrates the importance of the role of kinetic factors in determining the rate of a chemical reaction. Lithium has a higher m.p., this increases the activation energy required for dissolution in aqueous solution. It does not melt during the reaction as Na and K do, and thus it has a smaller area of contact with water. 

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Variation in properties of the compounds * Reactions of oxides and hydroxides * Reactions of chlorides * Reactions of hydrides * Relative thermal stability of carbonates and hydroxides * Relative solubility of sulphate(VI and hydroxde 

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Reactions of oxides and hydroxides 1. All group I oxides reacts with water to form hydroxides Oxide: O> + H,O > 20H: Peroxide: 0,2 + 2H,O > H,O, + 20H ‎WAL 8 ATTA 7 ATI. +‏ حك عع سويح ‎2. All group I oxides/hydroxides are basic and the basicity increases down the group. 

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Reactions of oxides and hydroxides 

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Reactions of chlorides 1. All group I chlorides are ionic and readily soluble in water. No hydrolysis occurs. 2. Group II chlorides show some degree of covalent character. Beryllium chloride is covalent and hydrolysis to form Be(OH),(s) and HCl(aq). Magnesium chloride is intermediate, it dissolves and hydrolysis slightly. Other group II chlorides just dissolve without hydrolysis. 

صفحه 38:
Reactions of hydrides They all react readily with water to give the metal hydroxide and hydrogen due to the strong basic property of the hydride ion, H: H:(s)+ H,O(1) > H,(g)+ OH (aq) Hydride ions are also good reducing agent. They can be used to prepare complex hydrides such as LiAlH, and NaBH, which are used to reduce C=O in organic chemistry. 

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Thermal Stability 

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Thermal Stability of carbonates Li,CO, > Li,O + CO, (at 700°C) All other group I carbonates are stable at ~800°C BeCO, > BeO + CO, (at 100°C) MgCO, > MgO + CO, (at 540°C) CaCO, 0620 + CO, (at 900°C) SrCO,>SrO+CO, (at 1290°C) BaCO,— BaO + CO, (at 1360°C) 

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Thermal Stability of hydroxides All group I hydroxides are stable except LiOH at Bunsen temperature. Be(OH),(s) > BeO(s) + H,O(g) AH = +54 kJ/mol Mg(OH),(s) > MgO(s) + H,O(g) AH = +81 kJ/mol Ca(OH),(s) > CaO(s) + H,O(g) AH = +109 kJ/mol Sr(OH),(s) > SrO(s) + H,O(g) AH = +127 kJ/mol Ba(OH),(s) > BaO(s) + H,O(g) AH = +146 kJ/mol 

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Thermal stability 1. Carbonates and hydroxides of Group I metals are as a whole more stable than those of Group II. 2. Thermal stability increases on descending the group. 3. Lithium often follow the pattern of Group II rather than Group I. This is an example of the diagonal relationship. 

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Explanation of Thermal Stability . Charge of the ions . Size of the ions . Compounds are more stable if the charge increases and size decreases. . For compounds with large polarizable anions, thermal stability is affected by the polarizing power of the cations. جر رحج تن 

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Explanation of Thermal Stability Decréasin Increasing polarizing stability pow 

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Explanation of Thermal Stability 9 _ >“ و00 + 07*0۶2 مت کت +92 ie OH Mgt |? ___. Mg?* 0? + H,O ‏تم‎ 

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Explanation of Thermal lan Stability ( Ww) ‏ووه ع‎ ‏یم‎ 

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Relative solubility of Group II hydroxides hydroxides wn the group. Solubility increases Solubility / mol per 100g water 0.020 x 10% 1.5x 10° 3.4x 103 15x 108 Compou nd Mg(OH), Ca(OH), Sr(OH), Ba(OH), 

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Solubility of Group II sulphates Compou | Solubility / mol per nd 100g water MgSO, 3600 x 10% 2 Solubility of sulphates CaSO, 11x 104 increases up the group. SrSO, 0.62 x 104 BaSO, 0.009 x 10+ 

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Explanation of solubility aque 1۳۹9 ‏ره‎ + X(aq) FT sotation -AH jattice AH iyaration M*(g) + X(g) AX sotution = ‏كلفد‎ pattie + AH pydration 

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Explanation of solubility AG sotution = “AH iattin + MAL ydration 

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Explanation of solubility ~AH jattic: + AH hydration AH solution For Group II sulpha than the anions. T| not cause a signifi to 1/(r, + 1). However, the chai / AH jyaration (PrOpOrti] 55 exothermic, and tl descending the Gr 

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Explanation of solubility AH ~AH jattic + MAL pydration solution = 3. For the smaller size anions, OH. Down the Group, less e| = break the lattice as the ‏کت‎ ۱ in| 024 smaller due to the larg 0 As a result, AH ‏مم © وووون.‎ and the solubility incre Mg(OH), Sr(OH), 

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Check point 39-4 

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Uses of s-block compounds * Sodium carbonate - Manufacture of glass - Water softening - Paper industry * Sodium hydrocarbonate - Baking powder - Soft drink 

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Uses of s-block compounds ¢ Sodium hydroxide - Manufacture of soaps, dyes, paper and drugs - To make rayon and important chemicals ¢ Magnesium hydroxide - Milk of magnesia, an antacid * Calcium hydroxide - To neutralize acids in waste water treatment ¢ Strontium compound - Fireworks, persistent intense red flame