Note for CHM 218 with Professor Berger at IPFW
Note for CHM 218 with Professor Berger at IPFW
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Date Created: 02/06/15
Chapter 12 The Alkaline Earth Metals 121 Beryllium has properties which distinguish it from the other members of the family and its chemistry is frequently discussed separately Common Features All of the allmline earth metals are silvery in color and have fairly low densities Density increases with increasing atomic number They have stronger metallic bonding than the alkali metals and as a result have higher melting points hardnesses and enthalpies of atomization The metals are less reactive than the neighboring allmli metals but increases in reactivity parallel the increase Element Density mp C AHmom gem 3 klmol 1 Mg 174 649 149 Ca 155 839 177 Sr 263 768 164 Ba 362 727 175 observed in the alkali metal family increasing reactivity with increasing atomic number They have high electrical and thermal conductivities which are characteristic of metals Unlike other metals however the allmline earths are so and this trend increases going down the group This so ness is the result of very weak metallic bonding There is a strong correlation between the so ness low melting point and small enthalpy of atomization All of them react with 02 Be Mg Ca and Sr give only MO Ba gives both BaO and BaOz M1202 MO Ba 02 H Ba02 Magnesium like its diagonal neighbor lithium reacts directly with N2 to yield the nitride Mg3N2 as do the heavier alkaline earths Be does not 3 M N2 4 M3N2 All react with halogens to give MXZ MX2MX2 Ca Sr and Ba react with cold water to give MOH2 and H2 Mg reacts only with hot water M H20 M0H2 H2 122 Ionic Compounds All of the metals form a stable 2 cation and most of the compounds are stable ionic solids colorless unless the anion imparts a color to the compound eg MnO439 CrO4239 Cr207239 Mg has a chargetovolume radius greater than that of LF so many of its compounds exhibit covalent characteristics Unlike the allmli metals salts are typically hydrated with the degree of hydration decreasing down the group Hydroxides however exhibit the opposite trendisrOH2 and BaOH2 are octahydrates while MgOH2 and CaOH2 are anhydrous Usual hydration numbers for several compounds are given in the table below M MCI2 MNO32 Mso4 M0H2 Mg 6 6 7 0 Ca 6 4 2 0 Sr 6 4 0 8 Ba 2 0 0 8 Solubility of Allmline Earth Metal Salts Compounds are typically less soluble than the corresponding alkali metal compounds Again solubility depends on AG for the process MXz S M 31 2 X 3 1 Two contributions to AG AG AH TAS Sulfates become less soluble descending the group but hydroxides become increasingly soluble This is consistent with the trend associated with cationanion size matching We can write a Hess Law cycle to relate AH for the process in terms of the lattice enthalpy U and the enthalpies of hydration of M2 and X39 sz s M2 g 2 x g AH U M g a M aq AH AHm M 2 X39 g r 2 X39 aq AH 2 x Athd 06 sz s M2 aq 2 x aq AH U Athd M2 2 x Athd X39 123 In general the lattice enthalpy and the hydration enthalpy for the cation will both be larger for MX2 than for MK Additionally we have twice the hydration enthalpy for the anion for a 1 anion Comparing NaCl with MgClZ compound AHgoln TASSoln AGSOOIH MgClZ 160 34 125 NaCl 4 13 79 For M gClz enthalpy factors favor dissolution while entropy factors oppose it This is the opposite of the behavior we saw for NaCl All else being equal solubilities of ionic compounds pairing 22 ions are typically less than those pairing 2l ions MY s a M2 aq Yz39 aq AH U Athd M2 Athd YZ39 U olt L 2 239 MZVYZ39 227 2 8 MW 2 X39 3271 76 Lattice enthalpies may be 33 greater but total hydration enthalpy may not compensate Furthermore AS term is less favorable for the divalent anion 124 The Elements um u The Fundamental Properties 0f Group 2A Elements Ref 32 Macyumquot Mnunululu Cnldlun Slmnrium llmiuln Mluln mm Eu Mquot a Sr m m Amwlu number 5 l2 J 3 51 KN N um xuxlnpex 9 mu 2mm mm mm mm m unimu Am Ahlmlhmzus zsmm mum synm umm znuul nn 135 57101 lul 1 44 z a suns usm Wu we 1567quot 45quot IX L 7ll 52 m M m m H mm r u u 15 m N c uugl 1 HI 24M MN R732 m7 Valence lcclmn 2v 1 1391 513 I 39 39lZ rum1 Snl 1w wlHZD 7701580 735an 1001140 Dnmlx xcm 17 l 2 MI J l 5 Ammlc main 5 m I on 107 115 2 2A 1mm Hmms Shannoanwwm A K J mm mun 132m 1 mm I nullnn u Ln Ln nu M mm mm A m m I 1 h neuynig mm mull thirA v 4 737 em em mm mama me e 1 v v u mm mm umnl m m m 54v 1 Lmlnuul lccunn m 1141 m m u Il nin Udmul Dunnunl ivydam lvlrrlluu Davy Dill Div Davy um um mm mm mm nun ml m quot 0 am Mm mo smv 50 m0 Rao 2 v m and mum w th um Emu Iluw an m we Ipw N None MgN Cn N SHN MVN MN mw halogen mx 1 MAX aux 5m le RnX rpw hydmnen NW Mm cm1 5ma runJ 39rwlll uucmm Dun llcn no ucv hm 395 u mnmdm Mumun wltmmlle39lwl MpL nun mam wxlh 125 Beryllium Steel gray in color fairly hard fairly high mp but low density High electrical conductivity resistant to corrosion strong nonmagnetic Occurs as the minerals bertrandite Be4Si207OH2 and beryl Be3AIZSi6018 The latter occurs in various colors due to the presence of trace impurities for example aquamarine and emerald with CrIII The element and its compounds are extremely toxic and inhalation causes berylliosis There are two forms of berylliuminduced lung disease acute and chronic Acute berylliosis has a sudden rapid onset and is characterized by severe in ammation of the lungs pneumonitis coughing increasing breathlessness dyspnea and other associated symptoms and ndings In addition in some individuals the skin or the eyes may be affected The more common chronic form of the disease develops more slowly and in some cases may not become apparent for many years a er initial beryllium exposure Chronic berylliosis is characterized by the abnormal formation of inflammatory masses or nodules granulomas Within certain tissues and organs and Widespread scarring and thickening of deep lung tissues interstitial pulmonary brosis Although granuloma development primarily affects the lungs it may also occur Within other bodily tissues and organs such as the skin and underlying subcutaneous tissues or the liver In individuals With chronic berylliosis associated symptoms and findings often include dry coughing fatigue weight loss chest pain and increasing shortness of breath From WebMD 126 The extremely large chargetoVolume ratio 1100 Cmm3 means that Be 2 is highly polarizing and covalent character dominates the bonding Even compounds that might be expected to be simple ionic compounds are usually tetrahydrates BeC12 4 H20 is actually BeH2042 2 or The anhydrous solids are typically in nite chains with 4coordinate Be atoms X x X x f Be X X X X X In the gas phase at higher temperatures Be halides are linear molecules Xi BeiX At lower temperatures but still in the gas phase 3coordinate dimers exist X X Be Be X X Be X X Although a metal it has some characteristics of nonmetals including the ability to form oxoanions Along with Al diagonal relationship and Zn Be is sometimes referred to as a weak metal near the metalsemimetal border Like Al and Zn its oxide and hydroxide are amph uteric That is they will react with acids as well as bases By comparison MgO and MgOH2 are strictly basic 1360 H20 2 H30 BeOH242 oxides BeO H20 2 OH39 a BeOH4239 BeOH2 2 H3O a BeOH242 hydroxides BeOH2 2 OH a BeOH4239 127 Magnesium There are some aspects of the chemistry of Mg that are suf ciently diiTerent from Ca Sr and Ba to discuss separately For example Hydrates typically decompose when heated rather than simply dehydrating MgC12 H20 a MgOHC1 HCl CaC12 2 H20 a CaCl2 2 H20 Compounds with covalent bonds are not uncommon du to the still relatively large chargetovolume radius of Mg2 120 Cmm3 vs Ca2 which is about 52 Cmm3 and comparable to L Mg reacts with alkyl halides and inserts in the CiX bond forming Grignard Reagents Like alkyl lithium compounds these are used as alkylating agents in organic chemistry RiX ether Mg s a Re MgiX ether Magnesium occurs in nature as camallite MgClZ 39 KCl 39 6 H20 and dolomite MgCO3 39 CaCO3 These are both actual stoichiometric compounds in which the indicated cations occupy holes in the lattice of the indicated anions rather than simple mixtures Mg2 occurs naturally in seawater and is the third most abundant ion a er Na and Cl39 seawater provides a major industrial source Extraction of Mg2 from seawater is accomplished by the Dow Chemical Extraction Process Mg2 ions are first precipitated as MgOH2 Ca0H2 S Mg 211 Mg0H2 S t Ca2 211 Following the ltration the MgOH2 s is dissolved in HCl MgOH2 s 2 HCl aq a MgC12 aq 2 H20 0 The solution is evaporated and the solid MgClZ is heated to melting in an electrolysis cell similar to the Downs Cell and electrolyzed MgCW Mg0Clzg 128 Maintaining an inert atmosphere above the reactive liquid Mg is problematic because unlike liquid Na Mg will react with N2 as discussed earlier SF6 or 802 are o en used to protect the Mg from atmospheric O2 and N2 Thermodynamically Mg is a potent reducing agent Mg22e eMg E 237V but its reactivity is less than might be expected because its surface become passivated by a very thin adherent coating of MgO that protects the metal from further oxidation Mg bums in 02 with the emission of a brilliant white light Powdered Mg was used in early ash photography and disposable ash bulbs and ash cubes made use of rapid combustion of brous Mg to provide a ash 2Mg02e2MgO Burning metals like Mg pose a special re hazard in laboratories Normal C02 re extinguishers cannot be used because burning magnesium reacts with C02 2MgC02 e2MgOC A Class D re extinguisher is required These use either graphite or very nely powdered NaCl Graphite reacts w1th Mg to form a magnes1um carb1de Class A B g solids which coats the magnesium and prevents further reaction Class B B g liquids w1th 92 NaCl melts and the 11qu1d coats the magnes1um Class C Electdcal effectively smothering the re Class D B g metals Industrially Mg is used in strong lowweight alloys with aluminum for structural materials Some lawn mower housings use this alloy Calcium and Barium Both are grayish metals that react with 02 slowly at room temperature but more rapidly at elevated temperatures Ca gives only the oxide while Ba give some peroxide dioxide 2 The charge to volume ratio of Ca2 52 Cmm3 is comparable to that of L while that of Ba 23 Cmm3 is comparable to that of Na 24 Cmm3 2 Ca s 02 g a 2 CaO s Ba s 02 g a Ba02 s 129 Because Xray absorption cross sections increase with the square of atomic number both Ca and Ba are Xray absorbers Slunies of the fairly insoluble BaSO4 are used in GI tract imaging Oxides and Hydroxides All M react with 02 to form MO with Ba also giving Ba02 MgO is insoluble in water and has an extremely high melting point 28250C and is refened to as a refractory compound High thermal conductivity but extremely small electrical conductivity make it useful in heating elements Other oxides react with water to give the corresponding hydroxide MO H20 CaOH2 CaO is prepared by the thermal decomposition of CaCO3 at T gt ll70 C CaCO3 Ca0 co2 CaO alm quicklime emits a bright white light when a ame is directed against it hence limelight similar to behavior of Th02 which is used in camping lantern mantles CaOH2 aka hydrated lime or slaked lime has been used to adjust pH of soil upwards but CaCO3 can accomplish the same job without raising it to much MOH2 become more soluble as we descend the group7 OH is a fairly small anion and this trend follows the size mismatch rule of thumb Solutions of CaOH2 and BaOH2 react with C02 to produce the corresponding carbonate CaOH2 aq C02 g a CaCO3 s H20 0 Calcium Carbonate S 1 b39l39 Compound 0 u 1 ty g 39L39 MgOH2 1 x 10 39 4 CaOH2 12 SrOH2 10 BaOH2 47 Ca is the 53911 most abundant element on earth found chie y as the carbonate in chalk limestone and marble deposits Chalk was formed is the seas mainly during the Cretaceous Period about 135 million years ago from the calcium carbonate skeletons of marine organisms Limestone formed as a simple precipitate and marble formed when subterranean limestone melted under intense pressure and temperature and subsequently cooled 1210 Underwater caverns like Carlsbad Cavems and Mammoth Cave occur in limestone beds Over geologic time scales rainwater with dissolved C02 entered small cracls and solubilized the limestone leaving vast underground voids CaCO3 s H20 0 C02 g a Ca aq 2 HCO339 aq Only allmli metal ions have suf ciently low charge densities to stabilize the large polarizable hydrogen carbonate ion so upon evaporation of water from solutions containing Ca2 and HCO339 ion CaCO3 is formed CaH0032 aq a Ca003 s H20 0 002 g Other Calcium Compounds Various silicate and aluminate compounds of calcium are important components of cement Hydrated calcium sulfate is the chief component in re resistant gypsum wallboard CaSO4 2 H20 s a CaSO4 12 H20 s 32 H20 g AH 298 83 1d Calcium chloride is an important dehydrating agent as well as icemelting agent CaC12 s H Ca2 aq 2 Cl39 aq AH 298 i 81 kJ Calcium carbide is actually a misnomer It s actually CaC2 dicarbide 2 formed in the reaction of CaO with C at 2000 C Ca0 3C a CaC2 CO This material reacts with water to produce acetylene CaC2 2 H20 a CaOH2 C2H2 This is a relatively safe and convenient source of acetylene for lamps for miners and spelunkers Calcium carbide also reacts with molecular nitrogen a relatively simple way of breaking the N E N triple bond 02102 N2 CaCN2 C The cyanamide ion is isoelectronic and isostructural with C02 and is a starting material in the synthesis of several organic compounds as well as being a slow release nitrogen containing fertilizer CaCN2 3 H20 a CaCO3 2 NH3
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