Bài giảng Hóa học - Chương 9: Nhóm IIA

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Bài giảng Hóa học - Chương 9: Nhóm IIAGROUP 2: THE ALKALINE EARTH METALSEmeraldisbasedon themineralberyl: 3BeO∙Al2O3∙6SiO2 Principle forms:  carbonates, sulfates and silicates Oxides and hydroxides only sparingly soluble.  Basic or “alkaline” Compounds do not decompose on heating.  Therefore named “earths” Heavier elements compounds are more reactive and are similar to Group I (also in other respects).Properties and Trends in Group 2A  Group 2A shows the same general trends of increasing atomic and ionic sizes and decreasing ionization energies from top to bottom as does group 1A.  The higher densities of the group 2A metals are mainly a consequence of the large differences in atomic sizes.  The group 2A metals are all good reducing agents.  Mg(OH)2 is virtually insoluble in water, however …  As the cation size increases from top to bottom on the periodic table, interionic attractions decrease in strength and the solubilities of the compounds in water increase.  Ba(OH)2 is sufficiently soluble to be used as a titrant in acid–base titrations. ĐẶC ĐIỂM CHUNG1. Là kim loại hoạt động, tăng dần từ BeRa. Kém so với KL kiềm do Z lớn, r nhỏ hơn2. Dễ mất 2e trở thành M2+ trong hợp chất và trong dung dịch3. Thế điện cực tương đương KL kiềm4. Thể hơi chỉ bao gồm phân tử 1 nguyên tử5. Ion không màu; nhiều hợp chất ít tan6. Be khác nhiều hơn so với Li và các nguyên tố trong nhóm; Be giống nhiều Al, Mg giống nhiều Zn. These were named alkaline earths because of their intermediate nature between the alkalis (oxides of the alkali metals) and the rare earths (oxides of rare earth metals).The alkaline earth metals are silvery colored, soft, low- density metals, which react readily with halogens to form ionic salts, and with water, though not as rapidly as the alkali metals, to form strongly alkaline (basic) hydroxides. For example, where sodium and potassium react with water at room temperature, magnesium reacts only with steam and calcium with hot water. Mg + 2H2O → Mg(OH)2 + H2Beryllium is an exception: It does not react with water or steam, and its halides are covalent. Reactions of Group 2A Metals Reactivity with water increases from beryllium to barium:  Berylliumdoes not react with water.  Magnesium reacts with steam but not with cold water.  Calcium reacts slowly with cold water.  Strontium and barium react more rapidly with cold water. Allthe alkaline earth metals react with dilute acids to displace hydrogen: M(s) + 2 H+(aq)  M2+(aq) + H2(g). The alkaline earth metals react with the halogens to form the corresponding halides, with oxygen to form the oxides, and with nitrogen to form the nitrides. Beryllium Unreactive toward air and water. BeO does not react with water, all others from hydroxides. Be and BeO dissolve in strongly basic solutions to form the BeO22- ion (therefore are acidic). BeCl2 and BeF2 melts are poor conductors:  Therefore they are covalent rather than ionic solids. The Special Case of Beryllium• Berylliumissomewhatdifferentfromtherestofgroup2A.• BeOdoesnotreactwithwater,whiletheothergroup2Ametal oxidesdoso:MO+H2OM(OH)2.• BeandBeOdissolveinstronglybasicsolutionstoformthe BeO22–ion.TheoxideBeOhasacidicproperties.Theother alkalineearthmetaloxidesarebasic.• MoltenBeF2andBeCl2arepoorconductorsofelectricity;they aremolecularsubstances(seebelow).TheothergroupIIA compoundsarealmostentirelyionic.Beryllium Chloride Important Compounds of Magnesium and Calcium Several magnesium compounds occur naturally, either in mineral form or in brines. These include the carbonate, chloride, hydroxide, and sulfate. Limestone is a naturally occurring form of calcium carbonate, containing clay and other impurities. Portlandcementismadebyheatinglimestone,clay, andsand.Whenthecementismixedwithsand, gravel,andwater,itsolidifiesintoconcrete. Ordinarysoda–limeglassisformedbyheating Important Compounds ofMagnesium and Calcium (cont’d) Limestoneisusedinthemetallurgyofironandsteelto produceaneasilyliquefiedmixtureofcalciumsilicates calledslag,whichcarriesawayimpuritiesfromthe moltenmetal. Precipitated(purified)calciumcarbonateisused extensivelyasafillerinpaint,plastics,printinginks, andrubber. Itisalsousedasamildabrasiveintoothpastes,food, cosmetics,andantacids. Addedtopaper,calciumcarbonatemakesthepaper bright,opaque,smooth,andcapableofabsorbingink well. Important Compounds of Magnesium and Calcium (cont’d) Quicklime(CaO)andslakedlime[Ca(OH)2]arethe cheapestandmostwidelyusedbases,andareusuallythe firstchoiceforneutralizingunwantedacids. Slakedlimeseesextensiveagriculturaluse. Quicklimeisusedtoneutralizesulfuroxidesformedwhen coalburns. Gypsum has the formula CaSO4·2 H2O. Another hydrate of calcium sulfate is plaster of paris which has the formula CaSO4 ·½ H2O and is obtained by heating gypsum. Gypsum is used to make the familiar “drywall” orDecomposition of CaCO3 (lime) Inthelimekiln: Δ CaCO3→CaO+CO2 burntlime or quicklime Inthelimeslaker: CaO+H2O→Ca(OH)2 slakedlime Stalactites and StalagmitesCO2+H2O→H3O++HCO3 Ka=4.4107HCO3+H2O→H3O++CO32 ...