Bài giảng Hóa học - Chương 9: Nhóm IIA

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Nội dung Text: Bài giảng Hóa học - Chương 9: Nhóm IIA

GROUP 2: THE ALKALINE EARTH METALS Emerald is based on the mineral beryl: 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 CHUNG 1. Là kim loại hoạt động, tăng dần từ BeRa. Kém so với KL kiềm do Z lớn, r nhỏ hơn 2. Dễ mất 2e trở thành M2+ trong hợp chất và trong dung dịch 3. Thế điện cực tương đương KL kiềm 4. Thể hơi chỉ bao gồm phân tử 1 nguyên tử 5. Ion không màu; nhiều hợp chất ít tan 6. 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 + H2 Beryllium 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 • Beryllium is somewhat different from the rest of group 2A. • BeO does not react with water, while the other group 2A metal oxides do so: MO + H2O  M(OH)2. • Be and BeO dissolve in strongly basic solutions to form the BeO22– ion. The oxide BeO has acidic properties. The other alkaline earth metal oxides are basic. • Molten BeF2 and BeCl2 are poor conductors of electricity; they are molecular substances (see below). The other group IIA compounds are almost entirely ionic.
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.  Portland cement is made by heating limestone, clay, and sand. When the cement is mixed with sand, gravel, and water, it solidifies into concrete.  Ordinary soda–lime glass is formed by heating
Important Compounds of Magnesium and Calcium (cont’d)  Limestone is used in the metallurgy of iron and steel to produce an easily liquefied mixture of calcium silicates called slag, which carries away impurities from the molten metal.  Precipitated (purified) calcium carbonate is used extensively as a filler in paint, plastics, printing inks, and rubber.  It is also used as a mild abrasive in toothpastes, food, cosmetics, and antacids.  Added to paper, calcium carbonate makes the paper bright, opaque, smooth, and capable of absorbing ink well.
Important Compounds of Magnesium and Calcium (cont’d)  Quicklime (CaO) and slaked lime [Ca(OH)2] are the cheapest and most widely used bases, and are usually the first choice for neutralizing unwanted acids.  Slaked lime sees extensive agricultural use.  Quicklime is used to neutralize sulfur oxides formed when coal burns.  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” or
Decomposition of CaCO3 (lime) In the lime kiln: Δ CaCO3 → CaO + CO2 burnt lime or quicklime In the lime slaker: CaO + H2O → Ca(OH)2 slaked lime
Stalactites and Stalagmites CO2 + H2O → H3O+ + HCO3 Ka = 4.4107 HCO3 + H2O → H3O+ + CO32 Ka = 4.71011 CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
Other Compounds  Gypsum, CaSO4·2H2O:  Plasterof paris CaSO4·½H2O by heating bypsum.  Used in drywall.  BaSO4 used in X-ray imaging .  Slaked lime used in mortar:  CaO absorbs water from the cement to form Ca(OH)2 which subsequently reacts with CO2 to form CaCO3.