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<p>1 Chapter Twenty The s-Block Elements (a) Cathode Anode Dilute COO (b) -COO Rb Brine H2O Limestone NH, (c) Na+ recovery Ca2+ NH,CI NH,CI Fe2+ Mg2+ (d) Product By product Copyright 2005 Pearson Prentice Hall, Inc. Copyright Inc. Prentice Hall © 2005 General Chemistry 4th Hill, McCreary, Perry Chapter Twenty</p><p>2 Introduction Modern chemical knowledge is based largely on principles that answer the "why" as well as the "how-to." In the remaining chapters, we will emphasize facts and applications, but we will refer to underlying principles repeatedly. Four of the 14 elements that comprise the s-block elements are somewhat unusual cases, although not all for the same reason. - Hydrogen is the simplest element, with one proton and one electron, and its behavior is rather special. - Helium lies in the s-block but its electron configuration fits with the noble gases, and it will be considered in the next chapter. - Francium and radium are highly radioactive and we will not consider them further. Prentice Hall © 2005 General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>3 Hydrogen 1671 Boyle: Flammable gas in metal-acid reaction. Hydrogen makes up 0.9% of the mass and 1766 Cavendish: Discovery of 15.1% of the atoms in Earth's crust, but makes up 89% of the atoms of the Sun, and 1803 Dalton: H chosen as 90% of all atoms in the universe as a whole. atomic weight standard. Hydrogen is an important by-product of 1810 Davy: H shown to be common to all acids. petroleum refining operations. In catalytic reforming, hydrogen is 1885 Balmer: Equation describing produced as a low-octane alkane is emission spectrum of H. converted to a higher-octane 1913 Bohr: Quantum mechanical unsaturated hydrocarbon. treatment of H atom. The most active of the metals, those of 1926 Schrödinger: Wave mechanics developed for Group 1A and the heavier members of H atom. Group 2A, displace H2(g) even from pure 1927 Heitler & London: Wave water. mechanics applied to Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>4 The Electrolysis of Water H2(g) This is the most direct method of producing hydrogen, but is an Cathode Anode expensive operation. The electricity that runs a 100-W lightbulb for an hour Dilute produces only about two grams of hydrogen. Copyright 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>5 Binary Compounds of Hydrogen Hydrogen reacts with other nonmetals to form molecular hydrides, such as ammonia, water, and HCI. Among the most important molecular hydrides are those of carbon (the beginning of organic chemistry!). Hydrogen reacts with the most active metals to form ionic hydrides, in which hydrogen exists as H. Ionic hydrides react with water to liberate CaH2 reacts with water, forming OH- and turning phenolphthalein pink. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>6 Binary Compounds of Hydrogen (cont'd) When hydrides are formed from transition elements, the products, called metallic hydrides, retain some metallic properties such as electrical conductivity. Some metallic hydrides are nonstoichiometric in that they have a variable formula. Hydrogen atoms occupy some but not all spaces between metal atoms. The formula of this hydride varies with the number of filled spaces. Copyright 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>7 Uses of Hydrogen Nearly half the hydrogen gas produced is used in the manufacture of ammonia (NH3). Hydrogen is used in the hydrogenation of vegetable oils and of unsaturated hydrocarbons such as benzene. Another chemical manufacturing process that uses hydrogen is the synthesis of methanol, an industrial solvent and starting material for making other organic materials. Hydrogen is used as a reducing agent in metallurgy. Liquid hydrogen is used as a rocket fuel. An oxyhydrogen welding torch using atomic hydrogen readily cuts through steel and can be used to melt tungsten, which has a melting point of about 3400 °C. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>8 The Alkali Metals Table 20.1 Some Properties of the Group 1A Metals Li Na K Rb Cs Atomic number, Z 3 11 19 37 55 Rb Valence-shell electron configuration 2s' Copyright 2005 Pearson Prentice Atomic (metallic) radius, pm 152 186 227 248 265 Ionic (M+ radius, pm 59 99 138 148 169 First ionization energy kJ/mol 520 496 419 403 376 Electronegativity 1.0 0.9 0.8 0.8 0.7 Flame color Carmine Yellow Violet Bluish red Blue Melting point, °C 180.54 97.81 63.65 39.1 28.40 Density, at 20 °C 0.534 0.971 0.862 1.532 1.873 Electrode potential, V M+(aq) + e M(s) -3.040 -2.713 -2.924 -2.924 -2.923 Electrical 18.6 37.9 25.9 12.7 8.0 Hardness 0.6 0.4 0.5 0.3 0.2 a These values are on a scale relative to silver, which is assigned a conductivity of 100; for comparison, the conductivity for copper on this scale is 95.0 and that of gold is 67.7. b On a scale called the Mohs scale that ranks hardness from 1 to with 10 being hardest. A material with a higher Mohs number is able to scratch a material with a lower Mohs number. Examples: fingernail (2.5), copper iron (4-5). glass (5-6). steel diamond (10). Notice that the alkali metals are softer than Copyright 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>9 Properties and Trends in Group 1A The Group 1A metals exhibit regular trends for a number of properties. Irregular trends suggest that factors are working against each other in determining a property (such as the density "discrepancy" between sodium and potassium). The alkali metals have two notable physical properties: they are all soft and have low melting points. When freshly cut, the alkali metals are bright and shiny- typical metallic properties. The metals quickly tarnish, however, as they react with oxygen in the air. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>10 Diagonal Relationships: The Special Case of Lithium In some of its properties, lithium and its compounds resemble magnesium and its compounds. - Lithium carbonate, fluoride, hydroxide, and phosphate are much less water soluble than those of other alkali metals. - Lithium is the only alkali metal that forms a nitride (Li3N). - When it burns in air, lithium forms a normal oxide rather than a peroxide or a superoxide. - Lithium carbonate and lithium hydroxide decompose to form the oxide on heating, while the carbonates and hydroxides of other Group 1A metals are thermally stable. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>11 Diagonal Relationships The elements in each encircled pair have several similar 1A 2A 3A 4A properties. Li Be B C Na Mg Al Si K Ca Ga Ge Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>12 Occurrence, Preparation, Use, and Reactions of the Alkali Metals Sodium and potassium are isolated primarily from brines (solutions of NaCl and KCI). Lithium is obtained mostly from the mineral spodumene, To convert an alkali metal ion into an alkali metal atom, the ion must take on an electron-a process of reduction. This is not easy with the alkali metals; they are excellent reducing agents. Potassium was the first alkali metal to be prepared by electrolysis. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>13 Occurrence, Preparation, Use, and Reactions of the Alkali Metals Liquid sodium is used as a heat transfer medium in some types of nuclear reactors and in automobile engine valves, and its vapor is used in lamps for outdoor lighting. Potassium is used in making KO, used as an oxygen source for miner's "self-rescuers" and similar devices: Lithium is used in lightweight batteries of the type found in heart pacemakers, cellular telephones, digital cameras, and portable computers. Prentice Hall © 2005 General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>14 Table 20.2 Some Typical Reactions of the Alkali Metals, M With halogens (group 7A), X2 2 M(s) + X2 2 MX(s) (e.g., LiF, KBr, CsI) With hydrogen, H2 2 M(s) + H2(g) 2 MH(s) (e.g., LiH, NaH) With excess oxygen, 4 Li(s) + 2 (plus some 2 Na(s) + (plus some M(s) + (where M = K, Rb, or Cs) With water, 2 M(s) + 2 2 MOH(aq) + H2(g) a is a normal oxide; is a peroxide; MO2 is a superoxide. These oxides are described further in Section 21.9. Under appropriate conditions, all the alkali metals can form M2O, M2O2, and Copyright 2005 Pearson Prentice Hall, Inc. Prentice Hall 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>15 Table 20.3 Some Chemicals Produced from NaCl U.S. Production, 2002, Chemical Thousands of Metric Tons Sodium hydroxide, NaOH 8984 Chlorine, Cl2 11362 Hydrochloric acid, HCI 4033 a Sodium sulfate, 515b a Most HCI is produced as a by-product of the chlorination of hydrocarbons; for exam- ple, in the chlorination of methane, CH4(g) + CH3Cl(g) + b Includes both and Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, McCreary, Perry Chapter Twenty</p><p>16 Important Compounds of Lithium, Sodium, and Potassium Lithium carbonate is the usual starting material for making other lithium compounds: - + Ca(OH)2(aq) CaCO(s) + 2 LiOH (aq) One use of LiOH is to remove CO2 from expired air in submarines and space vehicles: - 2 LiOH(s) + NaCl is the most important industrial sodium compound (50 million tons/yr). It is used to prepare a number of other chemicals and consumer products, including plastics, paper, bleach, soap, and laundry detergent. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>17 Preparation of Sodium Compounds from NaCI C A H2SO4 CaCO3 (concd aq) + NH3 + Electrolysis A NaCl Na NaCl(1) O2 Electrolysis A H2 NaCl(aq) NaOH(aq) NaH Cl2 SO2 NaOCI NaNO Copyright 2005 Pearson Prentice Hall, Inc. The methods of preparation suggested by this diagram are not necessarily the preferred industrial methods. Prentice Hall © 2005 General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>18 Example 20.1 The pathway in Figure 20.5 running from NaCl to to is called the Leblanc process. Write equations for the reactions involved. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>19 The Solvay Process-One Way to Diagram an Industrial Process Brine Ammoniated Ammonia NaCl, brine NH3 NaCl Limestone NH3 NH3 Lime kiln CO2 Carbonating tower CaO + CO2 NaCl(aq) + NH3 + + NH4Cl(aq) CaO Ammonia recovery Ca(OH)2 + 2 Filter CaCl2 + 2 + 2 NH3 Lime slaker Ca(OH)2 CaO + Ca(OH)2 Product By-product CaCl2 Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>20 The Alkali Metals and Living Matter Hydrogen, oxygen, carbon, and nitrogen are the most abundant elements in the human body, in the order listed. Sodium and potassium ions are in a second tier of seven elements that account for about 0.9% of the atoms. Sodium ions are found primarily in fluids outside cells and potassium ions are abundant in fluids within cells. Because most alkali metal compounds are water soluble, many drugs that are weak acids are administered in the form of their sodium or potassium salts. Lithium carbonate is used in medicine to level out the dangerous manic "highs" that occur in manic-depressive psychoses. Prentice Hall © 2005 General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>21 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. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>22 Table 20.4 Some Properties of the Group 2A Metals Be Mg Ca Sr Ba Atomic number, Z 4 12 20 38 56 Valence-shell electron configuration 4s2 5s2 6s2 Atomic (metallic) radius, pm 111 160 197 215 217 Ionic radius, pm 31 65 99 113 135 Ionization energy, kJ/mol 900 738 590 550 503 1757 1451 1145 1064 965 Electronegativity 1.5 1.2 1.0 1.0 0.9 Flame color None None Orange-red Scarlet Green Melting point, °C 1278 649 839 769 729 Density, 3 at 20 °C 1.848 1.738 1.550 2.540 3.594 Electrode potential, E°, V + M(s) - 1.70 - 2.356 2.84 -2.89 2.92 Electrical 40 36 46 6.9 3.2 ~5 2.0 1.5 1.8 a These values are on a scale relative to silver, which is assigned a conductivity of 100; for comparison, the conductiv- ity for copper on this scale is 95.0 and that of gold is 67.7. b On a scale called the Mohs scale that ranks hardness from 1 to 10, with 10 being hardest. Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, McCreary, Perry Chapter Twenty</p><p>23 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 + M(OH)2. Be and BeO dissolve in strongly basic solutions to form the ion. The oxide BeO has acidic properties. The other alkaline earth metal oxides are basic. Molten 2 and BeCl2 are poor conductors of electricity; they are molecular substances (see below). The other group IIA compounds are almost entirely ionic. CI Be CI 3p sp sp 3p Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>24 Occurrence, Preparation, Uses, and Reactions of Group 2A Metals Calcium and magnesium rank just ahead of sodium and potassium in abundance in the Earth's crust. Limestone is mainly dolomite is MgCO Barium and strontium are found in the Earth's crust at about 400 ppm, and beryllium is found at 2 ppm. An important mineral source of beryllium is the mineral beryl, Some familiar gemstones, including aquamarine and emerald, are beryl, distinctively colored by impurities. Prentice Hall © 2005 General Chemistry 4th edition, Hill, McCreary, Perry Chapter Twenty</p><p>25 Occurrence, Preparation, Uses, and Reactions of Group 2A Metals (cont'd) To obtain beryllium metal, beryl is first converted to BeF2. Then the BeF2 is reduced to beryllium, using magnesium as the reducing agent. Calcium is generally obtained by electrolysis of molten calcium chloride. Strontium and barium can also be obtained by electrolysis, but are usually obtained by the high-temperature reduction of their oxides, using aluminum as the reducing agent. Until recently, magnesium was obtained by the electrolysis of molten in the Dow process. Less expensive methods of obtaining magnesium are now available. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>26 Occurrence, Preparation, Uses, and Reactions of Group 2A Metals (cont'd) Alloys of beryllium with other metals have many applications such as springs, clips, and lightweight structural materials. Beryllium is nonsparking, and tools that must be used in flammable atmospheres are sometimes made of beryllium. Magnesium has a lower density than any other structural metal and is an important metallurgical reducing agent. Magnesium is also used in batteries and fireworks. Calcium is used to reduce the oxides or fluorides of less common metals to the free metals. Calcium is also alloyed with lead in lead-acid batteries, and is used to form other alloys with aluminum and silicon. Prentice Hall © 2005 General Chemistry 4th Hill, McCreary, Perry Chapter Twenty</p><p>27 Reactions of Group 2A Metals Reactivity with water increases from beryllium to barium: - Beryllium does 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. All the alkaline earth metals react with dilute acids to displace hydrogen: M(s) + 2 H+(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. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>28 Example 20.2 A Conceptual Example The photograph in the margin is a graphic illustration of what happens when we heat a strip of magnesium ribbon in air. If we know that the strip has a mass of 1.000 g, can we calculate with certainty the mass of the product obtained? Prentice Hall © 2005 General Chemistry 4th Hill, McCreary, Perry Chapter Twenty</p><p>29 Table 20.5 Important Magnesium Compounds Compound Uses Refractory bricks, glass, inks, rubber reinforcing agents, dentifrices, cosmetics, antacids, laxatives MgCl2 Magnesium metal, textiles, paper, fireproofing agents, cements, refrigeration brine MgO Refractories (furnace linings), ceramics, cements, removal from stack gases MgSO4 Fireproofing, textiles, ceramics, fertilizers, cosmetics, dietary supplements Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>30 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 limestone, sand, and sodium carbonate together. Prentice Hall © 2005 General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>31 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. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>32 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 2 Another hydrate of calcium sulfate is plaster of paris which has the formula CaSO4 H2O and is obtained by heating gypsum. Gypsum is used to make the familiar "drywall" or "plaster board" wall material. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>33 The Group 2A Metals and Living Matter Persons of average size have approximately 25 g of magnesium in their bodies. The recommended daily intake of magnesium for adults is 350 mg. Calcium is essential to all living matter. The human body typically contains from 1 to 1.5 kg of calcium. Strontium is not essential to living matter, but it is of interest because of its chemical similarity to calcium. Barium also has no known function in organisms; in fact, the Ba2+ ion is toxic. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>34 Chemistry of Groundwater Rainwater containing dissolved is acidic due to formation of Acidic rainwater converts CaCO to CaCO(s) + + + CaCO(s) + As the water evaporates from the the somewhat-soluble salt forms again. Deposited CaCO leads to stalactites, stalagmites, and other cave formations. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>35 Hard Water and Water Softening Hard water is groundwater that contains significant concentrations of ions Mg2+, from natural sources. Hard water tends to precipitate soaps, reducing their effectiveness. If the primary anion is the hydrogen carbonate ion, the hardness is said to be temporary hardness. If the primary anions are other than bicarbonate ion, then the hardness is called permanent hardness. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>36 Hard water Water Softening with ions (a) by lon Exchange An ion-exchange resin with acidic (b) groups bound to Na+ ~COO ~COO ~COO ~COO COO COO COO (c) As hard water passes through, hard-water Na+ cations are exchanged for Na+ Ca2+ Fe2+ Mg2+ (d) Copyright 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>37 Soaps and Detergents A soap acts by dispersing grease and oil films into microscopic droplets. The droplets detach themselves from the surfaces being cleaned, become suspended in water, and are removed by rinsing. The alkali metal soaps are water soluble; the alkaline earth metal soaps are not. A soap can function well in hard water only after a part of it is used up to precipitate all the alkaline earth metal ions present; in other words, the soap softens the water first. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>38 Cleaning Action of a Soap A soap has a hydrocarbon "tail" (a) and an ionic "head" An oil droplet is attracted to the hydrocarbon tails, and the ionic ends permit the droplet to be solubilized in water. (b) Copyright © 2005 Pearson Prentice Hall, Inc. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p><p>39 Cumulative Example Assume that all the cations in a container holding 1.00 L of hard water are A 25.00-mL sample of this water is passed through a cation-exchange resin having H3O+ as counterions. The water leaving the column is found to have a pH of 2.37. Determine the hardness of the water, expressed as ppm Ca2+. Prentice Hall © 2005 General Chemistry 4th Hill, Petrucci, McCreary, Perry Chapter Twenty</p>