The s -Block Elements

1. Chapter Twenty The s-Block Elements

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.

3. Hydrogen • Hydrogen makes up 0.9% of the mass and 15.1% of the atoms in Earth’s crust, but makes up 89% of the atoms of the Sun, and 90% of all atoms in the universe as a whole. • Hydrogen is an important by-product of petroleum refining operations. In catalytic reforming, hydrogen is produced as a low-octane alkane is converted to a higher-octane unsaturated hydrocarbon. • The most active of the metals, those of Group 1A and the heavier members of Group 2A, displace H2(g) even from pure water.

4. The Electrolysis of Water • This is the most direct method of producing hydrogen, but is an expensive operation. • The electricity that runs a 100-W lightbulb for an hour produces only about two grams of hydrogen.

5. Binary Compounds of Hydrogen • Hydrogen reacts with other nonmetals to form molecular hydrides, such as ammonia, water, and HCl. • 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 H2(g). CaH2 reacts with water, forming OH– and turning phenolphthalein pink.

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.

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.

8. The Alkali Metals

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.

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 (Li2O) 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.

11. Diagonal Relationships The elements in each encircled pair have several similar properties.

12. Occurrence, Preparation, Use, and Reactions of the Alkali Metals • Sodium and potassium are isolated primarily from brines (solutions of NaCl and KCl). • Lithium is obtained mostly from the mineral spodumene, LiAl(SiO3)2. • 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.

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 KO2, 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.

16. Important Compounds of Lithium, Sodium, and Potassium • Lithium carbonate is the usual starting material for making other lithium compounds: • Li2CO3(aq) + Ca(OH)2(aq)  CaCO3(s) + 2 LiOH(aq) • One use of LiOH is to remove CO2 from expired air in submarines and space vehicles: • 2 LiOH(s) + CO2(g)  Li2CO3(aq) + H2O • 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.

17. Preparation of Sodium Compounds from NaCl The methods of preparation suggested by this diagram are not necessarily the preferred industrial methods.

18. Example 20.1 The pathway in Figure 20.5 running from NaCl to Na2SO4 to Na2CO3 is called the Leblanc process. Write equations for the reactions involved.

19. The Solvay Process—One Way to Diagram an Industrial Process

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.

21. Properties and Trendsin 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.

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 + 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.

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 CaCO3; dolomite is MgCO3· CaCO3. • 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, Be3Al2Si6O18. • Some familiar gemstones, including aquamarine and emerald, are beryl, distinctively colored by impurities.

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 MgCl2, in the Dow process. Less expensive methods of obtaining magnesium are now available.

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.

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)  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.

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?

30. Important Compounds ofMagnesium 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.

31. Important Compounds ofMagnesium 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.

32. Important Compounds ofMagnesium 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 “plaster board” wall material.

33. The Group 2A Metalsand 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.

34. Chemistry of Groundwater • Rainwater containing dissolved CO2 is acidic due to formation of H2CO3. • Acidic rainwater converts CaCO3 to Ca(HCO3)2: • As the water evaporates from the Ca(HCO3)2, the somewhat-soluble salt forms CaCO3 again. Deposited CaCO3 leads to stalactites, stalagmites, and other cave formations.

35. Hard Water and Water Softening • Hard water is groundwater that contains significant concentrations of ions (Ca2+, Mg2+, Fe2+) 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.

36. Hard water with ions Water Softeningby Ion Exchange An ion-exchange resin with acidic groups bound to Na+ As hard water passes through, hard-water cations are exchanged for Na+

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.

38. Cleaning Action of a Soap A soap has a hydrocarbon “tail” … … 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.

39. Cumulative Example Assume that all the cations in a container holding 1.00 L of hard water are Ca2+. 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+.