Chapter 21Main Group of Elements: H Li Na Mg K Ca B C Al Si N P O S F Cl Br Element Abundances • • • Note low abundance of Li, Be, and B See also alternation of abundance with atomic number. Even atomic number = more abundant Ionic Compounds • Elements form ions with electron configurations that are the same as those for the previous or following noble gases (ns2np6) are crystalline solids with high melting points and conduct electricity in the molten state. • Metal + Nonmetal Product / Name Ba + Cl2 BaCl2 / Barium chloride + S8 8 Na2S / Sodium sulfide 16 Na + Al O2 Al2O3 Molecular Compounds • Metalloids and nonmetals generally form molecular compounds. • Molecular compounds have covalent bonds. • Nonmetal + Nonmetal Product / Name As + F2 AsF5 / ________________ Ge + O2 GeO2 / Germanium dioxide + P4 4 PH3 / Phosphine 6 H2 Recognize the Incorrect Formula and give the Correct Formula for each of the following: PF5 CsSO4 MgO Ba2SO4 CO3 CO K2Cl CaCH3CO2 Hydrogen - Properties • The rocket engine in the Shuttle itself is fueled by H2 + O2. • 1H 1.007825 amu protium • 2H = D 2.014102 deuterium • 3H = T 3.016049 tritium Half-life of tritium = 12.35 years Hydrogen - Preparation • Synthesis gas: C + H2O --> H2 + CO • Steam reforming process: C3H8 + 3 H2O --> 3 CO + 7 H2 • Water gas shift reaction: CO + H2O ---> CO2 + H2 • Electrolysis of water: H2O ---> H2 + 1/2 O2 • Metal + Acid: Mg + 2 HCl --> MgCl2 + H2 • Metal + KOH: 2 Al + 2 KOH + 6 H2O 2 KAl(OH)4 + H2 Hydrogen - Reactions • Virtually every element (except Group 8) will form compounds with H. H2 + Br2 --> 2 HBr. • Haber Process: N2 + 3 H2 --> 2 NH3 • Fuel cell - reactants are supplied continuously from an external source. • Cars can use electricity generated by H2/O2 fuel cells. • One way to store H2 is to adsorb the gas onto a metal or metal alloy (in interstitial sites). • Metal hydrides: Ca + H2 CaH2 Alkali Metals – Group IA • Highly reactive. • The important characteristic of Group 1A elements is their vigorous reaction with water. • Solids are usually stored under mineral oil. • All of the Group 1A metals are relatively soft and can be cut with a knife. http://video.google.com/videoplay?docid=-2134266654801392897 Sodium and Potassium - Properties • 2 Na (s) + 2 H2O(l) -- > 2 Na+ (aq) + 2 OH- (aq) + H2 (g) • 2 K (s) + Br2 (l) --> 2 KBr (s) • 2 Na(s) + O2(g) ---> Na2O2 (s) Sodium peroxide • K(s) + O2(g) ---> KO2(s) Potassium superoxide • KO2 used in breathing apparatus: 4 KO2(g) + 2 CO2 (g) ---> 2 K2CO3(s) + 3 O2(g) Sodium and Potassium - Preparation • Na is prepared by the electrolysis of molten Na in a “Downs cell” Operating at 7-8 V with 25k-40kA at T = 600 oC. Na mp = 97.8oC. • K is prepared from molten KCl: Na(g) + KCl(l) --> K (g) + NaCl (l) Li, Na, K – Important Compounds • Chlor-alkali industry: electrolysis of NaCl(aq) - brine 2 NaCl(aq) + 2 H2O(l) Cl2(g) + 2 NaOH(aq) + H2(g) • Na2CO3: soda ash or washing soda, used as an industrial base, in making soap, and in making glass. It is mined as trona: Na2CO3•NaHCO3•2H2O • Soda-lime process: to produce NaOH from inexpensive lime (CaO) and soda. CaO(s) + Na2CO3(aq) + H2O(l) 2 NaOH(aq) + CaCO3(s) • NaHCO3: baking soda, used in table salt: MgCl2(s) + 2 NaHCO3(s) MgCO3(s) + 2 NaCl(s) + H2O(l) + CO2(g) • KNO3: used as oxidant in gunpower (mixed with C and S). 2 KNO3(s) + 4C(s) K2CO3(s) + 3 CO(g) + N2(g) 2 KNO3(s) + 2S(s) K2SO4(s) + SO2(g) + N2(g) Alkaline Earth Metals – Group 2A • Their compounds neutralize acids, have a very high melting point. • Ca and Mg 5th and 7th in abundance on Earth. • Very reactive – only found in compounds. Many of these of low solubility: they are present as minerals. • Be is toxic. Celestite: SrSO4 Calcium and Magnesium - Properties • High melting, silvery metals, their ions have a 2+ charge. • React with oxygen: Ca + O2 CaO halogens: Ca + Cl2 CaCl2 water: Mg + 2 H2O Mg(OH)2 + H2 acids: Mg + 2 HCl MgCl2 + H2 • Ca: limestone gypsum fluoride CaCO3 CaSO4 2 H2O CaF2 • Mg: magnesite talc MgCO3 3 MgO . 4 SiO2 . H2O asbestos dolomite 3 MgO 4 SiO2 2 H2O MgCa(CO3)2 . . Magnesium - Production Ca, Mg - Applications • Production of HF: CaF2(s) + H2SO4(l) 2 HF(g) + CaSO4 (s) • Production of steel. • Production of H3PO4: Ca5F(PO4)3(s) + 5 H2SO4(l) 5 CaSO4 + fluoroapatite 3 H3PO4(aq) + HF(g) • CaO (lime) • CaCO3 (limestone) • Mortar from slaked lime: Ca(OH)2(s) + CO2 (g) CaCO3(s) + H2O(l) • Hard water: CaCO3(s) + H2O(l) + CO2(g) Ca2+(aq) + 2 HCO3-(aq) CO2 can be removed by evaporation and CaCO3 precipitates. • Very important biological elements: chlorophyll (contains Mg2+). • Barium compounds have also medical applications. Group 3A • Boron: metalloid. • Aluminum, Gallium, Indium, Thallium: metals. • Al is the third most abundant element in the earth crust. All other elements are very rare. • Diagonal relationship: Li Mg; Be Al; B Si • B2O3 boric oxide, B(OH)3 boric acid, in general borates ~ silicates. • Be(OH)2 and Al(OH)3 are both amphoteric. • Chlorides, bromides, iodides react vigorously with water (same as silicon compounds). • Hydrides of boron and silicon are volatile and flammable. • Hydrides of beryllium and aluminum are colorless, nonvolatile solids extensively polymerized: Al-H-Al bonds. • Electronic configuration is ns2np1 (generally have an oxidation number = 3+, but heavier elements have 1+). Boron • Borax: Na2B4O7 10. H2O • Pure B can be obtained electrochemically from the oxide or halide. • Mg can be used for chemical reductions: B2O3(s) + 3 Mg(s) 2 B(s) + 3 MgO(s) • B has several allotropes, all have an icosahedron (2-sided polyhedron) for 12 covalently linked boron atoms. Due to this B is very hard, refractory, an a semiconductor. • Al, Ga, In, and Tl are all relatively lowmelting, soft metals with high electrical conductivity. Boron - Therapy • 10B isotope (not 11B) has the ability to capture slow neutrons • In BNCT, tumor cells preferentially take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B --> 7Li neutron capture reaction (that produces a photon and an alpha particle) • 10B + 1n ---> 7Li + 4He + photon Boron - Compounds • Borax: Na2B4O7 . 10 H2O is used to dissolve other metal oxides – cleaning metallic surfaces. • A better formula for the ion of borax is: B4O5(OH)42• Can produce boric acid (a very weak acid): Na2B4O7 10 H2O(s) + H2SO4(aq) 4 B(OH)3(aq) + Na2SO4(aq) + 5 H2O(l) O HO B B OH O O B HO O O B OH Boron - Compounds • Boric acid has both Lewis and Brownsted acid behavior. • Used as antiseptic. • Boric acid is dehydrated to boric oxide: 2 B(OH)3(s) B2O3(s) + 3 H2O(l) • Borosilicate glass: 76% SiO2, 13% B2O3, Al2O3, Na2O. Gives the glass higher melting T, better resistance to acids. • B is less electronegative and H, in these hydrides H has a negative charge. Boranes: BxHy: • Diborane B2H6 is a colorless, gaseous compound. Boron - Compounds • Boranes are electron-deficient molecules, hydrogens have to two bonds, to to B atoms. • Diborane has a large endothermic DHof = +41 kJ/mol • Considered a good fuel: B2H6(g) + 3 O2(g) B2O3(s) + 3 H2O(g) DHo = -2038 kJ sp3 H H H B B H H 2e- spread over 3 orbitals H Boron - Compounds • Synthesized from sodium borohydride (white, crystalline, water-soluble solid made from NaH and borate). 2 NaBH4(s) + I2(s) B2H6(g) + 2 NaI(s) + H2(g) 4 NaH(s) + B(OCH3)3(g) NaBH4(s) + 3 NaOCH3(s) • Sodium borohydride is used to bleach wood pulp. • It is also used in the electrode-less plating of metals onto plastics. It is a reducing agent used to reduce aldehydes, carboxylic acids, and ketones. Boron - Compounds H H C H Element Valence eElectroneg. Radius •• B C H B 3 2.0 88 N B—N š bonding C 4 2.5 77 N 5 3.0 70 pm Boron - Compounds H B2H6 + NH3 NaBH4 Borazine: Mol. wt. = 80.5 Mp = -57 ˚C; Bp = 55 ˚C B—N = 144 pm H B H •• N N •• BCl3 + NH4Cl B H N •• B H H Benzene: Mol. wt. = 78.1 Mp = 6 ˚C; Bp = 80 ˚C C—C = 142 pm Boron - Compounds • Boron halides BX3 are monomeric and volatile. • Boron halides form Lewis acid complexes. BF3 BCl3 BBr3 BI3 Mp, ˚C -127.1 -107 -46 49.9 Bp, ˚C -99.9 12.5 91.3 210 Unused p orbital F B F F Low p bonding F B F F Order of Lewis acidity: BF3 < BCl3 < BBr3 < BI3 Aluminum • Low cost, alloys with other metals, pure is soft and weak. • Low density, strength, inert to corrosion. • Aluminum foil, cans, parts of aircraft. • Alloy: 4% copper and some Si, Mg, and Mn. • Softer aluminum like for window frames contain only Mn. • Al is easily oxidized and forms a protective coating of Al2O3 preventing further corrosion. 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) ∆H˚ = – 3351 kJ Aluminum - Production • Al is obtained electrochemically: Hall-Heroult method. • In nature: aluminosilicates (Al, . Si, O), can be break down in aluminum oxides: Al2O3 . nH2O Al2 O3 Fe2O3 SiO2 Amphoteric Basic bauxite. Acidic • Al2O3 is separed from iron 190 ūC, 30% NaOH and silicon by the Bayer process (based on amphoteric in solution Na2Si(OH)6 NaAl(OH)4 Fe2O3, solid properties): add CO2 (acidic oxide) Al2 O3 H2O + CO2 + 2 NaAl(OH)4 Na2CO3 + Al2O3 + 5 H2O Aluminum - Compounds Compound Mp, ˚C Subl. Temp. AlF3 1290 1272 AlCl3 192.4 180 AlBr3 97.8 256 AlI3 189.4 382 Aluminum - Compounds • • • • • • F F Al F F AlF3 is a lattice of Al3+ and F- ions F F F Al Octahedral Al3+ F F F F bridges F Found in cryolite, Na2AlF6 Solid AlCl3 is a layer lattice of 6-coordinate Al3+ ions. At mp the solid volume increases 85% and electrical conductivity decreases. 221 pm •• •• Cl •• Cl •• Cl •• •• Cl Al 101Þ Al Cl 118Þ Cl 206 pm In liquid and gas phase AlCl3 is dimer. AlBr3 and AlI3 are dimers in all phases. Aluminum - Compounds • Aluminum sulfate is the most important Al compound after Al(OH)3 and Al2O3. • Used in paper industry and as a flocculant in water purification. • As pH increases, associated species form. Their large charge nucleates fine, suspended dirt particles. 4+ H O Al(H2O)4 (OH2)4Al O H • Aluminum hydroxide and oxides have many different forms: a-Al2O3 Corundum – Very hard – used as abrasive in sandpaper and toothpaste a -AlO(OH) Diaspore a -Al(OH)3 Bayerite g -Al2O3 g -AlO(OH) Boehmite g -Al(OH)3 Gibbsite Group 4A • C, Si, Ge, Sn, Pb • General features: Moving away from metallic character ns2np2 configurations “inert pair” effect leads to Ge2+, Sn2+, Pb2+ Carbon • Layers of 6-member carbon rings. • sp2 C atoms. • Extended π bonding throughout the layers. Allotrope Density Hardness ∆H˚f Graphite 2.266 <1 0 Diamond 3.514 g/cm3 10 Mohs +1.90 kJ/mol • Graphite has high electrical conductivity • Diamond—has highest thermal conductivity of any known material Carbon • Uses of natural graphite: Steelmaking, refractories, crucibles, lubricants, brake linings, pencil lead (75,000 tons/year) • Uses of artificial graphite: electrodes, crucibles, motor brushes, fibers (350,000 tons/year). SiO2 + C --> (SiC) + CO2 SiC (2500 ˚C) --> Si + C • Heat coal in absence of air --> coke used in steelmaking (370 x 106 tons/year) • Incomplete combustion of hydrocarbons - > carbon black used 93% in tires (~3 kg in car tire), 3% in printing ink (>10 million tons/day). • Burn carbon in high oxygen atmosphere - > activated charcoal used in water and air filters (holes of 1-8 nm diameter -> high surface area: 1000m2/g) Carbon • Allotrope: Fullerene 5- and 6-member carbon rings. C atoms are bound into a sphere with 60 C atoms. Silicon • Quartz or sand + high purity coke --> Si – SiO2 + 2 C --> Si + 2 CO *Silicon is (oxidized or reduced) *Carbon is (oxidized or reduced) • Making very pure silicon: – 1) Si + Cl2 --> SiCl4 – 2) SiCl4 + Mg --> MgCl2 + Si * Identify the substances reduced and the substances oxidized on these reactions. • To purify the silicon, it is zone-refined Tin • Sn is relatively expensive, but used because it resists corrosion. • Pure Sn can be obtained electrochemically from SnCl2 – About 40% used in “tin plate” • “Tin cans” have 0.0004 - 0.025 mm layer of Sn on iron • About 30 x 109 cans plated annually in US • Alloys: Solder: 1/3 Sn and 2/3 Pb Bronze: 5-10% Sn + Cu Pewter: 90-95% Sn, 1-8% Sb, and < 3% Cu Bearing metal: 80-90% Sn, 5% Cu, and Pb Lead • Most abundant of the “heavy metals” • Romans used it in “plumbing” – the word comes from the Latin name for the element • • • • Main ore is galena, PbS 2 PbS + 3 O2 --> 2 PbO + 2 SO2(g) PbO + C --> Pb + CO About 60% of the batteries sold are Pb storage batteries ANODE: Pb(s) + HSO4- --> PbSO4(s) + H+(aq) + 2eCATHODE: PbO2(s) + 3 H+(aq) + HSO4-(aq) + 2e- --> PbSO4(s) + 2 H2 O Carbon - Compounds • CO2 — over 30 x 106 tons produced in US/year. – 1/2 used as refrigerant and propellant in aerosols – 1/4 used to “carbonate” soft drinks • CO2 is a “greenhouse” gas. Silicon - Compounds SiO2 is not like CO2 • Reason is that 2 Si=O bonds are weaker (~640 kJ each) than 4 Si-O bonds (464 kJ each) • Also orbital overlap to form Si=O is not efficient. • Most common form is alpha-quartz • Less pure forms are rose quartz, smoky quartz, amethyst, citrine. • All silicon-oxygen compounds have corner-shared SiO4 tetrahedra • a-Quartz has interlinked helical chains of SiO4 tetrahedra. • Helices can be right- or left-handed, so crystals are optically active. Silicon - Compounds • Quartz is a key electronic material, 2nd only to Si in volume. • Citrine and amethyst have Fe2+/Fe3+ impurities in quartz that give color. • Quartz consists of interlinked chains of SiO4 units. Silicon - Compounds • Quartz exhibits the property of piezoelectricity. – The production of an electric dipole when the crystal is deformed. • Piezoelectric effect is used to control oscillators in electric circuits such as watches and radios. • Most quartz used commercially is synthetic. • At 350-400 ˚C and 1-4 kilobars, SiO2 dissolves slightly in 1 M NaOH 3 SiO2 + 2 OH- ---> Si3O72- + H2O • SiO2 crystallizes on quartz seed crystals. Blue from Co2+ ions and brown from Fe2+ ions. mica Silicon - Compounds • Silicates have chains of SiO4 tetrahedra, often linked into a sheet structure. • Clays have sheets of SiO4 tetrahedra bound to sheets of AlO6 octahedra (large variety of clays). Used as remedies for stomach upset because they absorb toxins. • The large disk is baked clay from Africa; used medicinally. Silicon - Compounds • When quartz is melted, it forms silica glass. • Mix: 60-75% silica, 12-18% soda (Na2CO3), 5-12% lime (CaO): most common type and least expensive. Poor resistance to sudden temperature changes and to corrosive chemicals. • Add about 20% PbO: glass is relatively soft with a high refractive index gives it brilliance, used for art glass and electrical applications. • Add at least 5% B2O3: high resistance to temperature change and chemical corrosion, used in pipelines, light bulbs, photochromic glasses, sealed-beam headlights, lab ware, baking ware. Silicon - Compounds • Add AgCl and CuCl2: obtain a photochromic glass Cl- + light --> Cl + e• Darkening reaction: e- + Ag+ --> Ag (s) • Reversing reactions: Cl + Cu+ --> Cu2+ + ClCu2+ + Ag --> Cu+ + Ag+ • Mix impurities: colored glass – – – – – – – – Blue-green: Fe2+ Yellow-green: Fe3+ Blue glass: Co2+ Purple: Mn2+ Fe2+ + Cr salts --> green wine bottles Fe2+ + S --> brown U2+: yellow Se2-: red (as in traffic lights) Dale Chihuly Art www.chihuly.com Silicon - Compounds • 2 CH3Cl + Si --> SiCl2(CH3)2 • Also produces SiCl3(CH3), SiCl (CH3)3, and SiCl4 • Patented by E. G. Rochow of GE in 1945 • (CH3)3SiCl + 2 H2O --> (CH3)3Si—O—Si(CH3)3 + 2 HCl • R2SiCl2 + 2 H2O --> HO—SiR2—OH + 2 HCl • 2 HO—SiR2—OH ---> HO—SiR2—O—SiR2—OH + H2O Units link to give polymers! • Properties of silicones – Good thermal and oxidative stability, resistant to high and low Temp’s – Water repellent, antistick and antifoam properties – Resistant to UV radiation and weathering – Physiologically inert (*see breast implant studies) Silicon - Compounds • Can be made into oils, greases, emulsions, elastomers, and resins: • Production of >350,000 tons annually: 1000 different products • 65-70% fluid silicones: Cosmetics — suntan lotion, lipstick, antifoams — sewage treatment, antifroth — cooking oil, car polish, lubricants, release agents. • 25-30% elastomers: SiO2 added to linear dimethylpolysiloxane: Retains inertness, flexibility, elasticity, and strength up to 250 ˚C and down to –100 ˚C. Used in industrial sealants, belts and gaskets, medical tubing, space suits, etc. • 5-10% resins: Pure silicone resins are poly(organosiloxanes) with a large proportion of branched siloxyl groups. Used as raw materials for paints, binders and in building preservation. Electrical industry: insulating lacquers and used as high temperature enamels. Group 5A • Nitrogen, phosphorus, arsenic, antimony, and bismuth • N exists as N2 molecules. Others have more complex forms. • N2 is quite unreactive owing to the NN triple bond. • Atmosphere is about 80% N2 • N2 easily liquefied. Boils at -196 0C. • Used as a refrigerant • Phosphorous originally prepared from human waste. • Now obtained from the reduction of Pcontaining minerals such as Ca3(PO4)2. Nitrogen - Compounds • NH4NO3 --> N2O + 2 H2O • N2O, nitrous oxide (dinitrogen oxide), used as an anesthetic. Soluble in fats. Used as propellant in whipped cream cans. • NO, nitrogen oxide, is present in polluted air. Has 11 valence e-, is implicated in biological processes (circulatory system). Reacts readily with O2 to give NO2. HNO3 --> 2 N2O + H2O + 1/2 O2 • NO2, nitrogen dioxide, is a brown gas in equilibrium with N2O4, a colorless gas. Is a common air pollutant Nitrogen - Compounds • Haber Process: N2 + 3 H2 --> 2 NH3 • HNO3 production: – NH3 gas is oxidized on Pt surface in air to NO and NO2. Pt wire catalyzes reaction. Heat of reaction causes wire to glow. 4 NH3 + 5 O2 ---> 4 NO + 6 H2O – NO2 in water gives HNO3 (and HNO2). – Better prep’d from: 2 NaNO3 + H2SO4 --> 2 HNO3 + Na2SO4 • HNO3 readily reacts with almost all metals - except Al - to give metal nitrate and NO2. Cu Al Phosphorus – Compounds • White = P4 tetrahedron • Red = polymer of P4 tetrahedra • Yellow phosphorus spontaneously burns in air. P4(s) + 5 O2(g) -->P4O10(s) • Product: tetraphosphorus decaoxide. Phosphorus – Compounds OXIDES: Phosphorus – Compounds • Phosphorus reacts readily (is oxidized) with chlorine to give PCl3 and PCl5. • A match head contains an oxidizing agent (KClO3) and P4S3. • The striking strip on a match box contains red P. • Redox reaction lights the match. Group 6A • Oxygen, Sulfur, Selenium, Tellurium, Polonium. • O2 condenses to a pale blue liquid at -183 oC • * List ways to obtain O2. • Liquid O2 is paramagnetic and clings to a magnet. • O2 allotrope: Ozone, O3, is made by passing O2 through electric discharge. • The most stable allotrope of S is a crown-shaped ring of 8 S atoms (S8). • Heating S8 to the melting point causes the rings to open and a polymeric allotrope forms. Sulfur - Compounds • Sulfur is found in pure form in underground deposits along the coast of the U.S. It is recovered by pumping superheated steam into the beds to melt the S. • Sulfur is burned in air to give SO2 and then SO3. • SO3 reacts with water ---> H2SO4 • H2SO4 is the chemical produced in the largest amount in the U.S. • Vents -- BLACK SMOKERS -- in the bottom of the world’s oceans are a source of metal sulfides. Sulfur - Compounds • Orpiment, As2S3 • Pyrite (fool’s gold), FeS • Stibnite, Sb2S3 • Lead sulfide (galena), PbS Sulfur - Compounds • SO2 is produced by burning sulfur in oxygen. • SO2 is produced by treatment of metal sulfides with O2. • 2 ZnS + 3 O2 --> 2 ZnO + 2 SO2 • Also produced by burning fossil fuels. • About 2 x 108 tons of sulfur oxides are released into the atmosphere by human activities annually. Halogens – Group 7A • Diatomic elements: Cl2 gas, liquid Br2, and solid I2 • Cl is the most abundant in Group 7A. • Occurs in the sea and in salt (NaCl) deposits. Chlorine - Preparation • Commercial: Electrolysis of NaCl(aq) in a membrane cell. Anode: 2 Cl-(aq) --> Cl2(g) + 2eCathode: 2 H2O(liq) + 2e- --> H2(g) + 2 OH-(aq) • Oxidation of NaCl with strong oxidant (K2Cr2O7). Cl2 gas bubbles into water. Iodine - Preparation 2 I- + 4 H+ + MnO2 --> Mn2+ + I2 + 2 H2O Mixture of NaI and MnSO4 Add H2SO4; reaction produces I2. Halogens Reactions • Halogens react with nonmetals and metals to give covalent or ionic halides. Chlorine - Reactions • Forms ionic compounds: Bromine - Reactions • With metals give metal bromides. • Aluminum reaction: End of Chapter • Go over all the contents of your textbook. • Practice with examples and with problems at the end of the chapter. • Practice with OWL tutor. • Practice with the quiz on CD of Chemistry Now. • Work on your OWL assignment for Chapter 21. 21.12. Place the following oxides in order of increasing basicity: Na2O, Al2O3, SiO2, SO3. 21.14. Complete and balance the equations for the following reactions. K(s) + I2(g) Ba(s) + O2(g) Al(s) + S8(s) Si(s) + Cl2(g) 21.24. (a) Write equations for the half-reactions that occur at the cathode and the anode when an aqueous solution of KCl is electrolyzed. Which chemical species is oxidized, and which chemical species is reduced in this reaction? (b) Predict the products formed when an aqueous solution of CsI is electrolyzed. 21.26. Calcium reacts with hydrogen gas at 300−400 °C to form a hydride. This compound reacts readily with water, so it is an excellent drying agent for organic solvents. (a) Write a balanced equation showing the formation of calcium hydride from Ca and H2. (b) Write a balanced equation for the reaction of calcium hydride with water (Figure 21.6). 21.36. (a) Write an equation for the reaction of Al and H2O to produce H2 and Al2O3. (b) Using thermodynamic data in Appendix L, calculate DH°, DS°, and DG° for this reaction. Do these data indicate that the reaction should favor the products? (c) Why is aluminum metal unaffected by water? 21.42. "Aerated" concrete bricks are widely used building materials. They are obtained by mixing gas-forming additives with a moist mixture of lime, sand, and possibly cement. Industrially, the following reaction is important: 2 Al(s) + 3 Ca(OH)2(s)+ 6 H2O(l) [3 CaO . Al2O3 . 6 H2O](s) + 3 H2(g) Assume that the mixture of reactants contains 0.56 g of Al for each brick. What volume of hydrogen gas do you expect at 26 °C and atmospheric pressure (745 mm Hg)? 21.54. Unlike carbon, which can form extended chains of atoms, nitrogen can form chains of very limited length. Draw the Lewis electron dot structure of the azide ion,N3-.