11_Electrochemistry3

11_Electrochemistry3 - Electrolysis 20.9 and Metallurgy...

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Unformatted text preview: Electrolysis 20.9 and Metallurgy Chapter 23.1 23.4: MJ Bojan Metallurgy and electrolysis 1 Chemistry of the elements Occurrence of elements Abundance Where in nature State of elements IsolaIon and purificaIon Metals (metallurgy) Nonmetals Trends in bonding Important OxidaIon States Acid base properIes MJ Bojan Metallurgy and electrolysis 2 The periodic table MJ Bojan Metallurgy and electrolysis 3 Abundance of elements Terrestrial abundance O 46.6% 10 9 Si 27.72% 8 7 % by weight 6 5 4 3 2 1 0 H B C N F Na Mg Al P O S Cl K Ca Ti V Cr Mn Fe Co Cu Zn Si Se Rb Mo Sn I MJ Bojan Metallurgy and electrolysis 4 Standard states of the elements The most stable form of an element at 298 K and 1 atm ("STP") Metals All solids except one (which one?) Nonmetals Atomic gases Noble gases He, Ne, Ar, Kr, Xe, Rn Diatomics halogens and H2, N2, O2 H2, N2, O2 (gas) F2 (gas) Cl2 (gas) Br2 (liquid) I2 (solid) Metalloids All solids Halogens (group 7) Other nonmetals solids C (graphite), S, P, Se MJ Bojan Metallurgy and electrolysis 5 Occurrence of Metals: Minerals Metals commonly found in their pure elemental form: Ag, Au, Pt noble metals Most metals are found in solid inorganic compounds known as minerals. Minerals are named by common, not chemical, names. MJ Bojan Metallurgy and electrolysis 6 Minerals Most important metals are found in minerals as oxides, sulfides, or carbonates. Aluminosilicates and silicates Metal + Al, Si, O difficult to extract metals (Beryl = Be3Al2Si6O18) Nonsilicate minerals Oxides Al2O3, TiO2, Fe2O3 Sulfides PbS, ZnS, CuFeS2 Carbonates CaCO3 Salts Active metals- Group I, II Not found in nature as pure metals, always combined with other elements (e.g. salts in the ocean, minerals) MJ Bojan Metallurgy and electrolysis 7 Metallurgy Science and technology of extracting metals from their natural sources and preparing them for practical use It involves Mining. ConcentraIng ores. Reducing ores to obtain free metals. Electrometallurgy. Pyrometallurgy Hydrometallurgy Purifying metals. Mixing metals to form alloys that have the properIes desired. MJ Bojan Metallurgy and electrolysis 8 ELECTROLYSIS Electrolysis: Driving non-spontaneous reactions by applying electrical energy. An electrolysis cell consists of two electrodes in either aqueous solution (of ions) or in a molten salt e.g. molten NaCl. The anode The cathode MJ Bojan Metallurgy and electrolysis 9 Electrolysis MJ Bojan Metallurgy and electrolysis 10 Electrolysis: Things to consider 1. What reactants are present? 2. What are possible half reacIons and their E or Eo? 3. Use the Nernst equaIon to account for non-standard condiIons. Example: hydrolysis of water Anode: 2H2O(l) O2(g) + 4H+(aq) + 4e- Eo = -1.23 V Cathode: 2 (2H2O(l) + 2e- H2(g) + 2OH-(aq)) Eo = -0.83 V Overall: 2H2O(l) 2H2(g) + O2(g) Eocell = -2.06 V MJ Bojan Metallurgy and electrolysis 11 Hydrolysis of water, conInued Use Nernst equaIon For electrolysis of pure water [H+] = [OH-] = 1 10-7 M O2(g) and H2(g) are at 1 atm so PO2 = PN2 = 1 atm At anode At cathode Ecell = MJ Bojan Metallurgy and electrolysis 12 FYI - Electrolysis: Things to consider We could have also used the following combinaIons of half reacIon to get the same result: Eo 2 (2H+(aq) + 2e- H2(g)) 0.00 V 2H2O(l) O2(g) + 4H+(aq) + 4e- -1.23 V 2H2O(l) 2H2(g) + O2(g) Eo cell = -1.23 V You should be able to prove that the same voltage (-1.23 V) would be obtained in pure water, pH 7, as in standard soluIon, [H+] = 1M. MJ Bojan Metallurgy and electrolysis 13 Analyzing the electrolysis of aqueous NaCl 1. What is in the soluIon? 2. What are the half reacIons and their Eo? Possible cathode reacIons: Possible anode reacIons: Eo Eo MJ Bojan Metallurgy and electrolysis 14 Analyzing electrolysis of aqueous NaCl, conInued 3. Use the Nernst EquaIon to account for [H+] = [OH-] = 1 x 10-7 M in pure water (changes half cell potenIals) when adjusted to pH 7. Cathode Eo Na+(aq) + e- Na(s) -2.71 V 2H2O(l) + 2e- H2(g) + 2OH-(aq) -0.42 V Anode Eo -1.36 V -0.83 V 2Cl-(aq) Cl2(g) + 2e- 2H2O(l) O2(g) + 4H+(aq) + 4e- Which reaction will occur at the electrodes? MJ Bojan Metallurgy and electrolysis 15 Analyzing electrolysis of aqueous NaCl, conInued 4. What are the reacIons at the electrodes? Cathode: Anode: Overall: MJ Bojan Metallurgy and electrolysis 16 Analyzing electrolysis of aqueous NaCl, conInued 5. Why is anode half reacIon not predicted from Eo? a. Because Cl- (aq) is ajracted to the anode and so the local concentraIon is >> 1M. b. The anode half reacIon is kineIcally controlled. What is left behind? MJ Bojan Metallurgy and electrolysis 17 ELECTROLYSIS OF AQUEOUS Na2SO4 What are the reactions at the electrodes? Cathode Anode Overall ___________________________ MJ Bojan Metallurgy and electrolysis 18 How much electricity is needed? (How many electrons?) Electrolysis of CuSO4 Cu2+ + 2e- Cu 1. How many moles of electrons are required to produce a mole of Cu(s)? 2. What is the charge on a mole of electrons? 3. How many Coulombs is this? 4. How long does it take for a mole of electrons to pass through a circuit? MJ Bojan Metallurgy and electrolysis 19 ELECTROLYSIS CALCULATIONS 1 mole of e- = 1 Faraday = 96,500 Coulombs = charge on 1 mole of e1 F = 96,500 coulombs/mol 1 Ampere = 1 coulomb/second 1 coulomb = 1 Amp-sec Electromotive Force (EMF) force that causes electrons to flow (voltage) 1 Watt = 1 Amp-Volt 1 Joule = 1 coul-Volt = 1 Amp-sec-Volt = 1 Watt-sec 1 kW-hour = (1000 Watt)(3600 sec) = 3.6 x 106 Watt-sec = 3.6 x 106 Joules MJ Bojan Metallurgy and electrolysis 20 ELECTROLYSIS CALCULATION Electrolysis of CuSO4 gives 1.00g of Cu. Reaction is: Cu2+ + 2e- Cu 1. How many Faradays (F) of charge are required? 2. How many Coulombs is this? MJ Bojan Metallurgy and electrolysis 21 ELECTROLYSIS CALCULATION 3. If 1.00g of Cu is obtained in 1 hour, how many amps are required? 4. If 2 amps were used, how long would it take to produce 5 g of Cu? MJ Bojan Metallurgy and electrolysis 22 Electrometallurgy Electrometallurgy is the process of obtaining metals through electrolysis. Two different starting materials: -molten salt -aqueous solution COMMERCIAL APPLICATIONS OF ELECTROLYSIS Production of metals Na, Al. Purification of Metals Cu. Electroplating. MJ Bojan Metallurgy and electrolysis 23 Electrolysis of Molten Sodium Chloride NaCl is electrolyzed in a Downs cell. Gaseous Cl2 is allowed to disperse. Molten Na is siphoned off. MJ Bojan Metallurgy and electrolysis 24 ELECTROLYSIS OF MOLTEN NaCl What are the reactions at the electrodes? Cathode Anode Overall PYROMETALLURGY Pyrometallurgy: using high temperatures to obtain the free metal. Several steps are employed: Calcination is heating of ore to cause decomposition and elimination of a volatile product: PbCO3(s) PbO(s) + CO2(g) Roasting is heating which causes chemical reactions between the ore and the furnace atmosphere: 1. Burns off organic matter. 2. Converts carbonates and sulfides to oxides: 2 ZnS(s)+ 3O2(g) 2ZnO(s) + SO2(g) 3. Less active metals are often reduced HgS(s) + O2(g) Hg(l) + SO2(g) Smelting is a melting process in which materials formed during reactions separate into two or more layers. Refining is the treatment of a crude, relatively impure metal to improve its purity and better define its composition. MJ Bojan Metallurgy and electrolysis 26 Pyrometallurgy of Iron sources of iron: hematite Fe2O3 and magnetite Fe3O4. Iron Ore: Fe2O3 and SiO2 Add limestone and coke Coke is coal that has been heated to drive off the volatile components. Purified iron exits the furnace at the bojom. MJ Bojan Metallurgy and electrolysis 27 Pyrometallurgy of Iron Reactions 2C(s) + O2(g) 2CO(g) + heat heat + C(s) + H2O(g) CO(g) + H2(g) Fe3O4(s) + 4CO(g) 3Fe(l) + 4CO2(g) Fe3O4(s) + 4H2(g) 3Fe(l) + 4H2O(g) Coke: 1) heats furnace 2) reduces iron Why is water added? Why is limestone CaCO3 added? MJ Bojan Metallurgy and electrolysis 28 Pyrometallurgy of Iron At high T CaCO3 CaO + CO2 CaO + SiO2 Metal + nonmetal oxide oxide basic acidic CaSiO3(l) slag Limestone (CaCO3) removes SiO2 (and other) impurities slag floats on Fe(l); protects it from oxidation by O2 Slag: cement cinder block building materials MJ Bojan Metallurgy and electrolysis 29 Pyrometallurgy of Iron Refining Product in blast furnace: pig iron Refining: done in brittle; not strong Bessemer Converter O2 (g) bubbled through molten iron to oxidize remaining impurities CaO slag still present to remove impurities Alloying elements added as liquid iron is being removed. Purified molten steel is poured into molds. MJ Bojan Metallurgy and electrolysis 30 Hydrometallurgy These are techniques in which metal is extracted from ore via the use of aqueous reacIons. Leaching Bayer process MJ Bojan Metallurgy and electrolysis 31 Hydrometallurgy: Leaching Leaching is a process in which a metal-containing compound is selecIvely dissolved. Metals like Au, Ag are found in nature as pure elements, however, large deposits of these metals are rare. We can remove small amounts from rock by leaching. One can use water if the metal-containing compound is water soluble, but more open one must use acid, base, or a salt soluIon. Example: dissolve gold by complexaIon with CN-: 4 Au(s) + 8 CN-(aq) + O2(g) + 2 H2O(l) 4 Au(CN)2-(aq) + 4 OH-(aq) Kf[Au(CN)2]- = 2 x 1038 Pure gold is then obtained by reducIon: 2 Au(CN)2-(aq) + Zn(s) Zn(CN)42-(aq) + 2 Au(s) MJ Bojan Metallurgy and electrolysis 32 Hydrometallurgy: Bayer Process Aluminum is second most useful metal. Bauxite: Al2O3.xH2O. primary ore for Al ~ 50 % Al2O3 impurities: SiO2 Fe2O3 Bayer Process: bauxite is concentrated to produce aluminum oxide. Al2O3 H2O(s) + 2 H2O(l) + 2 OH-(aq) 2 Al(OH)4-(aq) hydrated metal complex Dissolve bauxite in strong base (NaOH) at high T, P (Al2O3 dissolves) Filter out solids, aluminate ion stays dissolved. Fe2O3, SiO2 do not dissolve Lower pH, Al(OH)3(s) precipitates Take advantage of amphoteric nature of Al MJ Bojan Metallurgy and electrolysis 33 Electrometallurgy of Aluminum Hall process electrolysis cell is used to produce aluminum. Problem: Al2O3 melts at 2000C and it is impractical to perform electrolysis on the molten salt. Hall: use purified Al2O3 in molten cryolite (Na3AlF6, melting point 1012C). Anode: C(s) + 2O2-(l) CO2(g) + 4e- Cathode: 3e- + Al3+(l) Al(l) The graphite rods are consumed in the reaction. MJ Bojan Metallurgy and electrolysis 34 Electrometallurgy of Aluminum MJ Bojan Metallurgy and electrolysis 35 Hydrometallurgy of Cu Copper containing ore (CuFeS2) is stirred with aqueous H2SO4 + O2 Redox Reaction 2CuFeS2(s)+2H+(aq)+SO42-(aq) + 4O2(g) 2Cu2+(aq) + 2SO42-(aq) + Fe2O3(s) + 3S(s) + H2O \ / 2CuSO4(aq) Electrolyzed to Cu MJ Bojan Metallurgy and electrolysis 36 Electrolysis to purify Copper Because of its good conductivity, Cu is used to make electrical wiring. Impurities reduce conductivity, therefore pure copper is required in the electronics industry. MJ Bojan Metallurgy and electrolysis 37 PurificaIon of Copper What are the reactions at the electrodes? Cathode thin sheet of pure copper Anode impure copper As the reaction proceeds, what happens to Cu? MJ Bojan Metallurgy and electrolysis 38 ...
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