Electrochemistry Electrochemistry is a very diverse area. It can be broadly divided into a) analytical electrochemistry which is concerned with methods of measurement.
Download ReportTranscript Electrochemistry Electrochemistry is a very diverse area. It can be broadly divided into a) analytical electrochemistry which is concerned with methods of measurement.
Electrochemistry Electrochemistry is a very diverse area. It can be broadly divided into a) analytical electrochemistry which is concerned with methods of measurement involving potentiometry (pH meters etc), voltammetry, and modern sensors (generally voltammetric in nature) b) physical electrochemistry is the basis of analytical electrochemistry, but is generally concerned with the effect of electric fields, charge balance, and diffusion. c) Chemical electrochemistry usually is less interested in quantitative analysis but is devoted to understanding the mechanisms of electron transfer based on chemical structure. d) Biologic electrochemistry can be understood as a form of physical electrochemistry (charge and fields around biomolecules) and of chemical electrochemistry (electron transfer events in biological systems) e) Geologic and environmental electrochemistry is concerned with all of the above as they take place in the natural environment . Of particular interest are the oxidation reduction reactions of iron, manganese, chromium, arsenic, sulfur, as these set the parameters in which life can grow and, as the oxidation state of iron has substantial implications for the stability of various minerals and their dissolution/formation. f) Technical - in this field fall the major areas of batteries, solar energy, fuel cells, and corrosion sciences. Electrochemistry Fundamental concepts Electron flow, nomenclature Energy in the electron flow Kinetics of the electron transfer event as influenced by a) energy b) homogeneous vs heterogeneous system Electrochemistry Energy : 1. 2. 3. Heat out/Heat in = enthalpy, H Equilibrium Constants, K Universal Entropy ~ Free energy, G Use total free energy – not just enthalpy aq 4 ,aq 8H MnO 2 aq 5Fe 2 aq Mn 3 aq 5Fe 4 H2 O This reaction is exothermic. How much? Hrxo species 8H+(aq) 4H2O(liquid) MnO4-(aq) Mn2+(aq) 5Fe2+(aq) 5Fe3+(aq) o n H i , products f ,i , products Hfo (kJ/mol) 0 -286 -543 -218.8 -47.69 -87.86 o n H i ,reac tan ts f ,i ,reac tan ts H rxo 1 218.8 5 87.86 4 286 8 0 1 543 5 47.69 Hrxo 2126 78145 . Hrxo 1344kJ This reaction could do a lot of work for us if we could get the energy aq 2 aq 8H 5Fe 4 ,aq MnO Fe2+ 2 aq Mn 3 aq 5Fe MnO4- Hrxo 1344kJ 4 H2 O Heat water to convert energy? inefficient clumsy heat loss Water Bath Fe2+ MnO4- Water circulation steam drive pistons Make electricity Alternative Strategy: Capture electrons directly Split into 2 partial reactions Fe2+ MnO4e Mn2+ Fe3+ To do this Need to balance Redox reactions Writing the Net Ionic Reaction of an Oxidation-Reduction Reaction Split into Reactions to be balanced Write oxidation half reaction Write reduction half reaction A. Balance atoms of element oxidized b. Balance Rx sides by adding e c. Balance charge by adding H+ or OHd. Balance hydrogen by adding H2O e. Balance oxygen A. Balance atoms of element reduced b. Balance Rx sides by adding e c. Balance charge by adding H+ or OHd. Balance hydrogen by adding H2O e. Balance oxygen Combine and balance electrons Writing the Net Ionic Reaction of an Oxidation-Reduction Reaction Reaction to be balanced Write oxidation half reaction Write reduction half reaction A. Balance atoms of element A. Balance atoms of element reduced oxidized b. Balance Ox number with e b. Balance Ox number with e c. Balance charge by adding H+ c. Balance charge by adding H+ or OHor OHProtons are added d. Balance hydrogen by adding d. Balance hydrogen by adding in an acidic solution H2O H2O and hydroxyls are e. Balance oxygen e. Balance oxygen added in a basic solution. Combine and balance electrons When in doubt add protons Example: Balance the following redox equation in an acidic solution 2 ( aq ) Fe 4( aq ) MnO 3 ( aq ) Fe Ox #balance 2+ 3+ 2 ( aq ) Fe 2 ( aq ) +7 Mn 4( aq ) MnO 3 ( aq ) Fe +2 2 ( aq ) Mn Mass balance Mass balance 2 ( aq ) Fe 3 ( aq ) Fe ox# balance e +2 -1 = 4(-2) +? 8-1 = ? 7=? 4( aq ) MnO 2 ( aq ) 5e Mn Example: Balance the following redox equation in an acidic solution 2 ( aq ) Fe 2+ 2 ( aq ) Fe 4( aq ) MnO 3 ( aq ) 2 ( aq ) Fe 3+ +7 Mn 4( aq ) MnO 3 ( aq ) Fe +2 2 ( aq ) Mn Mass balanced Mass balanced 2 ( aq ) Fe 3 ( aq ) Fe e Ox numbers 2 4( aq ) ( aq ) MnO 5e Mn +2 -1 + -5 = -6 4( aq ) MnO Charge balance 2 ( aq ) 5e 8Haq Mn Example: Balance the following redox equation in an acidic solution 2 ( aq ) Fe 2 ( aq ) Fe 3 ( aq ) Fe 4( aq ) MnO Fe 4( aq ) e MnO 3 ( aq ) 2 ( aq ) Mn ( aq ) 5e 8H 2 ( aq ) Mn Balance hydrogen by adding water 4( aq ) MnO ( aq ) 5e 8H Check oxygen balance 4O 2 ( aq ) Mn =+2 Check total charge balance: (-1)+5(-1)+8(+1)=+2 4H2 O =4O Example: Balance the following redox equation in an acidic solution 2 ( aq ) Fe 2 ( aq ) Fe 3 ( aq ) Fe 4( aq ) MnO 3 ( aq ) Fe 2 ( aq ) Mn e 4( aq ) MnO ( aq ) 5e 8H Mn Recombine while balancing electrons 2 ( aq ) 5Fe 3 ( aq ) 5Fe 5e 2 ( aq ) 4H2 O Example: Balance the following redox equation in an acidic solution 2 ( aq ) Fe 2 ( aq ) Fe 3 ( aq ) Fe 4( aq ) MnO 3 ( aq ) Fe Mn e 4( aq ) MnO ( aq ) 5e 8H 2 ( aq ) 5Fe 4( aq ) MnO 2 ( aq ) 2 ( aq ) 5Fe ( aq ) 8H 4H2 O 3 ( aq ) 5e 2 ( aq ) 4H2 O Mn 5Fe 3 ( aq ) 5Fe 2 ( aq ) Mn Final equation does not show any electrons because electrons “do not exist” in solution OJO: Important Point The reactions 4( aq ) MnO ( aq ) 5e 8H 2 ( aq ) 5Fe 1. 2. 3. 2 ( aq ) Mn 3 ( aq ) 5Fe 4H2 O 5e do not really occur by themselves they are linked through the electrons DEFINITION: = 1/2 reactions If all reactions are considered Half reactions where does the Electron go? aq e nH2 O e aq e nH3O H H2 O eaq H2 O H HO 10 1 1 k , 2.3x10 M s k , 19 . x101 M 1 s 1 Hydrated electrons not only react with water But with other species including biological, Hence it is a good way to sterilize water aq e RSH HS R eaq O2 O2 HOO HO pK HA 4.9 H2 O 10 1 1 k , 11 . x10 M s k , ~ 1010 M 1 s 1 Hydrated electrons in aerobic biology will produce Finite fluxes of the soft radical HOO Not only have to consider rate But energy aq e nH2 O e eaq nH3 O H H2 O E , 30 . VvsNHE o k , 2.3x1010 M 1 s 1 E o , 2.10V vs NHE eaq H2 O H HO H-O bond Water 464 kJ Hydronium 301 kJ k , 19 . x101 M 1 s 1 E , 2.930V vs NHE o Some other practical considerations Do you think the Mn/Fe reaction will continue for long? e + anion cation MnO4( aq ) 5e 8H(aq ) Mn(2aq ) 4 H2 O 5Fe(2aq ) 5Fe(3aq ) 5e 1MnO4- 5Fe2+ Net charge =0 Net charge =10(-1)+5(+3) =+5 - - - - - - -+ - + Net charge =0 e Fe3+ Mn2+ Net charge = (+2)+2(+1 +(-1)=+3 Will want to let spectator ions flow (but not the reactants!) e (current) Fe3+ 5+ - +1 “jelly” (salt bridge) retards motion of Fe3+/2+ MnO4“jelly” allows motion of spectators which produces Charge balance Weird Grammar Rules: Those Italians! Volta discovered this process 1. 2. 3. Always make electrons flow to right Electrons flow down to the cathode (cat = Greek for down). Electrons flow up into the anode (an = Greek for up) Count Alessandro Volta, Italy ~1800, first battery e (current) Rsb anode Oxidation electrons taken Out (up = anode) oxidation Cl- There is Resistance In the system, We will come Back to this cathode Rsoln Reduction electrons accepted In (down = cathode) reduction An An ox ox jumped jumped over over aa red red cat Vocabulary for Work when using electrons instead of heat. Ohm’s Law: Voltage = current x resistance V IR Voltage = energy required to move charge = Joules/Coulomb Georg Simon Ohm, 1789-1854 German physicist having a good hair day a coulomb is a unit of charge F=Faraday = 96,485 coulombs of charge/mole of e Coulomb Joule Coulomb Joule Coulomb mole e Coulomb mole mole e e Coulomb molese FFF FF VVV V J nn molese molese nnn molese molese FF V Ohm nFV J Joule Coulomb Nernst neg sign accounts for negative electron nFV G V directly relates to free energy because we are not Separately the work terms into heat and changing surrounding randomness with the heat Galen, 170 Marie the Jewess, 300 Charles Augustin James Watt Coulomb 1735-1806 1736-1819 Justus von Thomas Graham Liebig (1803-1873 1805-1869 Ludwig Boltzman 1844-1906 Gilbert N Lewis 1875-1946 Henri Louis LeChatlier 1850-1936 Johannes Bronsted 1879-1947 Jabir ibn Hawan, 721-815 Luigi Galvani 1737-1798 Richard AC E Erlenmeyer 1825-1909 An alchemist Count Alessandro G A A Volta, 1747-1827 James Joule (1818-1889) Henri Bequerel 1852-1908 Lawrence Henderson 1878-1942 Galileo Galili Evangelista Torricelli 1564-1642 1608-1647 Amedeo Avogadro 1756-1856 Rudolph Clausius 1822-1888 Jacobus van’t Hoff 1852-1911 Niels Bohr 1885-1962 John Dalton 1766-1844 William Thompson Lord Kelvin, 1824-1907 Johannes Rydberg 1854-1919 William Henry 1775-1836 Johann Balmer 1825-1898 J. J. Thomson 1856-1940 Erwin Schodinger Louis de Broglie 1887-1961 (1892-1987) Fitch Rule G3: Science is Referential Jean Picard 1620-1682 Jacques Charles 1778-1850 Francois-Marie Raoult 1830-1901 Heinrich R. Hertz, 1857-1894 Friedrich H. Hund 1896-1997 Daniel Fahrenheit 1686-1737 Max Planck 1858-1947 Rolf Sievert, 1896-1966 Blaise Pascal 1623-1662 Georg Simon Ohm 1789-1854 James Maxwell 1831-1879 Robert Boyle, 1627-1691 Isaac Newton 1643-1727 Michael Faraday 1791-1867 B. P. Emile Clapeyron 1799-1864 Dmitri Mendeleev 1834-1907 Svante Arrehenius Walther Nernst 1859-1927 1864-1941 Fritz London 1900-1954 Wolfgang Pauli 1900-1958 Johannes D. Van der Waals 1837-1923 Marie Curie 1867-1934 Anders Celsius 1701-1744 Germain Henri Hess 1802-1850 J. Willard Gibbs 1839-1903 Fritz Haber 1868-1934 Thomas M Lowry 1874-1936 Werner Karl Linus Pauling Louis Harold Gray 1905-1965 Heisenberg 1901-1994 1901-1976 For standard conditions (1 mole, 1 atm, 25C): nFV G o Greek G Komodo (Indonesia) K Vamale (Polynesia) Vo o nFV G RT ln K o o RT V ln K nF o Different languages, same information. Represent Total energy (heat + entropy) associated with a reaction Relationship G, K, V Example Problem 1 : What are K and the standard voltage associated with the Fire oxidation of lead given tabulated free energies? Grxo n G 2 Pbs O2( g) 2 PbOs 0 f , products n G of ,reac tan ts 0kJ 0kJ 187.9 kJ 1molO2 G 2molPbOs 2molPbs molPbO molPbs molO2 o rx Grxo 357.8kJ G RT ln K o Ke G 0 RT kJ 357 .8 mol Ke kJ 8.314 x 10 3 298 K mol K K 5.23x10 62 Relationship G, K, V Example Problem 1 : What are K and the standard voltage associated with the Fire oxidation of lead given tabulated free energies? 2 Pbs O2( g) 2 PbOs G 357.8kJ o rx K 5.23x10 -nFV0 = Go ? 62 Compound neutral O usually -2 +4 Ox # = 0 So Pb = +2 2(+2)=4 =-4electrons Relationship G, K, V Example Problem 1 : What are K and the standard voltage associated with the Fire oxidation of lead given tabulated free energies? 2 Pbs O2( g) 2 PbOs G 357.8kJ o rx K 5.23x10 62 -nFV0 = Go ? n=4 nFV o G o 357.8 357.8kJ 1000 J o mol rx as written kJ G 1 V 0.92V Vo J nF 4mol electrons 96487coulombs mol reaction mol electron coulomb 2 Pbs O2( g) 2 PbOs G 357.8kJ o rx K 5.23x10 62 V 0.92V o All tell us that reaction Will spontaneously Proceed to the right Favoring products How will we conveniently store info? Vo values for 1/2 reactions Compared to protons Reaction Cs+ + e K+ + e Na+ + e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Br2 + 2e Cl2 + 2e F2 + 2e Cs K Na Fe Pb H2(gas) Cu 4OHH2O2 2Br2Cl2F- Observations, Please!!!! Vo ? -2.95 -2.71 -0.44 -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 1. 2. 3. 4. 5. 6. 7. 8. 9. What seems to be the “grammar” for the reactions? What is the zero point? What do you expect the value for Cs to be? How do the values for the halogens compare to the group I elements? Is there a trend in the halogens? How does this relate to the periodic chart? How does this relate to “charge density”? Who wants the electrons? Where are the guys that want the electrons located on the chart? We said that electrons are rapidly Aquated and, rapidly react What about their energy? aq e nH2 O e eaq nH3 O H H2 O H-O bond Hydronium 301 kJ Water 464 kJ eaq H2 O H HO E , 30 . VvsNHE o k , 2.3x1010 M 1 s 1 E o , 2.10V vs NHE k , 19 . x101 M 1 s 1 E o , 2.930V vs NHE don’t have e Reaction e + H2O Cs+ + e K+ + e Na+ + e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Br2 + 2e Cl2 + 2e F2 + 2e want most have e want least eaq Cs K Na Fe Pb H2(gas) Cu 4OHH2O2 2Br2Cl2F- Vo -3.0 ? -2.95 -2.71 -0.44 -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 Start arrow on right hand side and end on left hand Reaction Cs+ + e K+ + e Na+ + e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Br2 + 2e Cl2 + 2e F2 + 2e Don’t have want most Have e want least Cs K Na Fe Pb H2(gas) Cu 4OHH2O2 2Br2Cl2F- Vo ? -2.95 -2.71 -0.44 -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 electrons flow down hill away from negative voltage Think of A water tower Start arrow on right hand side and end on left hand Uphill reactions: not probable Reaction Cs+ + e K+ + e Na+ + e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Br2 + 2e Cl2 + 2e F2 + 2e Cs K Na Fe Pb H2(gas) Cu 4OHH2O2 2Br2Cl2F- Vo ? -2.95 -2.71 -0.44 -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 Start arrow on right hand side and end on left hand Can I react F2 with K+? Reaction Cs+ + e Cs K+ + e K Na+ + e Na No, there Fe2+ + 2e Fe is nobody electrons, Pb2+ + 2e Pb 2H+ + 2e H2(gas) source! no electron Cu2+ + 2e Cu O2 + 2H2O + 4e 4OHO2 + 2H+ + 2e H2O2 Br2 + 2e 2BrCl2 + 2e 2ClF2 + 2e 2F- to Vo ? -2.95 -2.71 give -0.44 away -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 Start arrow on right hand side and end on left hand Can I exchange e between Cs with Pb? Reaction Cs+ + e K+ + e Na+ + e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Br2 + 2e Cl2 + 2e F2 + 2e Cs K Na Fe Pb H2(gas) Cu 4OHH2O2 2Br2Cl2F- Vo ? -2.95 -2.71 -0.44 -0.13 0 0.34 0.40 0.68 1.09 1.36 2.87 There is nobody to accept electrons! Example problem Standard V (good exam prototypes) Which reactions will go? a) b) c) d) Cs metal plus KBr? F2 gas plus PbCl2 Na metal plus chlorine gas Na+ + Cl- Strategy: 1. Pick one who has electrons 2. Pick one who doesn’t 3. Draw an arrow, starting where the electron is. 4. Is it up or downhill? Reaction e + H2O K+ + e Na+ + e NCl3_4H+ + 6e Fe2+ + 2e Pb2+ + 2e 2H+ + 2e N2(g) + 8H+ + 6e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Ag+ + e NO3- + 4H+ + 3e Br2 + 2e 2NO3- + 12H+ + 10e Cl2 + 2e Au+ + e F2 + 2e eaq K Na 3Cl- + NH4+ Fe Pb H2(gas) 2NH4+ Cu 4OHH2O2 Ag NO(g) +2H2O 2BrN2(g) +6H2O 2ClAu 2F- Vo -3.0 -2.95 -2.71 -1.37 -0.44 -0.13 0 0.275 0.34 0.40 0.68 0.799 0.957 1.09 1.246 1.36 1.83 2.87 Who rusts most? Pb Fe Cu Ag Au Why? What do we use for plumbing? Why was gold considered the sacred material? Reaction Fe2+ + 2e Pb2+ + 2e 2H+ + 2e Sn4++2e N2(g) + 8H+ + 6e Cu2+ + 2e O2 + 2H2O + 4e O2 + 2H+ + 2e Fe3+ +e Hg22+ +2e Ag+ + e NO3- + 4H+ + 3e Br2 + 2e 2NO3- + 12H+ + 10e Cl2 + 2e Au+ + e F2 + 2e Fe Pb H2(gas) Sn2+ 2NH4+ Cu 4OHH2O2 Fe2+ 2Hg(l) Ag NO(g) +2H2O 2BrN2(g) +6H2O 2ClAu 2F- Vo -0.44 -0.13 0 0.154 0.275 0.34 0.40 0.68 0.769 0.796 0.799 0.957 1.09 1.246 1.36 1.83 2.87 Saturday Friday Thursday Tuesday Wednesday Monday Sunday ~100 B.C (Context Slide 1) Change in metal sequence occurs at time that acids were developed (1100-1400AD) (Islamic Chemists) xM solid 2 O2 M 2 Oy , solid y G Air oxidation n MLnx,aqueous M aqueous xLaqueous1/K n M aqueous ne M solid ~1300A.D f V MLnx,aqueous ne M solid xLaqueous Chemical oxidation (Context Slide 2) 1. 2. 3. Some Rules Voltages sum Reversed reactions =change of sign Don’t worry about #electrons (n) since V = Joule/coulomb of charge Example Calculation: Summing V equations What is the voltage for the reaction: 2 A A B V? Given that A e A A e B A A e A e B 2 A A B Vao Vao/b Vao Vao/b V Vao/b Vao Example Summing V equations: If your lab partner attempts to add fluorine gas to a beaker containing potassium metal what should you do? Justify by calculating the reaction voltage and the free energy F2 , g 2e 2F K e K 2 K 2e 2K 2K 2 K 2e F2, g 2e 2 F 2 K F2 2 K 2 F V 2.87 V o 2.95 o Say your prayers and duck. V 2.95 o V o 2.95 V o 2.87 V o 582 . G o nFV o Go 2 F 2.95 2 F 2.87 2 F 582 . 29.648x104 582 . 1123kJ How does concentration fit In? G G RT ln Q o nFV G G nFV 0 o nFV nFV RT ln Q o nFV o RT V ln Q nF nF RT V V ln Q nF o RT C D V V ln a b nF A B c o d Nernst Equation: RT C D V V ln nF A a B b c d o At 25 oC C D 0.0592 V V log a b n A B c d o When the reaction favors products, it is Spontaneous, or Galvanic Luigi Galvani: “Frog leg Guy” 1780 More accurately: M a a Activity coefficient CM a concentration activity M a log 0509 . z u 1 2 CZ i 2 i 2 u 1 u Ionic strength Galen, 170 Marie the Jewess, 300 Charles Augustin James Watt Coulomb 1735-1806 1736-1819 Justus von Thomas Graham Liebig (1803-1873 1805-1869 Ludwig Boltzman 1844-1906 Gilbert N Lewis 1875-1946 Henri Louis LeChatlier 1850-1936 Johannes Bronsted 1879-1947 Jabir ibn Hawan, 721-815 Luigi Galvani 1737-1798 Richard AC E Erlenmeyer 1825-1909 An alchemist Count Alessandro G A A Volta, 1747-1827 James Joule (1818-1889) Henri Bequerel 1852-1908 Lawrence Henderson 1878-1942 Galileo Galili Evangelista Torricelli 1564-1642 1608-1647 Amedeo Avogadro 1756-1856 Rudolph Clausius 1822-1888 Jacobus van’t Hoff 1852-1911 Niels Bohr 1885-1962 John Dalton 1766-1844 William Thompson Lord Kelvin, 1824-1907 Johannes Rydberg 1854-1919 William Henry 1775-1836 Johann Balmer 1825-1898 J. J. Thomson 1856-1940 Erwin Schodinger Louis de Broglie 1887-1961 (1892-1987) Fitch Rule G3: Science is Referential Jean Picard 1620-1682 Jacques Charles 1778-1850 Francois-Marie Raoult 1830-1901 Heinrich R. Hertz, 1857-1894 Friedrich H. Hund 1896-1997 Daniel Fahrenheit 1686-1737 Max Planck 1858-1947 Rolf Sievert, 1896-1966 Blaise Pascal 1623-1662 Georg Simon Ohm 1789-1854 James Maxwell 1831-1879 Robert Boyle, 1627-1691 Isaac Newton 1643-1727 Michael Faraday 1791-1867 B. P. Emile Clapeyron 1799-1864 Dmitri Mendeleev 1834-1907 Svante Arrehenius Walther Nernst 1859-1927 1864-1941 Fritz London 1900-1954 Wolfgang Pauli 1900-1958 Johannes D. Van der Waals 1837-1923 Marie Curie 1867-1934 Anders Celsius 1701-1744 Germain Henri Hess 1802-1850 J. Willard Gibbs 1839-1903 Fritz Haber 1868-1934 Thomas M Lowry 1874-1936 Werner Karl Linus Pauling Louis Harold Gray 1905-1965 Heisenberg 1901-1994 1901-1976 Electrochemistry Electrochemistry is a very diverse area. It can be broadly divided into a) analytical electrochemistry which is concerned with methods of measurement involving potentiometry (pH meters etc), voltammetry, and modern sensors (generally voltammetric in nature) b) physical electrochemistry is the basis of analytical electrochemistry, but is generally concerned with the effect of electric fields, charge balance, and diffusion. c) Chemical electrochemistry usually is less interested in quantitative analysis but is devoted to understanding the mechanisms of electron transfer based on chemical structure. d) Biologic electrochemistry can be understood as a form of physical electrochemistry (charge and fields around biomolecules) and of chemical electrochemistry (electron transfer events in biological systems) e) Geologic and environmental electrochemistry is concerned with all of the above as they take place in the natural environment . Of particular interest are the oxidation reduction reactions of iron, manganese, chromium, arsenic, sulfur, as these set the parameters in which life can grow and, as the oxidation state of iron has substantial implications for the stability of various minerals and their dissolution/formation. f) Technical - in this field fall the major areas of batteries, solar energy, fuel cells, and corrosion sciences. A preview…… http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/redox.htm See also:Awesome site Biological “Galvanic” (Spontaneous) Cell: Respiration Note the negative To positive Arrangement of Voltages. Electrons flow away From the Negative sign. Note also very Small voltage steps, 0.01 V is a large driver! CoQ = Coenzyme Q What is the role of the long tail? Ubiquinone, Q 2e, 2H+ Ubiquinol, QH2 Q 2 H 2e H2 Q ~ 0.6999V 0 Open browser to see and rotate molecule http://www.reciprocalnet.org/recipnet/showsample.jsp?sampleId=27344188&sampleHistoryId=13823 Biological Electrochemistry Cytochrome C Cytochrome c oxidase Fe Containing Heme group in 8H O2,g 4H Membrane out Hemeglobin: Oxygen carrier Fe is square planar with 2 more coordination sites top and bottom. One is used for oxygen transport http://www.elmhurst.edu/~chm/vchembook/568globularprotein.html Review: Module 18: Complex Ions Review: Module 17B: Acid Bases Biological “Electrolytic” or Non-spontaneous cell: Photosynthesis Electrons are “pumped” up towards More negative voltage The pump chemistry is Similar (but not identical) to metal ligand crystal Field splitting light 700nm 680nm Photosystems I and II (Context Slide 1) Electrochemistry in Mining The Conquest of Mexico In 1550 the Viceroy wrote to the King “In just a few years a large area of forest has been destroyed [near the Taxco silver mines], and it appears that the wood supply will be depleted sooner than the ore. Ordinances have been made regarding the conservation of the forest, and likewise regarding the paths that the Indian workers use for making charcoal, cutting wood, and on the maximum loads that may carry.” Requires a less fuel Intensive method Mercury consumed in New World Spanish silver mines (1560-1820):170,000 tons; USA gold rush (1850-1900): 70,000 tons Amalgamation was introduced in the 1550s in M exico by a Spanish immigrant, Bartolome de M edina, who wrote Dec. 29, 1555 (1): 1. (Context Slide 2) I, Bartolome de Medina do declare that I learned in Spain through discussion with a German, that silver can be extracted from ore without the necessity for s melting it, or refining it, or incurring any other considerable expense. With this information I resolved to come to New Spain. Leaving my home, my wife and my children in Spain, I came to test it, knowing that if I were successful, I would render a great service to Our Lord, and to his Majesty and to all this realm. And having spent much time and money and suffered mental anguish, and seeing that I was not going to be able to make it work, I commended myself to Our Lady and I begged Her to enlighten me and guide me, so that I might be successful and it pleased Our Lady to enlighten me and put me on the right path so that I could make it work. Probert, A. Bartolome de Medina: The Patio P rocess and the Sixteenth Century Silver Crisis. In M ines of Silver and Gold in the Americas. A description of the process 1555. Grind the ore fine. Steep it in strong brine. Add mercury and mix thoroughly. Repeat mixing daily for several weeks. Every day take a pinch of ore mud and examine the mercury. See? It is bright and glistening. As times passes, it should darken as silver minerals are decomposed by salt and the silver forms an alloy with mercury. Amalgam is pasty. Wash out the spent ore in water. Retort residual amalgam; mercury is driven off and silver remains. Solubility Ag2 S 2 Ag S 2 2 3 2 Ag 4S2 O 2 Ag S2 O3 2 3 K insoluble 8.41x1026 1026.9 Could drive solubility f Complexation Ksp 1051 2 2.9 x10 13 2 8SO42 16e 32 H 8H2 SO3 8H2 O V10 0158 . 8H2 SO3 16e 8H 4S2 O32 12 H2 O V2o 0.400 S 2 Ss 2e V3o 0.447 Reduction Of S 8SO 30e 40H (how to log K Get the ligand) 2 4 4S2 O32 20H2 O Ss o Vnet 1005 . n o V net 0.0592 net 30 1002 log Knet . 507.7 0.0592 Kvoltage,net 10507.7 (Context Slide 2) 2 4 Ag2 S 8SO 30e 40H 2 Ag S2 O3 2 20H2 O S s 3 K Ktotal Knet K f 2 sp 10507.7 1026.9 1050.1 10484.5 Couple Reactions Example • Most native silver has long since been used: 2. but we still mine silver dust. 3. How is this economically feasible? 4. How could we get rich with a new process involving CN extraction? What is the voltage, free energy, and K Associated with this reaction? Ag s CN aq O 2,g Ag CN 2 ,aq Ag s CN aq Ag s CN O 2,g aq Ag CN 2 ,aq 1. Balance the equation a. Split into ½ reactions b. Balance each ½ reaction c. Recombine Ag CN 2,aq Ag s 2CN aq Ag CN O2, g ? O2 , g 2 H2 O 2,aq Ag s 2CN aq Ag CN 2,aq 1e 4 Ag s 8CN aq 4 Ag CN 2 ,aq 4 Ag s 8CN aq 4e aq O2 , g 4 Haq 2 H 2 O O2 , g 4 Haq 4e 2 H2 O 4 Ag CN 2 ,aq 4e V o 0.31 V o 123 . O2, g 4 H 4e 2 H2 O o 4 Ag s 8CN aq O2 , g 4 Haq 4 Ag CN 2 ,aq 2 H2 O V 154 . rx The free energy for the reaction is a mere: G = -nFVorx = -4(96485)(1.53) = -5.9x105 J G = -RTln K K = e(-G/RT) = e(-(-590000/(298x8.314)) = e238 = 10238/2.3 = 10103 all you need is: CN (cheap) O2 (air is cheap (an aerator)) Hypothetical Modern Silver/Gold Mine O2 Bulldozer (Context Slide) CNaerator Tibor Kocsis Same process used to recover silver at photography studios, in silver plating. Major cyanide spills: Czech, Elbe River, Jan. 2006; Romania, Tisza River, Nov. 2005; Laos, June, 2005; Ghana River Kubreko, Jan, 2005; China, Papua New Guinea, Ghana, Romania (10 tons Danube River, Mar. 2004), Ghana, Honduras, Nicaragua, China, 2002: Nevada, USA (Context Slide) Using Bugs to Mine Cu from CuS Biomining for Gold and Copper in Botswana collect Ksp=10-36 Cu 2 S2 2 s 8Fe 3 4 H2 O 8Fe 2 Cu 2 2SO42 16H -14e 8Fe 2 8Fe bugs 3 Catalytic reagent, supplied courtesy of bugs, Thiobacillus ferridoxin (Context Slide) Coupled Chemical Equation Example Calculate the formal potential for the reaction to form the initial corrosion product, Fe(OH)3,s reaction at pH 7 2 Fe 2 Fe 2 V 0.44 4e O2 2 H2 O 4e 4OH VOo2 0.40 V 0.771 3 2 Fe 2 2 Fe 2e 1 2 O2 H2 O 2e 2OH Fe 3 3OH Fe(OH ) 3 2Fe(OH) 3 3H2 O Fe2 O3 slow VOo2 0.40 Ksp 6.3x10 38 Introduction: Key Concepts 1. Grammer Rules: write all reactions as reductions 2. Half reactions 3. Aquated electrons (carry electrons from electrode to solution species) (disinfectants) 4. Voltage is an energy term 5. Applications 1. Disinfectants 2. Biology 3. Geology 4. Industry