Electrochemistry

Electrochemistry

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. Heat out/Heat in = enthalpy, H 2. Equilibrium Constants, K 3. Universal Entropy ~ Free energy, G

Use total free energy – not just enthalpy This reaction is exothermic. How much? species Hfo(kJ/mol) 8H+(aq) 0 4H2O(liquid) -286 MnO4-(aq) -543 Mn2+(aq) -218.8 5Fe2+(aq) -47.69 5Fe3+(aq) -87.86

This reaction could do a lot of work for us if we could get the energy Fe2+ MnO4-

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+ MnO4- To do this Need to balance Redox reactions e Mn2+ Fe3+

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 reduced b. Balance Rx sides by adding e c. Balance charge by adding H+ or OH- d. Balance hydrogen by adding H2O e. Balance oxygen A. Balance atoms of element oxidized b. Balance Rx sides by adding e c. Balance charge by adding H+ or OH- d. 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 reduced b. Balance Ox number with e c. Balance charge by adding H+ or OH- d. Balance hydrogen by adding H2O e. Balance oxygen A. Balance atoms of element oxidized b. Balance Ox number with e c. Balance charge by adding H+ or OH- d. Balance hydrogen by adding H2O e. Balance oxygen Protons are added in an acidic solution and hydroxyls are added in a basic solution. When in doubt add protons Combine and balance electrons

Example: Balance the following redox equation in an acidic solution Ox #balance 2+ 3+ +7 +2 Mass balance Mass balance ox# balance +2 -1 = 4(-2) +? 8-1 = ? 7 = ?

Example: Balance the following redox equation in an acidic solution 2+ 3+ +7 +2 Mass balanced Mass balanced Ox numbers +2 -1 + -5 = -6 Charge balance

Example: Balance the following redox equation in an acidic solution Balance hydrogen by adding water Check oxygen balance 4O =4O =+2 Check total charge balance: (-1)+5(-1)+8(+1)=+2

Example: Balance the following redox equation in an acidic solution Recombine while balancing electrons

Example: Balance the following redox equation in an acidic solution Final equation does not show any electrons because electrons “do not exist” in solution

OJO: Important Point The reactions 1. do not really occur by themselves 2. they are linked through the electrons 3. DEFINITION: = 1/2 reactions

If all reactions are considered Half reactions where does the Electron go?

Hydrated electrons not only react with water But with other species including biological, Hence it is a good way to sterilize water Hydrated electrons in aerobic biology will produce Finite fluxes of the soft radical HOO

Not only have to consider rate But energy H-O bond Water 464 kJ Hydronium 301 kJ

Some other practical considerations

Do you think the Mn/Fe reaction will continue for long? e anion - cation + 1MnO4- 5Fe2+ Net charge =0 - - Net charge =0 - - + - - - + - - e - - Fe3+ Net charge =10(-1)+5(+3) =+5 Net charge = (+2)+2(+1) +(-1)=+3 Mn2+

Will want to let spectator ions flow (but not the reactants!) (current) e 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. Always make electrons flow to right 2. Electrons flow down to the cathode (cat = Greek for down). 3. Electrons flow up into the anode (an = Greek for up) Count Alessandro Volta, Italy ~1800, first battery

oxidation reduction (current) e There is Resistance In the system, We will come Back to this Rsb cathode Cl- anode Rsoln Reduction electrons accepted In (down = cathode) Oxidation electrons taken Out (up = anode) An ox jumped over a red cat An An ox An ox jumped over An ox jumped over a red

Vocabulary for Work when using electrons instead of heat. Ohm’s Law: Voltage = current x resistance 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 Ohm Joule Coulomb Nernst neg sign accounts for negative electron V directly relates to free energy because we are not Separately the work terms into heat and changing surrounding randomness with the heat

Thomas M Lowry 1874-1936 James Watt 1736-1819 Galen, 170 Marie the Jewess, 300 Jabir ibn Hawan, 721-815 Galileo Galili 1564-1642 Evangelista Torricelli 1608-1647 Jean Picard 1620-1682 Daniel Fahrenheit 1686-1737 Blaise Pascal 1623-1662 Robert Boyle, 1627-1691 Isaac Newton 1643-1727 Anders Celsius 1701-1744 An alchemist Charles Augustin Coulomb 1735-1806 Luigi Galvani 1737-1798 Count Alessandro G A A Volta, 1747-1827 Amedeo Avogadro 1756-1856 John Dalton 1766-1844 William Henry 1775-1836 Jacques Charles 1778-1850 Georg Simon Ohm 1789-1854 Michael Faraday 1791-1867 B. P. Emile Clapeyron 1799-1864 Germain Henri Hess 1802-1850 Dmitri Mendeleev 1834-1907 Johannes D. Van der Waals 1837-1923 J. Willard Gibbs 1839-1903 Justus von Liebig (1803-1873 Thomas Graham 1805-1869 Richard AC E Erlenmeyer 1825-1909 James Joule (1818-1889) Rudolph Clausius 1822-1888 William Thompson Lord Kelvin, 1824-1907 Johann Balmer 1825-1898 Francois-Marie Raoult 1830-1901 James Maxwell 1831-1879 Ludwig Boltzman 1844-1906 Henri Louis LeChatlier 1850-1936 Henri Bequerel 1852-1908 Jacobus van’t Hoff 1852-1911 Johannes Rydberg 1854-1919 J. J. Thomson 1856-1940 Heinrich R. Hertz, 1857-1894 Max Planck 1858-1947 Svante Arrehenius 1859-1927 Walther Nernst 1864-1941 Marie Curie 1867-1934 Fritz Haber 1868-1934 Gilbert N Lewis 1875-1946 Johannes Bronsted 1879-1947 Lawrence Henderson 1878-1942 Niels Bohr 1885-1962 Erwin Schodinger 1887-1961 Louis de Broglie (1892-1987) Friedrich H. Hund 1896-1997 Rolf Sievert, 1896-1966 Fritz London 1900-1954 Wolfgang Pauli 1900-1958 Werner Karl Heisenberg 1901-1976 Linus Pauling 1901-1994 Louis Harold Gray 1905-1965 Fitch Rule G3: Science is Referential

For standard conditions (1 mole, 1 atm, 25C): Greek G Komodo (Indonesia) K Vamale (Polynesia) Vo 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?

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? -nFV0 = Go ? Compound neutral O usually -2 So Pb = +2 2(+2)=4 +4 Ox # = 0 =-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? -nFV0 = Go ? n=4

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 Vo Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87 Observations, Please!!!!

1. What seems to be the “grammar” for the reactions? 2. What is the zero point? 3. What do you expect the value for Cs to be? 4. How do the values for the halogens compare to the group I elements? 5. Is there a trend in the halogens? 6. How does this relate to the periodic chart? 7. How does this relate to “charge density”? 8. Who wants the electrons? 9. 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? H-O bond Hydronium 301 kJ Water 464 kJ

have e don’t have e want least Reaction Vo e + H2O eaq -3.0 Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87 want most

Start arrow on right hand side and end on left hand Have e want least Reaction Vo Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87 electrons flow down hill away from negative voltage Think of A water tower Don’t have want most

Start arrow on right hand side and end on left hand Uphill reactions: not probable Reaction Vo Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87

Start arrow on right hand side and end on left hand Can I react F2 with K+? Reaction Vo Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87 No, there is nobody to give away electrons, no electron source!

Start arrow on right hand side and end on left hand Can I exchange e between Cs with Pb? Reaction Vo Cs+ + e Cs ? K+ + e K -2.95 Na+ + e Na -2.71 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Br2 + 2e 2Br- 1.09 Cl2 + 2e 2Cl- 1.36 F2 + 2e 2F- 2.87 There is nobody to accept electrons!

Example problem Standard V (good exam prototypes) Which reactions will go? a) Cs metal plus KBr? b) F2 gas plus PbCl2 c) Na metal plus chlorine gas d) 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 Vo e + H2O eaq -3.0 K+ + e K -2.95 Na+ + e Na -2.71 NCl3_4H+ + 6e 3Cl- + NH4+ -1.37 Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 N2(g) + 8H+ + 6e 2NH4+ 0.275 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Ag+ + e Ag 0.799 NO3- + 4H+ + 3e NO(g) +2H2O 0.957 Br2 + 2e 2Br- 1.09 2NO3- + 12H+ + 10e N2(g) +6H2O 1.246 Cl2 + 2e 2Cl- 1.36 Au+ + e Au 1.83 F2 + 2e 2F- 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 Vo Fe2+ + 2e Fe -0.44 Pb2+ + 2e Pb -0.13 2H+ + 2e H2(gas) 0 Sn4++2e Sn2+ 0.154 N2(g) + 8H+ + 6e 2NH4+ 0.275 Cu2+ + 2e Cu 0.34 O2 + 2H2O + 4e 4OH- 0.40 O2 + 2H+ + 2e H2O2 0.68 Fe3+ +e Fe2+ 0.769 Hg22+ +2e 2Hg(l) 0.796 Ag+ + e Ag 0.799 NO3- + 4H+ + 3e NO(g) +2H2O 0.957 Br2 + 2e 2Br- 1.09 2NO3- + 12H+ + 10e N2(g) +6H2O 1.246 Cl2 + 2e 2Cl- 1.36 Au+ + e Au 1.83 F2 + 2e 2F- 2.87 Saturday Friday Thursday Tuesday Wednesday Monday Sunday

(Context Slide 1) ~100 B.C Change in metal sequence occurs at time that acids were developed (1100-1400AD) (Islamic Chemists) DG Air oxidation 1/Kf V ~1300A.D Chemical oxidation

(Context Slide 2)

Some Rules 1. Voltages sum 2. Reversed reactions =change of sign 3. Don’t worry about #electrons (n) since V = Joule/coulomb of charge

Example Calculation: Summing V equations What is the voltage for the reaction: Given that

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 Say your prayers and duck.

How does concentration fit In?

Nernst Equation: At 25 oC When the reaction favors products, it is Spontaneous, or Galvanic “Frog leg Guy” 1780 Luigi Galvani:

Electrochemistry