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Electrochemistry Part 1

Electrochemistry Part 1 . Ch. 20 in Text (Omit Sections 20.7 and 20.8). redoxmusic.com. I) Overview. electrochemistry deals with: electricity and chemistry electron transfers oxidation: loss of electrons reduction: gain of electrons changes in oxidation number

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Electrochemistry Part 1

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  1. ElectrochemistryPart 1 Ch. 20 in Text (Omit Sections 20.7 and 20.8) redoxmusic.com

  2. I) Overview • electrochemistry deals with: • electricity and chemistry • electron transfers • oxidation: loss of electrons • reduction: gain of electrons • changes in oxidation number • single replacement, synthesis, and decomposition hovr.com

  3. A) Oxidation LEO sez GER!! • the loss of negatively charged electrons results in an INCREASE in oxidation number • often involves metals • because it CAUSES reduction, the species oxidized is known as the reducing agent flickr.com

  4. B) Reduction LEO sez GER!! • the gain of negatively-charged electrons results in a DECREASE in oxidation number • often involves nonmetals • because it CAUSES oxidation, the species reduced is known as the oxidizing agent forums.joeuser.com

  5. C) An Example • the following reaction occurs in a nickel-cadmium battery: • Cd(s) + NiO2(s) + 2H2O(l) → Cd(OH)2(s) + Ni(OH)2(s) HW: 20.6 failblog.org

  6. D) Half-Reactions • show a redox reaction as 2 separate processes • atoms and charge must be conserved • oxidation represents electrons as products • reduction represents electrons as reactants • the number of electrons lost in oxidation must equal that gained in reduction sciencenotes.wordpress.com

  7. Cd(s) + NiO2(s) + 2H2O(l) → Cd(OH)2(s) + Ni(OH)2(s)

  8. E) Balancing Equations by the Half-Reaction Method • may be in acidic or basic solution • see textbook amnihilist.blogspot.com

  9. II) Voltaic Cells (Galvanic Cells) • a spontaneous redox reaction performs electrical work • a half cell consists of a metal strip placed in a solution of its ion; when connected they form a voltaic cell • the metal strips are connected with a wire for the movement of electrons • the solutions are connected with a salt bridge for the movement of ions (complete circuit) ask.metafilter.com

  10. electrons move from the more active metal (anode) where oxidation occurs to the less active metal (cathode) where reduction occurs • ions move through the salt bridge to maintain electrical neutrality ask.metafilter.com

  11. the anode gets smaller over time since the solid metal is breaking down into aqueous ions • the cathode gets bigger over time since the aqueous ions are building up as a solid metal Link ask.metafilter.com HW: 20.14

  12. III) Cell EMFA) Potential Energy • the flow of electrons from anode to cathode is spontaneous due a difference in potential energy between the two metals • the potential difference drives the reaction • the potential difference in energy per unit charge is measured in volts • 1 V = 1 J/C where J = Joules and C = Coulombs todayscampus.com

  13. B) EMF • the potential difference between the two electrodes is also known as the electromotive force or emf • standard emf or standard cell potential is measured at 1 atm and 25 ºC for 1 M solutions • denoted Eºcell ucl.ac.uk

  14. C) SHE • although the standard cell potential could be measured for ALL different metal combinations, it is easier to measure all standard cell potential values in reference to a COMMON standard cell • the standard hydrogen electrode (SHE) is designated as this standard • 2H+(aq, 1 M) + 2 e- → H2(g, 1 atm) is given a cell potential of 0 V (arbitrarily) • all other half-cells are measured against this standard mikeblaber.org

  15. D) Standard Reduction Potentials • by convention, the potential of an electrode is chosen to be the potential for reduction to occur (in relation to the SHE) • thus, tabulated standard emf potential values = Eºred • Eºcell = Eºred(cathode) - Eºred(anode) bouman.chem.georgetown.ed

  16. the more positive the Eºred, the greater the driving force for reduction • the more negative the Eºred, the greater the driving force for oxidation • FYI, changing the coefficient in a half-rxn to balance the net equation does NOT change the value of Eºred!!! titanmag.com

  17. Ex) Calculate the standard emf for a cell involving the following rxn: • 2Al(s) + 3I2(s) → 2Al+3(aq) + 6I-(aq) monado.wordpress.com HW: 20.24, 20.26 (a) & (b), 20.32

  18. E) Oxidizing/Reducing Agents old.he-man.org hsc.csu.edu.au HW: 20.40

  19. Electrochemistry Part 2 descargarmp3.net Ch. 20 in Text (Omit Sections 20.7 and 20.8)

  20. IV) SpontaneityA) Basics • a “+” value for E°cell means that the rxn is spontaneous • a “-” value for E°cell means that the rxn is nonspontaneous • Ex) Determine the spontaneity of the following: • Cu(s) + 2H+(aq) → Cu+2(aq) + H2(g) nightsintodreams.com

  21. B) Free Energy Change • ΔG = -nFE where: • ΔG is the free energy change in J/mol • n is the # of electrons transferred in the rxn (no units) • F is Faraday’s constant (1 F = 96,500 C/mol = 96,500 J/V•mol) • E is the emf in V wired.com

  22. Ex) If E°cell = 0.43 V for the following rxn, what is the standard free energy change? • 4Ag(s) + O2(g) + 4H+(aq) → 4Ag+(aq) + 2H2O(l) cheezburger.com HW: 20.46 (a), 20.50

  23. V) Concentration and Cell EMFA) The Nernst Equation • E = E° - RT ln Q OR nF • E = E° - 0.0592 V log Q n (at 25 °C) • we can use the above equations to determine the voltage under nonstandard conditions photos.aip.org

  24. Ex) If E°cell = 1.10 V for the following reaction, what is Ecell if [Cu+2] = 5.0 M and [Zn+2] = 0.050 M at 298 K? • Zn(s) + Cu+2(aq) → Zn+2(aq) + Cu(s) flickr.com HW: 20.54

  25. B) Concentration Cells • emf generated solely by a difference in concentration is called a concentration cell • although the E°cell = 0, this occurs under nonstandard conditions • driven by equalization of concentrations • the more dilute solution is the anode and the more concentrated solution is the cathode sparknotes.com Link

  26. Ex) Write half reactions for the previously depicted cell and calculate the voltage at 25 °C. debate.org HW: 20.58

  27. C) Cells and Equilibrium • cells eventually “die” out as reactants are converted to products, Q increases, and thus, E decreases • when E = 0, ΔG = 0 and we are equilibrium • log K = nE°/0.0592 at 298 K sbnation.com

  28. Ex) O2(g) + 4H+(aq) + 4Fe+2(aq) → 4Fe+3(aq) + 2H2O(l) • If E° = 0.46 V at 298 K, what is the value of the equilibrium constant? mathfail.com

  29. VI) Electrolytic CellsA) Set Up • electrical energy is used to cause a nonspontaneous rxn to occur • a power source (like a battery) must be added to force the rxn to occur chemed.chem.purdue.edu Link Link

  30. B) Quantitative Aspects • I = q/t where: • I is the current in amperes (amps), A • q is the charge in Coulombs • t is time in seconds • knowing the current can allow us to convert to grams of solid plated in an electrolytic cell canasianinvasion.com

  31. Ex) What mass of Al is produced in 1.00 hr by the electrolysis of molten AlCl3 if the electrical current is 10.0 A? failblog.org HW: 20.82 (a)

  32. flickr.com

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