C118DiscussionWorksheet1Key09 - C118 Discussion Worksheet 1...

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Unformatted text preview: C118 Discussion Worksheet 1 Week of September 7, 2009 Name __________________ Useful Rules: Rule 1: The formal reduction potential with the most negative E value contains the species that gets oxidized in a spontaneous galvanic cell Rule 2: If you reverse the direction of a reduction half reaction then reverse the sign on the E value Rule 3: Spontaneous galvanic cells proceed forward when E for the cell is positive. If the E is negative, rewrite the reaction in the opposite direction and reverse the sign of E to make the reaction spontaneous in the forward direction again. Question 1 Using the table of reduction potentials listed to the right hand side, answer the following questions. (a) Which has more negative formal reduction potential out of the following pairs of elements: Al and Pb Al Pb and K K Cu and Pb Pb 1 Ag and Cu Cu Cu and H2 H2 Zn and Cu Zn (b) For each pair, write the half reactions for the species you expect to get oxidized (this is the one that had the most negative formal reduction potential) and for the reduction Al and Pb Al = Al3+ + 3e Pb2+ + 2e = Pb Pb and K K = K+ + 1e Pb2+ + 2e = Pb Cu and Pb Cu 2+ + 2e= Cu Pb = Pb2+ + 2e Ag and Cu Cu = Cu2+ + 2e Ag+ +e = Ag Cu and H2 Cu 2+ + 2e = Cu H2 = 2H+ + 2e Zn and Cu Zn = Zn2+ + 2e Cu2+ + 2e= Cu 2 (c) For each reaction, write down the balanced overall cell reaction, calculate the voltage that would be expected from a galvanic cell made of the two materials. Al and Pb 2Al = 2Al3+ + 6e 3Pb2+ + 6e = 3Pb 2Al + 3Pb2+ = 2Al3+ + 3Pb E(cell) = –0.13 + (+1.66) = +1.53 V Pb and K 2K + Pb2+ = 2K+ + Pb E(cell) = 2.92 + ‐0.13 = +2.79V Cu and Pb Cu 2+ + Pb = Cu + Pb2+ E(cell) = 0.13 + 0.34= +0.47V Ag and Cu Cu + 2Ag+ = Cu2+ + 2Ag E(cell) = ‐0.34 + 0.80 = +0.46V Cu and H2 Cu 2+ + H2 = Cu+ 2H+ E(cell) = 0 + 0.34 = +0.34V Zn and Cu Zn + Cu2+ = Zn2+ + Cu E(cell) = 0.76 + 0.34 = +1.10V 3 2. Discuss in groups Pick any two elements you like from the table of data, the elemental abundance and other chemical properties you might know in order to consider how you would make the “best” galvanic cell. Possible examples of pros & cons Li, Na, Ca and K are not safe in water and tend to explode! Au and Ag are not cheap Best voltage, safest and cheapest might be Mg + Cu2+ = Mg2+ + Cu: E = +2.71 V Could you make any to be rechargeable? Al3+ can be a solid as Al2O3 Ag+ can be a solid as AgCl (rechargeable solids) E(Cell) = +2.46V 4 ...
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