lec 3 chem 6c

lec 3 chem 6c - Review: Solved a redox reaction in a basic...

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Solved a redox reaction in a basic solution Learned the basics of a Galvanic cell Cell potential ( ξ ) or electromotive force (emf) Standard cell potentials ( ξ °) Faraday’s Law Review:
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Preview Nernst Equation Applications of Galvanic cells Electrolysis Revisit some of the things that we have already learned Combining Faraday’s Law with the equilibrium constant
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? G° = -nF ξ ° Faraday’s Law It states that the emf is proportional to the negative of the Gibb’s Free Energy It also states that a galvanic cell will proceed in the direction of a positive cell potential
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Example: From Appendix 2B in the back of your book: Cu 2+ (aq) + 2e - Cu(s) ξ ° = + 0.34V Zn 2+ (aq) + 2e - Zn(s) ξ ° = 0.76V amount of oxidation = amount of reduction We need to move the electrons from one ½ reaction to the other side (flip a ½ reaction)
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Recall that when a reaction is switched it changes the sign of ξ ° Example: From Appendix 2B in the back of your book: Cu 2+ (aq) + 2e - Cu(s) ξ ° = + 0.34V Zn 2+ (aq) + 2e - Zn(s) ξ ° = 0.76V Switching the Zn ½ reaction will yield a positive ξ ° for the overall reaction ? G° = -nF ξ ° Faraday’s Law
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Example: Cu 2+ (aq) + 2e - Cu(s) ξ ° = 0.34V Zn(s) Zn 2+ (aq) + 2e - ξ ° = 0.76V Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) ξ ° = 1.10V ? G° = -nF ξ ° = 2 (96 485 C / mol)(1.10V) = -193 kJ / mol Using Faraday’s Law:
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Quick word of caution: Although ? G° is proportional to ξ °, ξ ° is independent of n, and ? G° is not. 2Cu 2+ (aq) + 4e - 2Cu(s) ξ ° = 0.34V 2Zn(s) 2Zn 2+ (aq) + 4e - ξ ° = 0.76V 2Zn(s) + 2Cu 2+ (aq) 2Zn 2+ (aq) + 2Cu(s) ξ ° = 1.10V ? G° = -nF ξ ° = 4 (96 485 C / mol)(1.10V) = -386 kJ / mol HW problem # 31
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Rules for determining a spontaneous electrochemical reaction from two ½ reactions: #1) Determine the greater of the two cell potentials #2) Flip the ½ reaction of the smaller cell potential
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This note was uploaded on 03/21/2009 for the course CHEM 6C taught by Professor Hoeger during the Spring '08 term at UCSD.

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lec 3 chem 6c - Review: Solved a redox reaction in a basic...

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