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Unformatted text preview: Lecture 13: The Nernst Equation • Reading: Zumdahl 11.4 • Recommended Problems:11.47, 11.49, 11.51, 11.55,11.57 • Outline: – Why would concentration matter in electrochem.? – The Nernst equation. – Applications Concentration and E cell • Consider the following redox reaction: Zn(s) + 2H + (aq) Zn 2+ (aq) + H 2 (g) E° cell = 0.76 V Δ G°= nFE° cell < 0 (spontaneous) • What if [H + ] = 2 M? Expect driving force for product formation to increase. Therefore Δ G decreases, and E cell increases How does E cell dependend on concentration? Concentration and E cell (cont.) • Recall, in general: Δ G = Δ G° + RTln(Q) • However: Δ G = nFE cellnFE cell = nFE° cell + RTln(Q) E cell = E° cell (RT/nF)ln(Q) E cell = E° cell (0.0591 V/n)log(Q) The Nernst Equation Summary of Relationships: Δ G & E cell Δ G = Δ G° + RTln(Q) Δ G = nFE cell E cell = E° cell (0.0591/n)log(Q) • None of these ideas is separate. They are all connected, and are all derived directly from thermodynamics. Concentration and E cell (cont.) • With the Nernst Eq., we can determine the effect of concentration on cell potentials. E cell = E° cell (0.0591 V/n)log(Q) • Example. Calculate the cell potential for the following: Fe(s) + Cu 2+ (aq) Fe 2+ (aq) + Cu(s) Where [Cu 2+ ] = 0.3 M and [Fe 2+ ] = 0.1 M Concentration and E cell (cont.) Fe(s) + Cu 2+ (aq) Fe 2+ (aq) + Cu(s) • First, need to identify the 1/2 cells Cu 2+ (aq) + 2e Cu(s) E° 1/2 = 0.34 V Fe 2+ (aq) + 2e...
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 Fall '12
 Chiu
 Redox

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