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handout_L11_NernstEq - Lecture 11 The Nernst Equation...

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Lecture 11: The Nernst Equation • Reading: Zumdahl 11.4 • 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) cell = 0.76 V G°= -nFE° cell < 0 (spontaneous) What if [H + ] = 2 M? Expect driving force for product formation to increase. Therefore G decreases, and E cell increases How does E cell dependend on concentration? Concentration and E cell (cont.) Recall, in general: G = G° + RTln(Q) • However: G = -nFE cell -nFE cell = -nFE° cell + RTln(Q) E cell = E° cell - (RT/nF)ln(Q) E cell = E° cell - (0.0591/n)log(Q) The Nernst Equation 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/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)
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