Electrochemistry (Ch 17) - AP Chemistry

4/1/2011
CONCENTRATION DEPENDENCE
• For the reaction,
Cu(s) + 2 Ce 4+ (aq) Cu 2+ (aq) + 2 Ce 3+ (aq)
has a potential of 1.36 V under standard conditions (1 M
concentrations all)
– What if [Ce 4+ ] is greater than 1 M
• Le Châtelier's principle predicts that the forward reaction will be favored,
increasing the cell potential.
– What if [Cu 2+ ] or [Ce 3+ ] is increased
• Cell potential is decreased
THE EFFECTS OF CONCENTRATION ON E
• For the cell reaction,
2 Al(s) + 3 Mn 2+ (aq) 2 Al 3+ (aq) + 3 Mn(s) E = 0.48 V
predict whether E cell is larger or smaller than E cell for the
following cases
– [Al 3+ ] = 2.0 M, [Mn 2+ ] = 1.0 M
• E cell < 0.48 V
– [Al 3+ ] = 1.0 M, [Mn 2+ ] = 3.0 M
• E cell > 0.48 V
Ch 17 - Electrochemistry 31
Ch 17 - Electrochemistry 32
CONCENTRATION CELLS
• A galvanic cell in which both
compartments have the same
components but at different
concentrations is called a
concentration cell (typically
with small voltages)
– What is the potential of this cell
and what is the direction of
electron flow
– The relevant half-reaction is,
Ag + + e - Ag
E = 0.80 V
CONCENTRATION CELLS
• The electrons will flow in the
direction that will equalize the
concentrations of Ag + in the
two compartments
– Accomplished by transferring
electrons from the 0.1 M Ag +
compartment to the 1 M Ag +
compartment
– This produces more Ag + in the
left compartment and consumes
Ag + in the right compartment
Ch 17 - Electrochemistry 33
Ch 17 - Electrochemistry 34
CONCENTRATION CELLS
• Determine the direction of electron flow and designate the anode and
cathode for the cell represented below.
THE NERNST EQUATION
• The concentration dependence of cell potential results directly from the
dependence of free energy on concentration
– Recall, ∆G = ∆G + RT lnQ where Q is the reaction quotient
– Also recall, ∆G = -nFE and ∆G = -nFE
– So, -nFE = -nFE + RT lnQ
• Solving for cell potential,
– Fe 2+ + 2 e - Fe
– Transferring electrons from left (anode) to right (cathode)
– This equation gives the relationship between the cell potential and the concentrations
of the cell components is commonly called the Nernst equation (German chemist
Walther Hermann Nernst, 1864-1941)
– At 25C,
Ch 17 - Electrochemistry 35
Ch 17 - Electrochemistry 36
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