Electrochemistry (Ch 17) - AP Chemistry

4/1/2011
STANDARD REDUCTION POTENTIALS
• The total potential (E cell ) can be measured (0.76 V), but there is no way to
measure the individual electrode potentials.
– To calculate potentials for each half-reaction, the reaction
2 H + 2 e - H 2
• where [H + ] = 1 M and P H2 = 1 atm
– has been assigned a potential of 0V
– E anode can be calculated because
E° cell = E° anode + E° cathode
• As a result, the reaction, ZnZn 2+ + 2 e - has a potential of 0.76 V
• Using the standard hydrogen potential as a reference, all other potentials
can be determined experimentally.
Ch 17 - Electrochemistry 13
STANDARD REDUCTION POTENTIALS
• What is the potential of the cathode
– Anode: Zn Zn 2+ + 2 e -
– Cathode: Cu 2+ + 2 e - Cu
• Since E° cell = E° anode + E° cathode
– E° cat = 1.10 V – 0.76 V = 0.34 V
• By convention, the potentials of
half reactions are given as reduction
potentials and are tabulated in
standard states in a list of standard reduction potentials
Ch 17 - Electrochemistry 14
STANDARD REDUCTION POTENTIALS
STANDARD REDUCTION POTENTIALS
• Combining two half-reactions to obtain a balanced oxidationreduction
reaction often requires two manipulations:
– One of the reduction half-reactions must be reversed (since redox reactions
must involve a substance being oxidized and a substance being reduced).
• The half-reaction with the largest positive potential will run as written (as a
reduction)
• The other half-reaction will be forced to run in reverse (as an oxidation)
• The net potential of the cell will be the difference between the two:
E° cell = E° cathode – E° anode
– Or “change the sign and add” (by changing the sign of the oxidation reaction)
Ch 17 - Electrochemistry 15
STANDARD REDUCTION POTENTIALS
• Combining two half-reactions to obtain a balanced oxidationreduction
reaction often requires two manipulations:
– Since the number of electrons lost must equal the number gained, the halfreactions
must be multiplied by integers as necessary to achieve the
balanced equation. However, the value of E° is not changed when a halfreaction
is multiplied by an integer.
• Since a standard reduction potential is an intensive property, the potential is not
multiplied by the integer.
STANDARD REDUCTION POTENTIALS
• Consider a galvanic cell based on the redox reaction
Fe 3+ (aq) + Cu(s) Cu 2+ (aq) + Fe 2+ (aq)
– The half reactions are:
Fe 3+ + e - Fe 2+ E° = 0.77 V (1)
Cu 2+ + 2 e - Cu E° = 0.34 V (2)
– To balance the cell reaction and calculate the standard cell potential, reaction (2)
must be reversed and multiply reaction (1) by two:
Cu Cu 2+ + 2 e -
– The cell reaction becomes:
-E° = -0.34 V
2 Fe 3+ + 2 e - 2 Fe 2+ E° = 0.77 V
Cu(s) + 2 Fe 3+ (aq) Cu 2+ (aq) + 2 Fe 2+ (aq)
E° cell = 0.43 V
Ch 17 - Electrochemistry 18
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