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Free Energy and Redox Reactions

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Free Energy and Redox Reactions
пЃ® The emf associated with any redox reaction
can be calculated.
 Eo = Eored (reduction) – Eored (oxidation)
пЃ® Spontaneous redox reaction
пЃ® positive Eo (standard conditions)
пЃ® positive E (non-standard conditions)
пЃ® negative DG
пЃ® Non-spontaneous redox reactions:
пЃ® negative Eo
пЃ® negative E
пЃ® positive DG
Free Energy and Redox Reactions
пЃ® The change in Gibbs free energy is related to
the emf of a redox reaction by the equation:
DG = -nFE
where DG = change in Gibbs free energy
n = number of electrons transferred
F = Faraday’s constant = 96,485 J/V.mol
E = emf under nonstandard conditions
(I will give you this equation and the value of F on your exam.)
Free Energy and Redox Reactions
пЃ® Under standard conditions, this equation
becomes:
DGo = -nFEo
where DGo = standard Gibbs free energy change
n = number of electrons transferred
F = Faraday’s constant = 96,485 J/V.mol
Eo = emf under standard conditions
(I will give you this equation and the value of F on your exam.)
Free Energy and Redox Reactions
пЃ® You can use the value of Eo to calculate the
value of DGo and the equilibrium constant, K,
for the reaction.
пЃ® DGo = -nFEo
пЃ® DGo = -RTlnK
Free Energy and Redox Reactions
Example: Use the standard reduction potentials
listed in Appendix E of your text to calculate the
equilibrium constant for the following reaction at
298K.
3 Ce4+ (aq) + Bi(s) + H2O (l) пѓ 3 Ce3+ (aq) + BiO+ (aq) + 2 H+
(aq)
Step 1: Calculate the value for Eo:
Free Energy and Redox Reactions
Step 2: Calculate the value of DGo:
Step 3: Calculate the value of K:
Answer: 2.42 x 1065
Free Energy and Redox Reactions
Example: What is the effect on the emf of the cell
described by the following equation when the following
changes are made:
2 Fe3+ (aq) + H2 (g) пѓ 2 Fe2+ (aq) + 2 H+ (aq)
The pressure of hydrogen gas in the anode compartment is
increased?
Iron (III) nitrate is added to the cathode compartment?
Sodium hydroxide is added to the anode compartment?
Free Energy and Redox Reactions
пЃ® The emf of a redox reaction varies with
temperature and with the concentrations of
reactants and products.
пЃ® The Nernst equation relates the emf under
nonstandard conditions to the standard emf and
the reaction quotient.
E = Eo- (RT/nF)lnQ
Free Energy and Redox Reactions
пЃ® Converting from natural log to log base 10 and
assuming that T = 298 K, the Nernst Equation
becomes:
E = Eo – 0.0592 log10 Q
n
where n = the number of electrons transferred
Q = reaction quotient
(I will give you this equation. You need to be able
to use this equation)
Free Energy and Redox Reactions
Example: Calculate the emf generated by the
following reaction when [Al3+] = 4.0 x 10-3 M and
[I-] = 0.010 M at 298K.
2 Al (s) + 3 I2 (s) пѓ 2 Al3+ (aq) + 6 I- (aq)
Step 1: Calculate Eo
Free Energy and Redox Reactions
пЃ® Step 2:
Calculate Q
пЃ® Step 3:
Calculate E using the Nernst Eq’n:
пЃ® Answer:
E = 2.36 V
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