Electrochemistry :  Standard-State Cell Potentials
Cell Potential - In a galvanic cell, the potential for the cell to do work on its surroundings by driving an electric current through a wire.
• One Joule (J) of energy is produced when one Coulomb (C) of electrical charge is transported across a potential of one Volt (V):

Standard-State Cell Potential(E) - A measure of the strength of the driving force behind the chemical reaction at standard-state conditions.

• Standard-State Conditions:
• All solutions are 1 M.
• All gases have a partial pressure of 1 MPa (0.9869 atm).
• Generally measured at 25C.
• The greater the difference between the oxidizing and reducing strengths of the reactants and products, the larger the cell potential.
Standard-state cell potentials are measured on an relative scale based upon the arbitrary assumption that the standard-state potential for the reduction of H+ ions to H2 gas is zero:
 2 H+ + 2 e-H2 E = 0.00 V

The cell potential for a reaction is the sum of the potentials for the oxidation and reduction half-reactions:
Eoverall= Eoxidation+Ereduction
Magnitude - Measures the driving force behind the reaction; the larger the magnitude of the cell potential, the further the reaction is from equilibrium.

Sign - Indicates in which direction the reaction will shift to reach equilibrium.

• If E is positive, the reaction is spontaneous and will proceed in the forward direction (right).
• If E is negative, the reaction is not spontaneous and will proceed in the reverse direction (left).
Example Calculation:

For the reaction:

Zn + Cu2+ Zn2+ + Cu

See Standard Reduction Potentials Table for standard-state reduction potential values for half-reactions.

Write oxidation and reduction the half-reactions and find the standard reduction potential for each:

• The half-reaction with the great reduction potential (further down on the table) will undergo reduction.
• The half-reaction with the smaller reduction potential (higher up on the table) will undergo oxidation.
• The reaction will be the reverse of what is on the table.
 Reduction: Cu2+ + 2 e- Cu Ered= 0.34 V Oxidation: Zn  Zn2+ + 2 e- Eox= + 0.76 V E= +1.10V

Line Notation

For the reaction:

Zn + Cu2+ Zn2+ + Cu

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