Embedded Files
Introduction
Ecell: The difference in electrical potential that an electrochemical cell has, or in other words, how much voltage the cell can generate.
E°cell (Standard Cell Potential) refers to the voltage a cell can generate under standard conditions (e.g. a temperature of 25°C or 298 K).
Calculating Standard Cell Potential
Standard Reduction Potentials: Guide to how much voltage is generated by half-reactions undergoing reduction at 25°C.
Half-reactions undergoing oxidation have the opposite voltage value of their reduction counterparts.
Step #1: Find standard reduction potentials of half reactions.
Step #2: Switch the sign of the oxidation reaction's standard reduction potential. Balance half-reactions if needed.
Step #3: Use the formula to calculate E°cell:
E°cell = E°reduction + E°oxidation
Step #4: Determine spontaneity if needed.
If E°cell > 0, the reaction is spontaneous and would generate electrical current. (e.g. Galvanic Cells)
If E°cell < 0, the reaction is not spontaneous and would require energy input. (e.g. Electrolysis Cells)
Examples of Calculating E°cell
Example #1: Cu²⁺ (aq) + Zn (s) → Cu (s) + Zn²⁺ (aq)
Step #1: Found standard reduction potentials.
Cu²⁺ + 2e⁻ → Cu +0.34 V
Zn²⁺ + 2e⁻ → Zn -0.76 V
Step #2: Flipped oxidation reaction and switched the sign of its electrical potential.
Zn → Zn²⁺ + 2e⁻ +0.76 V
Step #3: Calculate E°cell:
0.34 + 0.76 = +1.10 V
Step #4: It is spontaneous.
Example #2: Ag⁺ (aq) + Al (s) → Ag (s) + Al³⁺ (aq)
Step #1: Found standard reduction potentials.
Ag⁺ + e⁻ → Ag +0.80 V
Al³⁺ + 3e⁻ → Al -1.66 V
Step #2: Flipped oxidation reaction, switched the sign of its electrical potential, and balanced the equation.
3Ag⁺ + 3e⁻ → 3Ag +1.66 V
Balanced Overall Equation: 3Ag⁺ (aq) + Al (s) → 3Ag (s) + Al³⁺ (aq)
Step #3: Calculate E°cell:
0.80 + 1.66 = +2.46 V
Step #4: It is spontaneous.
Example #3: Na⁺ (aq) + Cu (s) → Na (s) + Cu²⁺ (aq)
Step #1: Found standard reduction potentials.
Cu²⁺ + 2e⁻ → Cu +0.34 V
Na⁺ + e⁻ → Na -2.71 V
Step #2: Flipped oxidation reaction, switched the sign of its electrical potential, and balanced the equation.
Cu → Cu²⁺ + 2e⁻ -0.34 V
2Na⁺ + 2e⁻ → 2Na -2.71 V
Balanced Overall Equation: 2Na⁺ (aq) + Cu (s) → 3Na (s) + Cu²⁺ (aq)
Step #3: Calculate E°cell:
-0.34 + (-2.71) = -3.05 V
Step #4: It is not spontaneous.
Nernst Equation
E°cell only measures the voltage of an electrochemical cell in standard conditions. Realistically, conditions may vary, which affects the cell voltage.
We use the Nernst Equation to determine the cell potential in non-standard conditions:
Ecell = E°cell - (RT/Fn)lnQ
Ecell: Cell Potential
E°cell: Standard Cell Potential
R: Gas Constant (8.314 J/mol·K)
T: Temperature (K)
F: Faraday’s constant (96,485 C/mol)
n: Moles of electrons transferred in reaction
Q: Reaction Quotient
Page updated
Google Sites
Report abuse