What are Standard Electrode Potentials?
Standard electrode potentials measure the tendency of a half-reaction to be reduced, comparing every electrode against hydrogen as a reference. They predict which half-reactions win and what voltage a cell can deliver.
A standard electrode potential (E°) is the voltage of a half-reaction at standard conditions (1 M concentration, 25°C, 1 atm), with hydrogen as reference (0 V). More positive = stronger oxidizer, more negative = stronger reducer.
- •F₂ → 2F⁻ : +2.87 V
- •Cl₂ → 2Cl⁻ : +1.36 V
- •O₂ + 4H⁺ + 4e⁻ → 2H₂O : +1.23 V
- •Li⁺ + e⁻ → Li : −3.04 V
- •K⁺ + e⁻ → K : −2.93 V
- •Ca²⁺ + 2e⁻ → Ca : −2.87 V
Step-by-step worked examples
Cell with Cu²⁺/Cu (+0.34 V) and Zn²⁺/Zn (−0.76 V). Which is cathode?
Cathode = higher E° = Cu²⁺/Cu Anode = lower E° = Zn²⁺/Zn Cell potential E°cell = 0.34 − (−0.76) = +1.10 V
Can Br₂ oxidize Fe²⁺ to Fe³⁺? (Br₂/Br⁻: +1.07 V; Fe³⁺/Fe²⁺: +0.77 V)
For oxidation: E°cell = E°cathode − E°anode = 0.77 − 1.07 = −0.30 V Negative → not spontaneous. No.
Rank Ag⁺, Cu²⁺, Zn²⁺ as oxidizing strength (Ag⁺/Ag: +0.80 V, Cu²⁺/Cu: +0.34 V, Zn²⁺/Zn: −0.76 V).
Higher E° = stronger oxidizer Ag⁺ (+0.80 V) > Cu²⁺ (+0.34 V) > Zn²⁺ (−0.76 V)
Flashcards
Quick quiz
Q1.Which half-reaction is the strongest oxidizer? F₂→2F⁻ (+2.87V) or Cl₂→2Cl⁻ (+1.36V)?
Q2.In a Zn–Cu cell, which is cathode? (Zn: −0.76V, Cu: +0.34V)
Q3.E°cell = −0.50 V. Is the reaction spontaneous?
Q4.Standard conditions include:
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Common mistakes
Higher E° means the metal reduces more easily. — Correct: Higher E° means the ion oxidizes the metal (the ion is reduced).
Anode is always positive. — Correct: Anode is negative in a galvanic cell.
E°cell = E°anode − E°cathode. — Correct: E°cell = E°cathode − E°anode.
Negative E° means no reaction happens. — Correct: Negative E°cell means the reverse is spontaneous.
FAQ
What is the standard electrode potential formula?
E°cell = E°cathode − E°anode. Positive = spontaneous, negative = non-spontaneous.
Why is hydrogen reference chosen as 0 V?
Hydrogen is common and arbitrarily set to 0 V as a universal benchmark for all potentials.
How do you predict which half-reaction is reduced?
The half-reaction with higher E° (more positive) is reduced; lower E° is oxidized.
Can you use electrode potentials to predict if a redox reaction works?
Yes — if E°cell > 0, the reaction is spontaneous. If E°cell < 0, it requires external energy.