What are Oxidation-Reduction Potentials?
Reduction potentials measure an atom's or ion's tendency to gain electrons and be reduced. The difference between half-cell potentials predicts whether a redox reaction will occur spontaneously.
Standard reduction potential (E°) in volts shows the tendency of a species to gain electrons. E°cell = E°cathode − E°anode; a positive E°cell indicates a spontaneous reaction.
- •F₂ / F⁻: +2.87 V
- •Cl₂ / Cl⁻: +1.36 V
- •Ag⁺ / Ag: +0.80 V
- •Li⁺ / Li: −3.04 V
- •K⁺ / K: −2.93 V
- •Zn²⁺ / Zn: −0.76 V
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Step-by-step worked examples
Calculate E°cell for a cell with Cu²⁺/Cu cathode (E° = +0.34 V) and Zn²⁺/Zn anode (E° = −0.76 V).
E°cell = E°cathode − E°anode E°cell = (+0.34) − (−0.76) E°cell = +0.34 + 0.76 = +1.10 V (spontaneous)
Is the reaction Ag⁺ + Fe → Ag + Fe²⁺ spontaneous? (Ag⁺/Ag: E° = +0.80 V; Fe²⁺/Fe: E° = −0.44 V).
Cathode: Ag⁺ + e⁻ → Ag (E° = +0.80 V) Anode: Fe → Fe²⁺ + 2e⁻ (E° = −0.44 V) E°cell = 0.80 − (−0.44) = +1.24 V (positive, spontaneous)
A cell has E°cathode = +1.36 V and E°anode = +0.80 V. Calculate E°cell.
E°cell = E°cathode − E°anode E°cell = 1.36 − 0.80 = +0.56 V (spontaneous)
Flashcards
Quick quiz
Q1.For a cell with Cathode E° = +1.50 V and Anode E° = +0.62 V, what is E°cell?
Q2.Which species is the strongest oxidising agent (most likely to be reduced)?
Q3.If E°cell = −0.25 V, is the reaction spontaneous?
Q4.What does the cathode reduction potential represent?
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Common mistakes
Confusing anode and cathode reduction potentials. — Correct: Both are reduction potentials; the one at the cathode where reduction actually happens is E°cathode.
Using E°anode directly without reversing its sign. — Correct: E°anode is written as a reduction potential; subtract it as-is: E°cathode − E°anode.
Thinking negative E° always means non-spontaneous. — Correct: Negative E°cell (not individual E°) means non-spontaneous.
Forgetting to subtract: E°cell = E°cathode + E°anode. — Correct: E°cell = E°cathode − E°anode (subtraction, not addition).
FAQ
What is standard reduction potential?
The potential (in volts) measured for a half-reaction under standard conditions (1 M, 1 atm, 25°C) vs. the standard hydrogen electrode (0.00 V).
How do you predict if a redox reaction is spontaneous?
Calculate E°cell = E°cathode − E°anode. If E°cell > 0, the reaction is spontaneous.
Why is hydrogen electrode (H⁺/H₂) set at 0.00 V?
It's the reference point. All other reduction potentials are measured relative to it.
What is the relationship between E° and ΔG?
ΔG° = −nFE°cell (n = electrons, F = Faraday constant). Negative ΔG (spontaneous) requires positive E°cell.